Fix: format string type mismatch
[lttng-tools.git] / src / bin / lttng-sessiond / ust-app.c
index 7f4e9f1e0c928df31c5b5a39b1614e54f22361f8..9fe54835b656c69844599e3b0bd7b51f69db482f 100644 (file)
@@ -17,6 +17,7 @@
 
 #define _GNU_SOURCE
 #include <errno.h>
+#include <inttypes.h>
 #include <pthread.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <sys/types.h>
 #include <unistd.h>
 #include <urcu/compiler.h>
+#include <lttng/ust-error.h>
+#include <signal.h>
 
 #include <common/common.h>
 #include <common/sessiond-comm/sessiond-comm.h>
 
+#include "buffer-registry.h"
 #include "fd-limit.h"
 #include "health.h"
 #include "ust-app.h"
 #include "ust-consumer.h"
 #include "ust-ctl.h"
+#include "utils.h"
+
+/* Next available channel key. Access under next_channel_key_lock. */
+static uint64_t _next_channel_key;
+static pthread_mutex_t next_channel_key_lock = PTHREAD_MUTEX_INITIALIZER;
+
+/* Next available session ID. Access under next_session_id_lock. */
+static uint64_t _next_session_id;
+static pthread_mutex_t next_session_id_lock = PTHREAD_MUTEX_INITIALIZER;
+
+/*
+ * Return the incremented value of next_channel_key.
+ */
+static uint64_t get_next_channel_key(void)
+{
+       uint64_t ret;
+
+       pthread_mutex_lock(&next_channel_key_lock);
+       ret = ++_next_channel_key;
+       pthread_mutex_unlock(&next_channel_key_lock);
+       return ret;
+}
+
+/*
+ * Return the atomically incremented value of next_session_id.
+ */
+static uint64_t get_next_session_id(void)
+{
+       uint64_t ret;
+
+       pthread_mutex_lock(&next_session_id_lock);
+       ret = ++_next_session_id;
+       pthread_mutex_unlock(&next_session_id_lock);
+       return ret;
+}
+
+static void copy_channel_attr_to_ustctl(
+               struct ustctl_consumer_channel_attr *attr,
+               struct lttng_ust_channel_attr *uattr)
+{
+       /* Copy event attributes since the layout is different. */
+       attr->subbuf_size = uattr->subbuf_size;
+       attr->num_subbuf = uattr->num_subbuf;
+       attr->overwrite = uattr->overwrite;
+       attr->switch_timer_interval = uattr->switch_timer_interval;
+       attr->read_timer_interval = uattr->read_timer_interval;
+       attr->output = uattr->output;
+}
+
+/*
+ * Match function for the hash table lookup.
+ *
+ * It matches an ust app event based on three attributes which are the event
+ * name, the filter bytecode and the loglevel.
+ */
+static int ht_match_ust_app_event(struct cds_lfht_node *node, const void *_key)
+{
+       struct ust_app_event *event;
+       const struct ust_app_ht_key *key;
+
+       assert(node);
+       assert(_key);
+
+       event = caa_container_of(node, struct ust_app_event, node.node);
+       key = _key;
+
+       /* Match the 3 elements of the key: name, filter and loglevel. */
+
+       /* Event name */
+       if (strncmp(event->attr.name, key->name, sizeof(event->attr.name)) != 0) {
+               goto no_match;
+       }
+
+       /* Event loglevel. */
+       if (event->attr.loglevel != key->loglevel) {
+               if (event->attr.loglevel_type == LTTNG_UST_LOGLEVEL_ALL
+                               && key->loglevel == 0 && event->attr.loglevel == -1) {
+                       /*
+                        * Match is accepted. This is because on event creation, the
+                        * loglevel is set to -1 if the event loglevel type is ALL so 0 and
+                        * -1 are accepted for this loglevel type since 0 is the one set by
+                        * the API when receiving an enable event.
+                        */
+               } else {
+                       goto no_match;
+               }
+       }
+
+       /* One of the filters is NULL, fail. */
+       if ((key->filter && !event->filter) || (!key->filter && event->filter)) {
+               goto no_match;
+       }
+
+       if (key->filter && event->filter) {
+               /* Both filters exists, check length followed by the bytecode. */
+               if (event->filter->len != key->filter->len ||
+                               memcmp(event->filter->data, key->filter->data,
+                                       event->filter->len) != 0) {
+                       goto no_match;
+               }
+       }
+
+       /* Match. */
+       return 1;
+
+no_match:
+       return 0;
+}
+
+/*
+ * Unique add of an ust app event in the given ht. This uses the custom
+ * ht_match_ust_app_event match function and the event name as hash.
+ */
+static void add_unique_ust_app_event(struct ust_app_channel *ua_chan,
+               struct ust_app_event *event)
+{
+       struct cds_lfht_node *node_ptr;
+       struct ust_app_ht_key key;
+       struct lttng_ht *ht;
+
+       assert(ua_chan);
+       assert(ua_chan->events);
+       assert(event);
+
+       ht = ua_chan->events;
+       key.name = event->attr.name;
+       key.filter = event->filter;
+       key.loglevel = event->attr.loglevel;
+
+       node_ptr = cds_lfht_add_unique(ht->ht,
+                       ht->hash_fct(event->node.key, lttng_ht_seed),
+                       ht_match_ust_app_event, &key, &event->node.node);
+       assert(node_ptr == &event->node.node);
+}
+
+/*
+ * Close the notify socket from the given RCU head object. This MUST be called
+ * through a call_rcu().
+ */
+static void close_notify_sock_rcu(struct rcu_head *head)
+{
+       int ret;
+       struct ust_app_notify_sock_obj *obj =
+               caa_container_of(head, struct ust_app_notify_sock_obj, head);
+
+       /* Must have a valid fd here. */
+       assert(obj->fd >= 0);
+
+       ret = close(obj->fd);
+       if (ret) {
+               ERR("close notify sock %d RCU", obj->fd);
+       }
+       lttng_fd_put(LTTNG_FD_APPS, 1);
+
+       free(obj);
+}
+
+/*
+ * Return the session registry according to the buffer type of the given
+ * session.
+ *
+ * A registry per UID object MUST exists before calling this function or else
+ * it assert() if not found. RCU read side lock must be acquired.
+ */
+static struct ust_registry_session *get_session_registry(
+               struct ust_app_session *ua_sess)
+{
+       struct ust_registry_session *registry = NULL;
+
+       assert(ua_sess);
+
+       switch (ua_sess->buffer_type) {
+       case LTTNG_BUFFER_PER_PID:
+       {
+               struct buffer_reg_pid *reg_pid = buffer_reg_pid_find(ua_sess->id);
+               if (!reg_pid) {
+                       goto error;
+               }
+               registry = reg_pid->registry->reg.ust;
+               break;
+       }
+       case LTTNG_BUFFER_PER_UID:
+       {
+               struct buffer_reg_uid *reg_uid = buffer_reg_uid_find(
+                               ua_sess->tracing_id, ua_sess->bits_per_long, ua_sess->uid);
+               if (!reg_uid) {
+                       goto error;
+               }
+               registry = reg_uid->registry->reg.ust;
+               break;
+       }
+       default:
+               assert(0);
+       };
+
+error:
+       return registry;
+}
 
 /*
  * Delete ust context safely. RCU read lock must be held before calling
 static
 void delete_ust_app_ctx(int sock, struct ust_app_ctx *ua_ctx)
 {
+       int ret;
+
+       assert(ua_ctx);
+
        if (ua_ctx->obj) {
-               ustctl_release_object(sock, ua_ctx->obj);
+               ret = ustctl_release_object(sock, ua_ctx->obj);
+               if (ret < 0 && ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
+                       ERR("UST app sock %d release ctx obj handle %d failed with ret %d",
+                                       sock, ua_ctx->obj->handle, ret);
+               }
                free(ua_ctx->obj);
        }
        free(ua_ctx);
@@ -57,53 +267,94 @@ static
 void delete_ust_app_event(int sock, struct ust_app_event *ua_event)
 {
        int ret;
-       struct lttng_ht_iter iter;
-       struct ust_app_ctx *ua_ctx;
 
-       /* Destroy each context of event */
-       cds_lfht_for_each_entry(ua_event->ctx->ht, &iter.iter, ua_ctx,
-                       node.node) {
-               ret = lttng_ht_del(ua_event->ctx, &iter);
-               assert(!ret);
-               delete_ust_app_ctx(sock, ua_ctx);
-       }
+       assert(ua_event);
+
        free(ua_event->filter);
-       lttng_ht_destroy(ua_event->ctx);
 
        if (ua_event->obj != NULL) {
-               ustctl_release_object(sock, ua_event->obj);
+               ret = ustctl_release_object(sock, ua_event->obj);
+               if (ret < 0 && ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
+                       ERR("UST app sock %d release event obj failed with ret %d",
+                                       sock, ret);
+               }
                free(ua_event->obj);
        }
        free(ua_event);
 }
 
 /*
- * Delete ust app stream safely. RCU read lock must be held before calling
- * this function.
+ * Release ust data object of the given stream.
+ *
+ * Return 0 on success or else a negative value.
  */
-static
-void delete_ust_app_stream(int sock, struct ltt_ust_stream *stream)
+static int release_ust_app_stream(int sock, struct ust_app_stream *stream)
 {
+       int ret = 0;
+
+       assert(stream);
+
        if (stream->obj) {
-               ustctl_release_object(sock, stream->obj);
+               ret = ustctl_release_object(sock, stream->obj);
+               if (ret < 0 && ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
+                       ERR("UST app sock %d release stream obj failed with ret %d",
+                                       sock, ret);
+               }
                lttng_fd_put(LTTNG_FD_APPS, 2);
                free(stream->obj);
        }
+
+       return ret;
+}
+
+/*
+ * Delete ust app stream safely. RCU read lock must be held before calling
+ * this function.
+ */
+static
+void delete_ust_app_stream(int sock, struct ust_app_stream *stream)
+{
+       assert(stream);
+
+       (void) release_ust_app_stream(sock, stream);
        free(stream);
 }
 
+/*
+ * We need to execute ht_destroy outside of RCU read-side critical
+ * section and outside of call_rcu thread, so we postpone its execution
+ * using ht_cleanup_push. It is simpler than to change the semantic of
+ * the many callers of delete_ust_app_session().
+ */
+static
+void delete_ust_app_channel_rcu(struct rcu_head *head)
+{
+       struct ust_app_channel *ua_chan =
+               caa_container_of(head, struct ust_app_channel, rcu_head);
+
+       ht_cleanup_push(ua_chan->ctx);
+       ht_cleanup_push(ua_chan->events);
+       free(ua_chan);
+}
+
 /*
  * Delete ust app channel safely. RCU read lock must be held before calling
  * this function.
  */
 static
-void delete_ust_app_channel(int sock, struct ust_app_channel *ua_chan)
+void delete_ust_app_channel(int sock, struct ust_app_channel *ua_chan,
+               struct ust_app *app)
 {
        int ret;
        struct lttng_ht_iter iter;
        struct ust_app_event *ua_event;
        struct ust_app_ctx *ua_ctx;
-       struct ltt_ust_stream *stream, *stmp;
+       struct ust_app_stream *stream, *stmp;
+       struct ust_registry_session *registry;
+
+       assert(ua_chan);
+
+       DBG3("UST app deleting channel %s", ua_chan->name);
 
        /* Wipe stream */
        cds_list_for_each_entry_safe(stream, stmp, &ua_chan->streams.head, list) {
@@ -117,7 +368,6 @@ void delete_ust_app_channel(int sock, struct ust_app_channel *ua_chan)
                assert(!ret);
                delete_ust_app_ctx(sock, ua_ctx);
        }
-       lttng_ht_destroy(ua_chan->ctx);
 
        /* Wipe events */
        cds_lfht_for_each_entry(ua_chan->events->ht, &iter.iter, ua_event,
@@ -126,14 +376,242 @@ void delete_ust_app_channel(int sock, struct ust_app_channel *ua_chan)
                assert(!ret);
                delete_ust_app_event(sock, ua_event);
        }
-       lttng_ht_destroy(ua_chan->events);
+
+       if (ua_chan->session->buffer_type == LTTNG_BUFFER_PER_PID) {
+               /* Wipe and free registry from session registry. */
+               registry = get_session_registry(ua_chan->session);
+               if (registry) {
+                       ust_registry_channel_del_free(registry, ua_chan->key);
+               }
+       }
 
        if (ua_chan->obj != NULL) {
-               ustctl_release_object(sock, ua_chan->obj);
-               lttng_fd_put(LTTNG_FD_APPS, 2);
+               /* Remove channel from application UST object descriptor. */
+               iter.iter.node = &ua_chan->ust_objd_node.node;
+               lttng_ht_del(app->ust_objd, &iter);
+               ret = ustctl_release_object(sock, ua_chan->obj);
+               if (ret < 0 && ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
+                       ERR("UST app sock %d release channel obj failed with ret %d",
+                                       sock, ret);
+               }
+               lttng_fd_put(LTTNG_FD_APPS, 1);
                free(ua_chan->obj);
        }
-       free(ua_chan);
+       call_rcu(&ua_chan->rcu_head, delete_ust_app_channel_rcu);
+}
+
+/*
+ * Push metadata to consumer socket.
+ *
+ * The socket lock MUST be acquired.
+ * The ust app session lock MUST be acquired.
+ *
+ * On success, return the len of metadata pushed or else a negative value.
+ */
+ssize_t ust_app_push_metadata(struct ust_registry_session *registry,
+               struct consumer_socket *socket, int send_zero_data)
+{
+       int ret;
+       char *metadata_str = NULL;
+       size_t len, offset;
+       ssize_t ret_val;
+
+       assert(registry);
+       assert(socket);
+
+       /*
+        * On a push metadata error either the consumer is dead or the metadata
+        * channel has been destroyed because its endpoint might have died (e.g:
+        * relayd). If so, the metadata closed flag is set to 1 so we deny pushing
+        * metadata again which is not valid anymore on the consumer side.
+        *
+        * The ust app session mutex locked allows us to make this check without
+        * the registry lock.
+        */
+       if (registry->metadata_closed) {
+               return -EPIPE;
+       }
+
+       pthread_mutex_lock(&registry->lock);
+
+       offset = registry->metadata_len_sent;
+       len = registry->metadata_len - registry->metadata_len_sent;
+       if (len == 0) {
+               DBG3("No metadata to push for metadata key %" PRIu64,
+                               registry->metadata_key);
+               ret_val = len;
+               if (send_zero_data) {
+                       DBG("No metadata to push");
+                       goto push_data;
+               }
+               goto end;
+       }
+
+       /* Allocate only what we have to send. */
+       metadata_str = zmalloc(len);
+       if (!metadata_str) {
+               PERROR("zmalloc ust app metadata string");
+               ret_val = -ENOMEM;
+               goto error;
+       }
+       /* Copy what we haven't send out. */
+       memcpy(metadata_str, registry->metadata + offset, len);
+       registry->metadata_len_sent += len;
+
+push_data:
+       pthread_mutex_unlock(&registry->lock);
+       ret = consumer_push_metadata(socket, registry->metadata_key,
+                       metadata_str, len, offset);
+       if (ret < 0) {
+               ret_val = ret;
+               goto error_push;
+       }
+
+       free(metadata_str);
+       return len;
+
+end:
+error:
+       pthread_mutex_unlock(&registry->lock);
+error_push:
+       free(metadata_str);
+       return ret_val;
+}
+
+/*
+ * For a given application and session, push metadata to consumer. The session
+ * lock MUST be acquired here before calling this.
+ * Either sock or consumer is required : if sock is NULL, the default
+ * socket to send the metadata is retrieved from consumer, if sock
+ * is not NULL we use it to send the metadata.
+ *
+ * Return 0 on success else a negative error.
+ */
+static int push_metadata(struct ust_registry_session *registry,
+               struct consumer_output *consumer)
+{
+       int ret_val;
+       ssize_t ret;
+       struct consumer_socket *socket;
+
+       assert(registry);
+       assert(consumer);
+
+       rcu_read_lock();
+
+       /*
+        * Means that no metadata was assigned to the session. This can happens if
+        * no start has been done previously.
+        */
+       if (!registry->metadata_key) {
+               ret_val = 0;
+               goto end_rcu_unlock;
+       }
+
+       /* Get consumer socket to use to push the metadata.*/
+       socket = consumer_find_socket_by_bitness(registry->bits_per_long,
+                       consumer);
+       if (!socket) {
+               ret_val = -1;
+               goto error_rcu_unlock;
+       }
+
+       /*
+        * TODO: Currently, we hold the socket lock around sampling of the next
+        * metadata segment to ensure we send metadata over the consumer socket in
+        * the correct order. This makes the registry lock nest inside the socket
+        * lock.
+        *
+        * Please note that this is a temporary measure: we should move this lock
+        * back into ust_consumer_push_metadata() when the consumer gets the
+        * ability to reorder the metadata it receives.
+        */
+       pthread_mutex_lock(socket->lock);
+       ret = ust_app_push_metadata(registry, socket, 0);
+       pthread_mutex_unlock(socket->lock);
+       if (ret < 0) {
+               ret_val = ret;
+               goto error_rcu_unlock;
+       }
+
+       rcu_read_unlock();
+       return 0;
+
+error_rcu_unlock:
+       /*
+        * On error, flag the registry that the metadata is closed. We were unable
+        * to push anything and this means that either the consumer is not
+        * responding or the metadata cache has been destroyed on the consumer.
+        */
+       registry->metadata_closed = 1;
+end_rcu_unlock:
+       rcu_read_unlock();
+       return ret_val;
+}
+
+/*
+ * Send to the consumer a close metadata command for the given session. Once
+ * done, the metadata channel is deleted and the session metadata pointer is
+ * nullified. The session lock MUST be acquired here unless the application is
+ * in the destroy path.
+ *
+ * Return 0 on success else a negative value.
+ */
+static int close_metadata(struct ust_registry_session *registry,
+               struct consumer_output *consumer)
+{
+       int ret;
+       struct consumer_socket *socket;
+
+       assert(registry);
+       assert(consumer);
+
+       rcu_read_lock();
+
+       if (!registry->metadata_key || registry->metadata_closed) {
+               ret = 0;
+               goto end;
+       }
+
+       /* Get consumer socket to use to push the metadata.*/
+       socket = consumer_find_socket_by_bitness(registry->bits_per_long,
+                       consumer);
+       if (!socket) {
+               ret = -1;
+               goto error;
+       }
+
+       ret = consumer_close_metadata(socket, registry->metadata_key);
+       if (ret < 0) {
+               goto error;
+       }
+
+error:
+       /*
+        * Metadata closed. Even on error this means that the consumer is not
+        * responding or not found so either way a second close should NOT be emit
+        * for this registry.
+        */
+       registry->metadata_closed = 1;
+end:
+       rcu_read_unlock();
+       return ret;
+}
+
+/*
+ * We need to execute ht_destroy outside of RCU read-side critical
+ * section and outside of call_rcu thread, so we postpone its execution
+ * using ht_cleanup_push. It is simpler than to change the semantic of
+ * the many callers of delete_ust_app_session().
+ */
+static
+void delete_ust_app_session_rcu(struct rcu_head *head)
+{
+       struct ust_app_session *ua_sess =
+               caa_container_of(head, struct ust_app_session, rcu_head);
+
+       ht_cleanup_push(ua_sess->channels);
+       free(ua_sess);
 }
 
 /*
@@ -141,65 +619,91 @@ void delete_ust_app_channel(int sock, struct ust_app_channel *ua_chan)
  * this function.
  */
 static
-void delete_ust_app_session(int sock, struct ust_app_session *ua_sess)
+void delete_ust_app_session(int sock, struct ust_app_session *ua_sess,
+               struct ust_app *app)
 {
        int ret;
        struct lttng_ht_iter iter;
        struct ust_app_channel *ua_chan;
+       struct ust_registry_session *registry;
 
-       if (ua_sess->metadata) {
-               if (ua_sess->metadata->stream_obj) {
-                       ustctl_release_object(sock, ua_sess->metadata->stream_obj);
-                       lttng_fd_put(LTTNG_FD_APPS, 2);
-                       free(ua_sess->metadata->stream_obj);
-               }
-               if (ua_sess->metadata->obj) {
-                       ustctl_release_object(sock, ua_sess->metadata->obj);
-                       lttng_fd_put(LTTNG_FD_APPS, 2);
-                       free(ua_sess->metadata->obj);
+       assert(ua_sess);
+
+       pthread_mutex_lock(&ua_sess->lock);
+
+       registry = get_session_registry(ua_sess);
+       if (registry && !registry->metadata_closed) {
+               /* Push metadata for application before freeing the application. */
+               (void) push_metadata(registry, ua_sess->consumer);
+
+               /*
+                * Don't ask to close metadata for global per UID buffers. Close
+                * metadata only on destroy trace session in this case. Also, the
+                * previous push metadata could have flag the metadata registry to
+                * close so don't send a close command if closed.
+                */
+               if (ua_sess->buffer_type != LTTNG_BUFFER_PER_UID &&
+                               !registry->metadata_closed) {
+                       /* And ask to close it for this session registry. */
+                       (void) close_metadata(registry, ua_sess->consumer);
                }
-               trace_ust_destroy_metadata(ua_sess->metadata);
        }
 
        cds_lfht_for_each_entry(ua_sess->channels->ht, &iter.iter, ua_chan,
                        node.node) {
                ret = lttng_ht_del(ua_sess->channels, &iter);
                assert(!ret);
-               delete_ust_app_channel(sock, ua_chan);
+               delete_ust_app_channel(sock, ua_chan, app);
+       }
+
+       /* In case of per PID, the registry is kept in the session. */
+       if (ua_sess->buffer_type == LTTNG_BUFFER_PER_PID) {
+               struct buffer_reg_pid *reg_pid = buffer_reg_pid_find(ua_sess->id);
+               if (reg_pid) {
+                       buffer_reg_pid_remove(reg_pid);
+                       buffer_reg_pid_destroy(reg_pid);
+               }
        }
-       lttng_ht_destroy(ua_sess->channels);
 
        if (ua_sess->handle != -1) {
-               ustctl_release_handle(sock, ua_sess->handle);
+               ret = ustctl_release_handle(sock, ua_sess->handle);
+               if (ret < 0 && ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
+                       ERR("UST app sock %d release session handle failed with ret %d",
+                                       sock, ret);
+               }
        }
-       free(ua_sess);
+       pthread_mutex_unlock(&ua_sess->lock);
+
+       call_rcu(&ua_sess->rcu_head, delete_ust_app_session_rcu);
 }
 
 /*
  * Delete a traceable application structure from the global list. Never call
  * this function outside of a call_rcu call.
+ *
+ * RCU read side lock should _NOT_ be held when calling this function.
  */
 static
 void delete_ust_app(struct ust_app *app)
 {
        int ret, sock;
-       struct lttng_ht_iter iter;
-       struct ust_app_session *ua_sess;
-
-       rcu_read_lock();
+       struct ust_app_session *ua_sess, *tmp_ua_sess;
 
        /* Delete ust app sessions info */
        sock = app->sock;
        app->sock = -1;
 
        /* Wipe sessions */
-       cds_lfht_for_each_entry(app->sessions->ht, &iter.iter, ua_sess,
-                       node.node) {
-               ret = lttng_ht_del(app->sessions, &iter);
-               assert(!ret);
-               delete_ust_app_session(app->sock, ua_sess);
+       cds_list_for_each_entry_safe(ua_sess, tmp_ua_sess, &app->teardown_head,
+                       teardown_node) {
+               /* Free every object in the session and the session. */
+               rcu_read_lock();
+               delete_ust_app_session(sock, ua_sess, app);
+               rcu_read_unlock();
        }
-       lttng_ht_destroy(app->sessions);
+
+       ht_cleanup_push(app->sessions);
+       ht_cleanup_push(app->ust_objd);
 
        /*
         * Wait until we have deleted the application from the sock hash table
@@ -222,8 +726,6 @@ void delete_ust_app(struct ust_app *app)
 
        DBG2("UST app pid %d deleted", app->pid);
        free(app);
-
-       rcu_read_unlock();
 }
 
 /*
@@ -242,26 +744,54 @@ void delete_ust_app_rcu(struct rcu_head *head)
 }
 
 /*
- * Alloc new UST app session.
+ * Delete the session from the application ht and delete the data structure by
+ * freeing every object inside and releasing them.
  */
-static
-struct ust_app_session *alloc_ust_app_session(void)
+static void destroy_app_session(struct ust_app *app,
+               struct ust_app_session *ua_sess)
 {
-       struct ust_app_session *ua_sess;
+       int ret;
+       struct lttng_ht_iter iter;
+
+       assert(app);
+       assert(ua_sess);
+
+       iter.iter.node = &ua_sess->node.node;
+       ret = lttng_ht_del(app->sessions, &iter);
+       if (ret) {
+               /* Already scheduled for teardown. */
+               goto end;
+       }
+
+       /* Once deleted, free the data structure. */
+       delete_ust_app_session(app->sock, ua_sess, app);
+
+end:
+       return;
+}
+
+/*
+ * Alloc new UST app session.
+ */
+static
+struct ust_app_session *alloc_ust_app_session(struct ust_app *app)
+{
+       struct ust_app_session *ua_sess;
 
        /* Init most of the default value by allocating and zeroing */
        ua_sess = zmalloc(sizeof(struct ust_app_session));
        if (ua_sess == NULL) {
                PERROR("malloc");
-               goto error;
+               goto error_free;
        }
 
        ua_sess->handle = -1;
        ua_sess->channels = lttng_ht_new(0, LTTNG_HT_TYPE_STRING);
+       pthread_mutex_init(&ua_sess->lock, NULL);
 
        return ua_sess;
 
-error:
+error_free:
        return NULL;
 }
 
@@ -270,7 +800,8 @@ error:
  */
 static
 struct ust_app_channel *alloc_ust_app_channel(char *name,
-               struct lttng_ust_channel *attr)
+               struct ust_app_session *ua_sess,
+               struct lttng_ust_channel_attr *attr)
 {
        struct ust_app_channel *ua_chan;
 
@@ -287,6 +818,8 @@ struct ust_app_channel *alloc_ust_app_channel(char *name,
 
        ua_chan->enabled = 1;
        ua_chan->handle = -1;
+       ua_chan->session = ua_sess;
+       ua_chan->key = get_next_channel_key();
        ua_chan->ctx = lttng_ht_new(0, LTTNG_HT_TYPE_ULONG);
        ua_chan->events = lttng_ht_new(0, LTTNG_HT_TYPE_STRING);
        lttng_ht_node_init_str(&ua_chan->node, ua_chan->name);
@@ -295,8 +828,16 @@ struct ust_app_channel *alloc_ust_app_channel(char *name,
 
        /* Copy attributes */
        if (attr) {
-               memcpy(&ua_chan->attr, attr, sizeof(ua_chan->attr));
+               /* Translate from lttng_ust_channel to ustctl_consumer_channel_attr. */
+               ua_chan->attr.subbuf_size = attr->subbuf_size;
+               ua_chan->attr.num_subbuf = attr->num_subbuf;
+               ua_chan->attr.overwrite = attr->overwrite;
+               ua_chan->attr.switch_timer_interval = attr->switch_timer_interval;
+               ua_chan->attr.read_timer_interval = attr->read_timer_interval;
+               ua_chan->attr.output = attr->output;
        }
+       /* By default, the channel is a per cpu channel. */
+       ua_chan->attr.type = LTTNG_UST_CHAN_PER_CPU;
 
        DBG3("UST app channel %s allocated", ua_chan->name);
 
@@ -306,6 +847,28 @@ error:
        return NULL;
 }
 
+/*
+ * Allocate and initialize a UST app stream.
+ *
+ * Return newly allocated stream pointer or NULL on error.
+ */
+struct ust_app_stream *ust_app_alloc_stream(void)
+{
+       struct ust_app_stream *stream = NULL;
+
+       stream = zmalloc(sizeof(*stream));
+       if (stream == NULL) {
+               PERROR("zmalloc ust app stream");
+               goto error;
+       }
+
+       /* Zero could be a valid value for a handle so flag it to -1. */
+       stream->handle = -1;
+
+error:
+       return stream;
+}
+
 /*
  * Alloc new UST app event.
  */
@@ -325,7 +888,6 @@ struct ust_app_event *alloc_ust_app_event(char *name,
        ua_event->enabled = 1;
        strncpy(ua_event->name, name, sizeof(ua_event->name));
        ua_event->name[sizeof(ua_event->name) - 1] = '\0';
-       ua_event->ctx = lttng_ht_new(0, LTTNG_HT_TYPE_ULONG);
        lttng_ht_node_init_str(&ua_event->node, ua_event->name);
 
        /* Copy attributes */
@@ -364,6 +926,29 @@ error:
        return ua_ctx;
 }
 
+/*
+ * Allocate a filter and copy the given original filter.
+ *
+ * Return allocated filter or NULL on error.
+ */
+static struct lttng_ust_filter_bytecode *alloc_copy_ust_app_filter(
+               struct lttng_ust_filter_bytecode *orig_f)
+{
+       struct lttng_ust_filter_bytecode *filter = NULL;
+
+       /* Copy filter bytecode */
+       filter = zmalloc(sizeof(*filter) + orig_f->len);
+       if (!filter) {
+               PERROR("zmalloc alloc ust app filter");
+               goto error;
+       }
+
+       memcpy(filter, orig_f, sizeof(*filter) + orig_f->len);
+
+error:
+       return filter;
+}
+
 /*
  * Find an ust_app using the sock and return it. RCU read side lock must be
  * held before calling this helper function.
@@ -388,54 +973,96 @@ error:
 }
 
 /*
- * Create the channel context on the tracer.
+ * Find an ust_app using the notify sock and return it. RCU read side lock must
+ * be held before calling this helper function.
  */
-static
-int create_ust_channel_context(struct ust_app_channel *ua_chan,
-               struct ust_app_ctx *ua_ctx, struct ust_app *app)
+static struct ust_app *find_app_by_notify_sock(int sock)
 {
-       int ret;
-
-       health_code_update(&health_thread_cmd);
+       struct lttng_ht_node_ulong *node;
+       struct lttng_ht_iter iter;
 
-       ret = ustctl_add_context(app->sock, &ua_ctx->ctx,
-                       ua_chan->obj, &ua_ctx->obj);
-       if (ret < 0) {
+       lttng_ht_lookup(ust_app_ht_by_notify_sock, (void *)((unsigned long) sock),
+                       &iter);
+       node = lttng_ht_iter_get_node_ulong(&iter);
+       if (node == NULL) {
+               DBG2("UST app find by notify sock %d not found", sock);
                goto error;
        }
 
-       ua_ctx->handle = ua_ctx->obj->handle;
-
-       DBG2("UST app context created successfully for channel %s", ua_chan->name);
+       return caa_container_of(node, struct ust_app, notify_sock_n);
 
 error:
-       health_code_update(&health_thread_cmd);
-       return ret;
+       return NULL;
+}
+
+/*
+ * Lookup for an ust app event based on event name, filter bytecode and the
+ * event loglevel.
+ *
+ * Return an ust_app_event object or NULL on error.
+ */
+static struct ust_app_event *find_ust_app_event(struct lttng_ht *ht,
+               char *name, struct lttng_ust_filter_bytecode *filter, int loglevel)
+{
+       struct lttng_ht_iter iter;
+       struct lttng_ht_node_str *node;
+       struct ust_app_event *event = NULL;
+       struct ust_app_ht_key key;
+
+       assert(name);
+       assert(ht);
+
+       /* Setup key for event lookup. */
+       key.name = name;
+       key.filter = filter;
+       key.loglevel = loglevel;
+
+       /* Lookup using the event name as hash and a custom match fct. */
+       cds_lfht_lookup(ht->ht, ht->hash_fct((void *) name, lttng_ht_seed),
+                       ht_match_ust_app_event, &key, &iter.iter);
+       node = lttng_ht_iter_get_node_str(&iter);
+       if (node == NULL) {
+               goto end;
+       }
+
+       event = caa_container_of(node, struct ust_app_event, node);
+
+end:
+       return event;
 }
 
 /*
- * Create the event context on the tracer.
+ * Create the channel context on the tracer.
+ *
+ * Called with UST app session lock held.
  */
 static
-int create_ust_event_context(struct ust_app_event *ua_event,
+int create_ust_channel_context(struct ust_app_channel *ua_chan,
                struct ust_app_ctx *ua_ctx, struct ust_app *app)
 {
        int ret;
 
-       health_code_update(&health_thread_cmd);
+       health_code_update();
 
        ret = ustctl_add_context(app->sock, &ua_ctx->ctx,
-                       ua_event->obj, &ua_ctx->obj);
+                       ua_chan->obj, &ua_ctx->obj);
        if (ret < 0) {
+               if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
+                       ERR("UST app create channel context failed for app (pid: %d) "
+                                       "with ret %d", app->pid, ret);
+               } else {
+                       DBG3("UST app disable event failed. Application is dead.");
+               }
                goto error;
        }
 
        ua_ctx->handle = ua_ctx->obj->handle;
 
-       DBG2("UST app context created successfully for event %s", ua_event->name);
+       DBG2("UST app context handle %d created successfully for channel %s",
+                       ua_ctx->handle, ua_chan->name);
 
 error:
-       health_code_update(&health_thread_cmd);
+       health_code_update();
        return ret;
 }
 
@@ -448,7 +1075,7 @@ int set_ust_event_filter(struct ust_app_event *ua_event,
 {
        int ret;
 
-       health_code_update(&health_thread_cmd);
+       health_code_update();
 
        if (!ua_event->filter) {
                ret = 0;
@@ -458,13 +1085,19 @@ int set_ust_event_filter(struct ust_app_event *ua_event,
        ret = ustctl_set_filter(app->sock, ua_event->filter,
                        ua_event->obj);
        if (ret < 0) {
+               if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
+                       ERR("UST app event %s filter failed for app (pid: %d) "
+                                       "with ret %d", ua_event->attr.name, app->pid, ret);
+               } else {
+                       DBG3("UST app filter event failed. Application is dead.");
+               }
                goto error;
        }
 
        DBG2("UST filter set successfully for event %s", ua_event->name);
 
 error:
-       health_code_update(&health_thread_cmd);
+       health_code_update();
        return ret;
 }
 
@@ -476,13 +1109,17 @@ static int disable_ust_event(struct ust_app *app,
 {
        int ret;
 
-       health_code_update(&health_thread_cmd);
+       health_code_update();
 
        ret = ustctl_disable(app->sock, ua_event->obj);
        if (ret < 0) {
-               ERR("UST app event %s disable failed for app (pid: %d) "
-                               "and session handle %d with ret %d",
-                               ua_event->attr.name, app->pid, ua_sess->handle, ret);
+               if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
+                       ERR("UST app event %s disable failed for app (pid: %d) "
+                                       "and session handle %d with ret %d",
+                                       ua_event->attr.name, app->pid, ua_sess->handle, ret);
+               } else {
+                       DBG3("UST app disable event failed. Application is dead.");
+               }
                goto error;
        }
 
@@ -490,7 +1127,7 @@ static int disable_ust_event(struct ust_app *app,
                        ua_event->attr.name, app->pid);
 
 error:
-       health_code_update(&health_thread_cmd);
+       health_code_update();
        return ret;
 }
 
@@ -502,13 +1139,17 @@ static int disable_ust_channel(struct ust_app *app,
 {
        int ret;
 
-       health_code_update(&health_thread_cmd);
+       health_code_update();
 
        ret = ustctl_disable(app->sock, ua_chan->obj);
        if (ret < 0) {
-               ERR("UST app channel %s disable failed for app (pid: %d) "
-                               "and session handle %d with ret %d",
-                               ua_chan->name, app->pid, ua_sess->handle, ret);
+               if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
+                       ERR("UST app channel %s disable failed for app (pid: %d) "
+                                       "and session handle %d with ret %d",
+                                       ua_chan->name, app->pid, ua_sess->handle, ret);
+               } else {
+                       DBG3("UST app disable channel failed. Application is dead.");
+               }
                goto error;
        }
 
@@ -516,7 +1157,7 @@ static int disable_ust_channel(struct ust_app *app,
                        ua_chan->name, app->pid);
 
 error:
-       health_code_update(&health_thread_cmd);
+       health_code_update();
        return ret;
 }
 
@@ -528,13 +1169,17 @@ static int enable_ust_channel(struct ust_app *app,
 {
        int ret;
 
-       health_code_update(&health_thread_cmd);
+       health_code_update();
 
        ret = ustctl_enable(app->sock, ua_chan->obj);
        if (ret < 0) {
-               ERR("UST app channel %s enable failed for app (pid: %d) "
-                               "and session handle %d with ret %d",
-                               ua_chan->name, app->pid, ua_sess->handle, ret);
+               if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
+                       ERR("UST app channel %s enable failed for app (pid: %d) "
+                                       "and session handle %d with ret %d",
+                                       ua_chan->name, app->pid, ua_sess->handle, ret);
+               } else {
+                       DBG3("UST app enable channel failed. Application is dead.");
+               }
                goto error;
        }
 
@@ -544,7 +1189,7 @@ static int enable_ust_channel(struct ust_app *app,
                        ua_chan->name, app->pid);
 
 error:
-       health_code_update(&health_thread_cmd);
+       health_code_update();
        return ret;
 }
 
@@ -556,13 +1201,17 @@ static int enable_ust_event(struct ust_app *app,
 {
        int ret;
 
-       health_code_update(&health_thread_cmd);
+       health_code_update();
 
        ret = ustctl_enable(app->sock, ua_event->obj);
        if (ret < 0) {
-               ERR("UST app event %s enable failed for app (pid: %d) "
-                               "and session handle %d with ret %d",
-                               ua_event->attr.name, app->pid, ua_sess->handle, ret);
+               if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
+                       ERR("UST app event %s enable failed for app (pid: %d) "
+                                       "and session handle %d with ret %d",
+                                       ua_event->attr.name, app->pid, ua_sess->handle, ret);
+               } else {
+                       DBG3("UST app enable event failed. Application is dead.");
+               }
                goto error;
        }
 
@@ -570,134 +1219,60 @@ static int enable_ust_event(struct ust_app *app,
                        ua_event->attr.name, app->pid);
 
 error:
-       health_code_update(&health_thread_cmd);
-       return ret;
-}
-
-/*
- * Open metadata onto the UST tracer for a UST session.
- */
-static int open_ust_metadata(struct ust_app *app,
-               struct ust_app_session *ua_sess)
-{
-       int ret;
-       struct lttng_ust_channel_attr uattr;
-
-       health_code_update(&health_thread_cmd);
-
-       uattr.overwrite = ua_sess->metadata->attr.overwrite;
-       uattr.subbuf_size = ua_sess->metadata->attr.subbuf_size;
-       uattr.num_subbuf = ua_sess->metadata->attr.num_subbuf;
-       uattr.switch_timer_interval =
-               ua_sess->metadata->attr.switch_timer_interval;
-       uattr.read_timer_interval =
-               ua_sess->metadata->attr.read_timer_interval;
-       uattr.output = ua_sess->metadata->attr.output;
-
-       /* We are going to receive 2 fds, we need to reserve them. */
-       ret = lttng_fd_get(LTTNG_FD_APPS, 2);
-       if (ret < 0) {
-               ERR("Exhausted number of available FD upon metadata open");
-               goto error;
-       }
-       /* UST tracer metadata creation */
-       ret = ustctl_open_metadata(app->sock, ua_sess->handle, &uattr,
-                       &ua_sess->metadata->obj);
-       if (ret < 0) {
-               ERR("UST app open metadata failed for app pid:%d with ret %d",
-                               app->pid, ret);
-               goto error;
-       }
-
-       ua_sess->metadata->handle = ua_sess->metadata->obj->handle;
-
-error:
-       health_code_update(&health_thread_cmd);
-       return ret;
-}
-
-/*
- * Create stream onto the UST tracer for a UST session.
- */
-static int create_ust_stream(struct ust_app *app,
-               struct ust_app_session *ua_sess)
-{
-       int ret;
-
-       health_code_update(&health_thread_cmd);
-
-       /* We are going to receive 2 fds, we need to reserve them. */
-       ret = lttng_fd_get(LTTNG_FD_APPS, 2);
-       if (ret < 0) {
-               ERR("Exhausted number of available FD upon metadata stream create");
-               goto error;
-       }
-       ret = ustctl_create_stream(app->sock, ua_sess->metadata->obj,
-                       &ua_sess->metadata->stream_obj);
-       if (ret < 0) {
-               ERR("UST create metadata stream failed");
-               goto error;
-       }
-
-error:
-       health_code_update(&health_thread_cmd);
+       health_code_update();
        return ret;
 }
 
 /*
- * Create the specified channel onto the UST tracer for a UST session.
+ * Send channel and stream buffer to application.
+ *
+ * Return 0 on success. On error, a negative value is returned.
  */
-static int create_ust_channel(struct ust_app *app,
+static int send_channel_pid_to_ust(struct ust_app *app,
                struct ust_app_session *ua_sess, struct ust_app_channel *ua_chan)
 {
        int ret;
+       struct ust_app_stream *stream, *stmp;
 
-       health_code_update(&health_thread_cmd);
-
-       /* TODO: remove cast and use lttng-ust-abi.h */
+       assert(app);
+       assert(ua_sess);
+       assert(ua_chan);
 
-       /* We are going to receive 2 fds, we need to reserve them. */
-       ret = lttng_fd_get(LTTNG_FD_APPS, 2);
-       if (ret < 0) {
-               ERR("Exhausted number of available FD upon create channel");
-               goto error;
-       }
+       health_code_update();
 
-       health_code_update(&health_thread_cmd);
+       DBG("UST app sending channel %s to UST app sock %d", ua_chan->name,
+                       app->sock);
 
-       ret = ustctl_create_channel(app->sock, ua_sess->handle,
-                       (struct lttng_ust_channel_attr *)&ua_chan->attr, &ua_chan->obj);
+       /* Send channel to the application. */
+       ret = ust_consumer_send_channel_to_ust(app, ua_sess, ua_chan);
        if (ret < 0) {
-               ERR("Creating channel %s for app (pid: %d, sock: %d) "
-                               "and session handle %d with ret %d",
-                               ua_chan->name, app->pid, app->sock,
-                               ua_sess->handle, ret);
-               lttng_fd_put(LTTNG_FD_APPS, 2);
                goto error;
        }
 
-       ua_chan->handle = ua_chan->obj->handle;
-
-       DBG2("UST app channel %s created successfully for pid:%d and sock:%d",
-                       ua_chan->name, app->pid, app->sock);
+       health_code_update();
 
-       health_code_update(&health_thread_cmd);
-
-       /* If channel is not enabled, disable it on the tracer */
-       if (!ua_chan->enabled) {
-               ret = disable_ust_channel(app, ua_sess, ua_chan);
+       /* Send all streams to application. */
+       cds_list_for_each_entry_safe(stream, stmp, &ua_chan->streams.head, list) {
+               ret = ust_consumer_send_stream_to_ust(app, ua_chan, stream);
                if (ret < 0) {
                        goto error;
                }
+               /* We don't need the stream anymore once sent to the tracer. */
+               cds_list_del(&stream->list);
+               delete_ust_app_stream(-1, stream);
        }
+       /* Flag the channel that it is sent to the application. */
+       ua_chan->is_sent = 1;
 
 error:
-       health_code_update(&health_thread_cmd);
+       health_code_update();
        return ret;
 }
 
 /*
  * Create the specified event onto the UST tracer for a UST session.
+ *
+ * Should be called with session mutex held.
  */
 static
 int create_ust_event(struct ust_app *app, struct ust_app_session *ua_sess,
@@ -705,18 +1280,18 @@ int create_ust_event(struct ust_app *app, struct ust_app_session *ua_sess,
 {
        int ret = 0;
 
-       health_code_update(&health_thread_cmd);
+       health_code_update();
 
        /* Create UST event on tracer */
        ret = ustctl_create_event(app->sock, &ua_event->attr, ua_chan->obj,
                        &ua_event->obj);
        if (ret < 0) {
-               if (ret == -EEXIST || ret == -EPERM) {
-                       ret = 0;
-                       goto error;
+               if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
+                       ERR("Error ustctl create event %s for app pid: %d with ret %d",
+                                       ua_event->attr.name, app->pid, ret);
+               } else {
+                       DBG3("UST app create event failed. Application is dead.");
                }
-               ERR("Error ustctl create event %s for app pid: %d with ret %d",
-                               ua_event->attr.name, app->pid, ret);
                goto error;
        }
 
@@ -725,7 +1300,15 @@ int create_ust_event(struct ust_app *app, struct ust_app_session *ua_sess,
        DBG2("UST app event %s created successfully for pid:%d",
                        ua_event->attr.name, app->pid);
 
-       health_code_update(&health_thread_cmd);
+       health_code_update();
+
+       /* Set filter if one is present. */
+       if (ua_event->filter) {
+               ret = set_ust_event_filter(ua_event, app);
+               if (ret < 0) {
+                       goto error;
+               }
+       }
 
        /* If event not enabled, disable it on the tracer */
        if (ua_event->enabled == 0) {
@@ -737,10 +1320,10 @@ int create_ust_event(struct ust_app *app, struct ust_app_session *ua_sess,
                         * just created it.
                         */
                        switch (ret) {
-                       case -EPERM:
+                       case -LTTNG_UST_ERR_PERM:
                                /* Code flow problem */
                                assert(0);
-                       case -EEXIST:
+                       case -LTTNG_UST_ERR_EXIST:
                                /* It's OK for our use case. */
                                ret = 0;
                                break;
@@ -752,7 +1335,7 @@ int create_ust_event(struct ust_app *app, struct ust_app_session *ua_sess,
        }
 
 error:
-       health_code_update(&health_thread_cmd);
+       health_code_update();
        return ret;
 }
 
@@ -762,10 +1345,6 @@ error:
 static void shadow_copy_event(struct ust_app_event *ua_event,
                struct ltt_ust_event *uevent)
 {
-       struct lttng_ht_iter iter;
-       struct ltt_ust_context *uctx;
-       struct ust_app_ctx *ua_ctx;
-
        strncpy(ua_event->name, uevent->attr.name, sizeof(ua_event->name));
        ua_event->name[sizeof(ua_event->name) - 1] = '\0';
 
@@ -776,24 +1355,8 @@ static void shadow_copy_event(struct ust_app_event *ua_event,
 
        /* Copy filter bytecode */
        if (uevent->filter) {
-               ua_event->filter = zmalloc(sizeof(*ua_event->filter) +
-                       uevent->filter->len);
-               if (!ua_event->filter) {
-                       return;
-               }
-               memcpy(ua_event->filter, uevent->filter,
-                       sizeof(*ua_event->filter) + uevent->filter->len);
-       }
-       cds_lfht_for_each_entry(uevent->ctx->ht, &iter.iter, uctx, node.node) {
-               ua_ctx = alloc_ust_app_ctx(&uctx->ctx);
-               if (ua_ctx == NULL) {
-                       /* malloc() failed. We should simply stop */
-                       return;
-               }
-
-               lttng_ht_node_init_ulong(&ua_ctx->node,
-                               (unsigned long) ua_ctx->ctx.ctx);
-               lttng_ht_add_unique_ulong(ua_event->ctx, &ua_ctx->node);
+               ua_event->filter = alloc_copy_ust_app_filter(uevent->filter);
+               /* Filter might be NULL here in case of ENONEM. */
        }
 }
 
@@ -804,7 +1367,6 @@ static void shadow_copy_channel(struct ust_app_channel *ua_chan,
                struct ltt_ust_channel *uchan)
 {
        struct lttng_ht_iter iter;
-       struct lttng_ht_node_str *ua_event_node;
        struct ltt_ust_event *uevent;
        struct ltt_ust_context *uctx;
        struct ust_app_event *ua_event;
@@ -814,10 +1376,24 @@ static void shadow_copy_channel(struct ust_app_channel *ua_chan,
 
        strncpy(ua_chan->name, uchan->name, sizeof(ua_chan->name));
        ua_chan->name[sizeof(ua_chan->name) - 1] = '\0';
-       /* Copy event attributes */
-       memcpy(&ua_chan->attr, &uchan->attr, sizeof(ua_chan->attr));
+
+       ua_chan->tracefile_size = uchan->tracefile_size;
+       ua_chan->tracefile_count = uchan->tracefile_count;
+
+       /* Copy event attributes since the layout is different. */
+       ua_chan->attr.subbuf_size = uchan->attr.subbuf_size;
+       ua_chan->attr.num_subbuf = uchan->attr.num_subbuf;
+       ua_chan->attr.overwrite = uchan->attr.overwrite;
+       ua_chan->attr.switch_timer_interval = uchan->attr.switch_timer_interval;
+       ua_chan->attr.read_timer_interval = uchan->attr.read_timer_interval;
+       ua_chan->attr.output = uchan->attr.output;
+       /*
+        * Note that the attribute channel type is not set since the channel on the
+        * tracing registry side does not have this information.
+        */
 
        ua_chan->enabled = uchan->enabled;
+       ua_chan->tracing_channel_id = uchan->id;
 
        cds_lfht_for_each_entry(uchan->ctx->ht, &iter.iter, uctx, node.node) {
                ua_ctx = alloc_ust_app_ctx(&uctx->ctx);
@@ -831,11 +1407,9 @@ static void shadow_copy_channel(struct ust_app_channel *ua_chan,
 
        /* Copy all events from ltt ust channel to ust app channel */
        cds_lfht_for_each_entry(uchan->events->ht, &iter.iter, uevent, node.node) {
-               struct lttng_ht_iter uiter;
-
-               lttng_ht_lookup(ua_chan->events, (void *) uevent->attr.name, &uiter);
-               ua_event_node = lttng_ht_iter_get_node_str(&uiter);
-               if (ua_event_node == NULL) {
+               ua_event = find_ust_app_event(ua_chan->events, uevent->attr.name,
+                               uevent->filter, uevent->attr.loglevel);
+               if (ua_event == NULL) {
                        DBG2("UST event %s not found on shadow copy channel",
                                        uevent->attr.name);
                        ua_event = alloc_ust_app_event(uevent->attr.name, &uevent->attr);
@@ -843,7 +1417,7 @@ static void shadow_copy_channel(struct ust_app_channel *ua_chan,
                                continue;
                        }
                        shadow_copy_event(ua_event, uevent);
-                       lttng_ht_add_unique_str(ua_chan->events, &ua_event->node);
+                       add_unique_ust_app_event(ua_chan, ua_event);
                }
        }
 
@@ -872,20 +1446,38 @@ static void shadow_copy_session(struct ust_app_session *ua_sess,
 
        DBG2("Shadow copy of session handle %d", ua_sess->handle);
 
-       ua_sess->id = usess->id;
-       ua_sess->uid = usess->uid;
-       ua_sess->gid = usess->gid;
-
-       ret = snprintf(ua_sess->path, PATH_MAX, "%s-%d-%s/", app->name, app->pid,
-                       datetime);
+       ua_sess->tracing_id = usess->id;
+       ua_sess->id = get_next_session_id();
+       ua_sess->uid = app->uid;
+       ua_sess->gid = app->gid;
+       ua_sess->euid = usess->uid;
+       ua_sess->egid = usess->gid;
+       ua_sess->buffer_type = usess->buffer_type;
+       ua_sess->bits_per_long = app->bits_per_long;
+       /* There is only one consumer object per session possible. */
+       ua_sess->consumer = usess->consumer;
+       ua_sess->output_traces = usess->output_traces;
+
+       switch (ua_sess->buffer_type) {
+       case LTTNG_BUFFER_PER_PID:
+               ret = snprintf(ua_sess->path, sizeof(ua_sess->path),
+                               DEFAULT_UST_TRACE_PID_PATH "/%s-%d-%s", app->name, app->pid,
+                               datetime);
+               break;
+       case LTTNG_BUFFER_PER_UID:
+               ret = snprintf(ua_sess->path, sizeof(ua_sess->path),
+                               DEFAULT_UST_TRACE_UID_PATH, ua_sess->uid, app->bits_per_long);
+               break;
+       default:
+               assert(0);
+               goto error;
+       }
        if (ret < 0) {
                PERROR("asprintf UST shadow copy session");
-               /* TODO: We cannot return an error from here.. */
                assert(0);
+               goto error;
        }
 
-       /* TODO: support all UST domain */
-
        /* Iterate over all channels in global domain. */
        cds_lfht_for_each_entry(usess->domain_global.channels->ht, &iter.iter,
                        uchan, node.node) {
@@ -900,15 +1492,24 @@ static void shadow_copy_session(struct ust_app_session *ua_sess,
 
                DBG2("Channel %s not found on shadow session copy, creating it",
                                uchan->name);
-               ua_chan = alloc_ust_app_channel(uchan->name, &uchan->attr);
+               ua_chan = alloc_ust_app_channel(uchan->name, ua_sess, &uchan->attr);
                if (ua_chan == NULL) {
                        /* malloc failed FIXME: Might want to do handle ENOMEM .. */
                        continue;
                }
-
                shadow_copy_channel(ua_chan, uchan);
+               /*
+                * The concept of metadata channel does not exist on the tracing
+                * registry side of the session daemon so this can only be a per CPU
+                * channel and not metadata.
+                */
+               ua_chan->attr.type = LTTNG_UST_CHAN_PER_CPU;
+
                lttng_ht_add_unique_str(ua_sess->channels, &ua_chan->node);
        }
+
+error:
+       return;
 }
 
 /*
@@ -919,7 +1520,7 @@ void __lookup_session_by_app(struct ltt_ust_session *usess,
                        struct ust_app *app, struct lttng_ht_iter *iter)
 {
        /* Get right UST app session from app */
-       lttng_ht_lookup(app->sessions, (void *)((unsigned long) usess->id), iter);
+       lttng_ht_lookup(app->sessions, &usess->id, iter);
 }
 
 /*
@@ -930,10 +1531,10 @@ static struct ust_app_session *lookup_session_by_app(
                struct ltt_ust_session *usess, struct ust_app *app)
 {
        struct lttng_ht_iter iter;
-       struct lttng_ht_node_ulong *node;
+       struct lttng_ht_node_u64 *node;
 
        __lookup_session_by_app(usess, app, &iter);
-       node = lttng_ht_iter_get_node_ulong(&iter);
+       node = lttng_ht_iter_get_node_u64(&iter);
        if (node == NULL) {
                goto error;
        }
@@ -945,164 +1546,257 @@ error:
 }
 
 /*
- * Create a UST session onto the tracer of app and add it the session
- * hashtable.
+ * Setup buffer registry per PID for the given session and application. If none
+ * is found, a new one is created, added to the global registry and
+ * initialized. If regp is valid, it's set with the newly created object.
  *
- * Return ust app session or NULL on error.
+ * Return 0 on success or else a negative value.
  */
-static struct ust_app_session *create_ust_app_session(
-               struct ltt_ust_session *usess, struct ust_app *app)
+static int setup_buffer_reg_pid(struct ust_app_session *ua_sess,
+               struct ust_app *app, struct buffer_reg_pid **regp)
 {
-       int ret;
-       struct ust_app_session *ua_sess;
-
-       health_code_update(&health_thread_cmd);
+       int ret = 0;
+       struct buffer_reg_pid *reg_pid;
 
-       ua_sess = lookup_session_by_app(usess, app);
-       if (ua_sess == NULL) {
-               DBG2("UST app pid: %d session id %d not found, creating it",
-                               app->pid, usess->id);
-               ua_sess = alloc_ust_app_session();
-               if (ua_sess == NULL) {
-                       /* Only malloc can failed so something is really wrong */
-                       goto end;
-               }
-               shadow_copy_session(ua_sess, usess, app);
-       }
+       assert(ua_sess);
+       assert(app);
 
-       health_code_update(&health_thread_cmd);
+       rcu_read_lock();
 
-       if (ua_sess->handle == -1) {
-               ret = ustctl_create_session(app->sock);
+       reg_pid = buffer_reg_pid_find(ua_sess->id);
+       if (!reg_pid) {
+               /*
+                * This is the create channel path meaning that if there is NO
+                * registry available, we have to create one for this session.
+                */
+               ret = buffer_reg_pid_create(ua_sess->id, &reg_pid);
                if (ret < 0) {
-                       ERR("Creating session for app pid %d", app->pid);
-                       /* This means that the tracer is gone... */
-                       ua_sess = (void*) -1UL;
                        goto error;
                }
+               buffer_reg_pid_add(reg_pid);
+       } else {
+               goto end;
+       }
 
-               ua_sess->handle = ret;
-
-               /* Add ust app session to app's HT */
-               lttng_ht_node_init_ulong(&ua_sess->node, (unsigned long) ua_sess->id);
-               lttng_ht_add_unique_ulong(app->sessions, &ua_sess->node);
-
-               DBG2("UST app session created successfully with handle %d", ret);
+       /* Initialize registry. */
+       ret = ust_registry_session_init(&reg_pid->registry->reg.ust, app,
+                       app->bits_per_long, app->uint8_t_alignment,
+                       app->uint16_t_alignment, app->uint32_t_alignment,
+                       app->uint64_t_alignment, app->long_alignment,
+                       app->byte_order, app->version.major,
+                       app->version.minor);
+       if (ret < 0) {
+               goto error;
        }
 
-end:
-       health_code_update(&health_thread_cmd);
-       return ua_sess;
+       DBG3("UST app buffer registry per PID created successfully");
 
+end:
+       if (regp) {
+               *regp = reg_pid;
+       }
 error:
-       delete_ust_app_session(-1, ua_sess);
-       health_code_update(&health_thread_cmd);
-       return NULL;
+       rcu_read_unlock();
+       return ret;
 }
 
 /*
- * Create a context for the channel on the tracer.
+ * Setup buffer registry per UID for the given session and application. If none
+ * is found, a new one is created, added to the global registry and
+ * initialized. If regp is valid, it's set with the newly created object.
+ *
+ * Return 0 on success or else a negative value.
  */
-static
-int create_ust_app_channel_context(struct ust_app_session *ua_sess,
-               struct ust_app_channel *ua_chan, struct lttng_ust_context *uctx,
-               struct ust_app *app)
+static int setup_buffer_reg_uid(struct ltt_ust_session *usess,
+               struct ust_app *app, struct buffer_reg_uid **regp)
 {
        int ret = 0;
-       struct lttng_ht_iter iter;
-       struct lttng_ht_node_ulong *node;
-       struct ust_app_ctx *ua_ctx;
+       struct buffer_reg_uid *reg_uid;
 
-       DBG2("UST app adding context to channel %s", ua_chan->name);
+       assert(usess);
+       assert(app);
 
-       lttng_ht_lookup(ua_chan->ctx, (void *)((unsigned long)uctx->ctx), &iter);
-       node = lttng_ht_iter_get_node_ulong(&iter);
-       if (node != NULL) {
-               ret = -EEXIST;
-               goto error;
-       }
+       rcu_read_lock();
 
-       ua_ctx = alloc_ust_app_ctx(uctx);
-       if (ua_ctx == NULL) {
-               /* malloc failed */
-               ret = -1;
-               goto error;
-       }
-
-       lttng_ht_node_init_ulong(&ua_ctx->node, (unsigned long) ua_ctx->ctx.ctx);
-       lttng_ht_add_unique_ulong(ua_chan->ctx, &ua_ctx->node);
+       reg_uid = buffer_reg_uid_find(usess->id, app->bits_per_long, app->uid);
+       if (!reg_uid) {
+               /*
+                * This is the create channel path meaning that if there is NO
+                * registry available, we have to create one for this session.
+                */
+               ret = buffer_reg_uid_create(usess->id, app->bits_per_long, app->uid,
+                               LTTNG_DOMAIN_UST, &reg_uid);
+               if (ret < 0) {
+                       goto error;
+               }
+               buffer_reg_uid_add(reg_uid);
+       } else {
+               goto end;
+       }
 
-       ret = create_ust_channel_context(ua_chan, ua_ctx, app);
+       /* Initialize registry. */
+       ret = ust_registry_session_init(&reg_uid->registry->reg.ust, NULL,
+                       app->bits_per_long, app->uint8_t_alignment,
+                       app->uint16_t_alignment, app->uint32_t_alignment,
+                       app->uint64_t_alignment, app->long_alignment,
+                       app->byte_order, app->version.major,
+                       app->version.minor);
        if (ret < 0) {
                goto error;
        }
+       /* Add node to teardown list of the session. */
+       cds_list_add(&reg_uid->lnode, &usess->buffer_reg_uid_list);
 
+       DBG3("UST app buffer registry per UID created successfully");
+
+end:
+       if (regp) {
+               *regp = reg_uid;
+       }
 error:
+       rcu_read_unlock();
        return ret;
 }
 
 /*
- * Create an UST context and enable it for the event on the tracer.
+ * Create a session on the tracer side for the given app.
+ *
+ * On success, ua_sess_ptr is populated with the session pointer or else left
+ * untouched. If the session was created, is_created is set to 1. On error,
+ * it's left untouched. Note that ua_sess_ptr is mandatory but is_created can
+ * be NULL.
+ *
+ * Returns 0 on success or else a negative code which is either -ENOMEM or
+ * -ENOTCONN which is the default code if the ustctl_create_session fails.
  */
-static
-int create_ust_app_event_context(struct ust_app_session *ua_sess,
-               struct ust_app_event *ua_event, struct lttng_ust_context *uctx,
-               struct ust_app *app)
+static int create_ust_app_session(struct ltt_ust_session *usess,
+               struct ust_app *app, struct ust_app_session **ua_sess_ptr,
+               int *is_created)
 {
-       int ret = 0;
-       struct lttng_ht_iter iter;
-       struct lttng_ht_node_ulong *node;
-       struct ust_app_ctx *ua_ctx;
+       int ret, created = 0;
+       struct ust_app_session *ua_sess;
 
-       DBG2("UST app adding context to event %s", ua_event->name);
+       assert(usess);
+       assert(app);
+       assert(ua_sess_ptr);
 
-       lttng_ht_lookup(ua_event->ctx, (void *)((unsigned long)uctx->ctx), &iter);
-       node = lttng_ht_iter_get_node_ulong(&iter);
-       if (node != NULL) {
-               ret = -EEXIST;
-               goto error;
+       health_code_update();
+
+       ua_sess = lookup_session_by_app(usess, app);
+       if (ua_sess == NULL) {
+               DBG2("UST app pid: %d session id %" PRIu64 " not found, creating it",
+                               app->pid, usess->id);
+               ua_sess = alloc_ust_app_session(app);
+               if (ua_sess == NULL) {
+                       /* Only malloc can failed so something is really wrong */
+                       ret = -ENOMEM;
+                       goto error;
+               }
+               shadow_copy_session(ua_sess, usess, app);
+               created = 1;
        }
 
-       ua_ctx = alloc_ust_app_ctx(uctx);
-       if (ua_ctx == NULL) {
-               /* malloc failed */
-               ret = -1;
+       switch (usess->buffer_type) {
+       case LTTNG_BUFFER_PER_PID:
+               /* Init local registry. */
+               ret = setup_buffer_reg_pid(ua_sess, app, NULL);
+               if (ret < 0) {
+                       goto error;
+               }
+               break;
+       case LTTNG_BUFFER_PER_UID:
+               /* Look for a global registry. If none exists, create one. */
+               ret = setup_buffer_reg_uid(usess, app, NULL);
+               if (ret < 0) {
+                       goto error;
+               }
+               break;
+       default:
+               assert(0);
+               ret = -EINVAL;
                goto error;
        }
 
-       lttng_ht_node_init_ulong(&ua_ctx->node, (unsigned long) ua_ctx->ctx.ctx);
-       lttng_ht_add_unique_ulong(ua_event->ctx, &ua_ctx->node);
+       health_code_update();
 
-       ret = create_ust_event_context(ua_event, ua_ctx, app);
-       if (ret < 0) {
-               goto error;
+       if (ua_sess->handle == -1) {
+               ret = ustctl_create_session(app->sock);
+               if (ret < 0) {
+                       if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
+                               ERR("Creating session for app pid %d with ret %d",
+                                               app->pid, ret);
+                       } else {
+                               DBG("UST app creating session failed. Application is dead");
+                       }
+                       delete_ust_app_session(-1, ua_sess, app);
+                       if (ret != -ENOMEM) {
+                               /*
+                                * Tracer is probably gone or got an internal error so let's
+                                * behave like it will soon unregister or not usable.
+                                */
+                               ret = -ENOTCONN;
+                       }
+                       goto error;
+               }
+
+               ua_sess->handle = ret;
+
+               /* Add ust app session to app's HT */
+               lttng_ht_node_init_u64(&ua_sess->node,
+                               ua_sess->tracing_id);
+               lttng_ht_add_unique_u64(app->sessions, &ua_sess->node);
+
+               DBG2("UST app session created successfully with handle %d", ret);
+       }
+
+       *ua_sess_ptr = ua_sess;
+       if (is_created) {
+               *is_created = created;
        }
 
+       /* Everything went well. */
+       ret = 0;
+
 error:
+       health_code_update();
        return ret;
 }
 
 /*
- * Set UST filter for the event on the tracer.
+ * Create a context for the channel on the tracer.
+ *
+ * Called with UST app session lock held and a RCU read side lock.
  */
 static
-int set_ust_app_event_filter(struct ust_app_session *ua_sess,
-               struct ust_app_event *ua_event,
-               struct lttng_filter_bytecode *bytecode,
+int create_ust_app_channel_context(struct ust_app_session *ua_sess,
+               struct ust_app_channel *ua_chan, struct lttng_ust_context *uctx,
                struct ust_app *app)
 {
        int ret = 0;
+       struct lttng_ht_iter iter;
+       struct lttng_ht_node_ulong *node;
+       struct ust_app_ctx *ua_ctx;
 
-       DBG2("UST app adding context to event %s", ua_event->name);
+       DBG2("UST app adding context to channel %s", ua_chan->name);
 
-       /* Copy filter bytecode */
-       ua_event->filter = zmalloc(sizeof(*ua_event->filter) + bytecode->len);
-       if (!ua_event->filter) {
-               return -ENOMEM;
+       lttng_ht_lookup(ua_chan->ctx, (void *)((unsigned long)uctx->ctx), &iter);
+       node = lttng_ht_iter_get_node_ulong(&iter);
+       if (node != NULL) {
+               ret = -EEXIST;
+               goto error;
+       }
+
+       ua_ctx = alloc_ust_app_ctx(uctx);
+       if (ua_ctx == NULL) {
+               /* malloc failed */
+               ret = -1;
+               goto error;
        }
-       memcpy(ua_event->filter, bytecode,
-               sizeof(*ua_event->filter) + bytecode->len);
-       ret = set_ust_event_filter(ua_event, app);
+
+       lttng_ht_node_init_ulong(&ua_ctx->node, (unsigned long) ua_ctx->ctx.ctx);
+       lttng_ht_add_unique_ulong(ua_chan->ctx, &ua_ctx->node);
+
+       ret = create_ust_channel_context(ua_chan, ua_ctx, app);
        if (ret < 0) {
                goto error;
        }
@@ -1113,6 +1807,8 @@ error:
 
 /*
  * Enable on the tracer side a ust app event for the session and channel.
+ *
+ * Called with UST app session lock held.
  */
 static
 int enable_ust_app_event(struct ust_app_session *ua_sess,
@@ -1171,7 +1867,8 @@ error:
 }
 
 /*
- * Lookup ust app channel for session and enable it on the tracer side.
+ * Lookup ust app channel for session and enable it on the tracer side. This
+ * MUST be called with a RCU read side lock acquired.
  */
 static int enable_ust_app_channel(struct ust_app_session *ua_sess,
                struct ltt_ust_channel *uchan, struct ust_app *app)
@@ -1184,8 +1881,8 @@ static int enable_ust_app_channel(struct ust_app_session *ua_sess,
        lttng_ht_lookup(ua_sess->channels, (void *)uchan->name, &iter);
        ua_chan_node = lttng_ht_iter_get_node_str(&iter);
        if (ua_chan_node == NULL) {
-               DBG2("Unable to find channel %s in ust session id %u",
-                               uchan->name, ua_sess->id);
+               DBG2("Unable to find channel %s in ust session id %" PRIu64,
+                               uchan->name, ua_sess->tracing_id);
                goto error;
        }
 
@@ -1201,256 +1898,878 @@ error:
 }
 
 /*
- * Create UST app channel and create it on the tracer.
+ * Ask the consumer to create a channel and get it if successful.
+ *
+ * Return 0 on success or else a negative value.
  */
-static struct ust_app_channel *create_ust_app_channel(
-               struct ust_app_session *ua_sess, struct ltt_ust_channel *uchan,
-               struct ust_app *app)
+static int do_consumer_create_channel(struct ltt_ust_session *usess,
+               struct ust_app_session *ua_sess, struct ust_app_channel *ua_chan,
+               int bitness, struct ust_registry_session *registry)
 {
-       int ret = 0;
-       struct lttng_ht_iter iter;
-       struct lttng_ht_node_str *ua_chan_node;
-       struct ust_app_channel *ua_chan;
+       int ret;
+       unsigned int nb_fd = 0;
+       struct consumer_socket *socket;
 
-       /* Lookup channel in the ust app session */
-       lttng_ht_lookup(ua_sess->channels, (void *)uchan->name, &iter);
-       ua_chan_node = lttng_ht_iter_get_node_str(&iter);
-       if (ua_chan_node != NULL) {
-               ua_chan = caa_container_of(ua_chan_node, struct ust_app_channel, node);
-               goto end;
-       }
+       assert(usess);
+       assert(ua_sess);
+       assert(ua_chan);
+       assert(registry);
 
-       ua_chan = alloc_ust_app_channel(uchan->name, &uchan->attr);
-       if (ua_chan == NULL) {
-               /* Only malloc can fail here */
+       rcu_read_lock();
+       health_code_update();
+
+       /* Get the right consumer socket for the application. */
+       socket = consumer_find_socket_by_bitness(bitness, usess->consumer);
+       if (!socket) {
+               ret = -EINVAL;
                goto error;
        }
-       shadow_copy_channel(ua_chan, uchan);
 
-       ret = create_ust_channel(app, ua_sess, ua_chan);
+       health_code_update();
+
+       /* Need one fd for the channel. */
+       ret = lttng_fd_get(LTTNG_FD_APPS, 1);
        if (ret < 0) {
-               /* Not found previously means that it does not exist on the tracer */
-               assert(ret != -EEXIST);
+               ERR("Exhausted number of available FD upon create channel");
                goto error;
        }
 
-       lttng_ht_add_unique_str(ua_sess->channels, &ua_chan->node);
+       /*
+        * Ask consumer to create channel. The consumer will return the number of
+        * stream we have to expect.
+        */
+       ret = ust_consumer_ask_channel(ua_sess, ua_chan, usess->consumer, socket,
+                       registry);
+       if (ret < 0) {
+               goto error_ask;
+       }
 
-       DBG2("UST app create channel %s for PID %d completed", ua_chan->name,
-                       app->pid);
+       /*
+        * Compute the number of fd needed before receiving them. It must be 2 per
+        * stream (2 being the default value here).
+        */
+       nb_fd = DEFAULT_UST_STREAM_FD_NUM * ua_chan->expected_stream_count;
 
-end:
-       return ua_chan;
+       /* Reserve the amount of file descriptor we need. */
+       ret = lttng_fd_get(LTTNG_FD_APPS, nb_fd);
+       if (ret < 0) {
+               ERR("Exhausted number of available FD upon create channel");
+               goto error_fd_get_stream;
+       }
+
+       health_code_update();
+
+       /*
+        * Now get the channel from the consumer. This call wil populate the stream
+        * list of that channel and set the ust objects.
+        */
+       if (usess->consumer->enabled) {
+               ret = ust_consumer_get_channel(socket, ua_chan);
+               if (ret < 0) {
+                       goto error_destroy;
+               }
+       }
 
+       rcu_read_unlock();
+       return 0;
+
+error_destroy:
+       lttng_fd_put(LTTNG_FD_APPS, nb_fd);
+error_fd_get_stream:
+       /*
+        * Initiate a destroy channel on the consumer since we had an error
+        * handling it on our side. The return value is of no importance since we
+        * already have a ret value set by the previous error that we need to
+        * return.
+        */
+       (void) ust_consumer_destroy_channel(socket, ua_chan);
+error_ask:
+       lttng_fd_put(LTTNG_FD_APPS, 1);
 error:
-       delete_ust_app_channel(-1, ua_chan);
-       return NULL;
+       health_code_update();
+       rcu_read_unlock();
+       return ret;
 }
 
 /*
- * Create UST app event and create it on the tracer side.
+ * Duplicate the ust data object of the ust app stream and save it in the
+ * buffer registry stream.
+ *
+ * Return 0 on success or else a negative value.
  */
-static
-int create_ust_app_event(struct ust_app_session *ua_sess,
-               struct ust_app_channel *ua_chan, struct ltt_ust_event *uevent,
-               struct ust_app *app)
+static int duplicate_stream_object(struct buffer_reg_stream *reg_stream,
+               struct ust_app_stream *stream)
 {
-       int ret = 0;
-       struct lttng_ht_iter iter;
-       struct lttng_ht_node_str *ua_event_node;
-       struct ust_app_event *ua_event;
+       int ret;
 
-       /* Get event node */
-       lttng_ht_lookup(ua_chan->events, (void *)uevent->attr.name, &iter);
-       ua_event_node = lttng_ht_iter_get_node_str(&iter);
-       if (ua_event_node != NULL) {
-               ret = -EEXIST;
-               goto end;
-       }
+       assert(reg_stream);
+       assert(stream);
 
-       /* Does not exist so create one */
-       ua_event = alloc_ust_app_event(uevent->attr.name, &uevent->attr);
-       if (ua_event == NULL) {
-               /* Only malloc can failed so something is really wrong */
-               ret = -ENOMEM;
-               goto end;
+       /* Reserve the amount of file descriptor we need. */
+       ret = lttng_fd_get(LTTNG_FD_APPS, 2);
+       if (ret < 0) {
+               ERR("Exhausted number of available FD upon duplicate stream");
+               goto error;
        }
-       shadow_copy_event(ua_event, uevent);
 
-       /* Create it on the tracer side */
-       ret = create_ust_event(app, ua_sess, ua_chan, ua_event);
+       /* Duplicate object for stream once the original is in the registry. */
+       ret = ustctl_duplicate_ust_object_data(&stream->obj,
+                       reg_stream->obj.ust);
        if (ret < 0) {
-               /* Not found previously means that it does not exist on the tracer */
-               assert(ret != -EEXIST);
+               ERR("Duplicate stream obj from %p to %p failed with ret %d",
+                               reg_stream->obj.ust, stream->obj, ret);
+               lttng_fd_put(LTTNG_FD_APPS, 2);
                goto error;
        }
+       stream->handle = stream->obj->handle;
+
+error:
+       return ret;
+}
+
+/*
+ * Duplicate the ust data object of the ust app. channel and save it in the
+ * buffer registry channel.
+ *
+ * Return 0 on success or else a negative value.
+ */
+static int duplicate_channel_object(struct buffer_reg_channel *reg_chan,
+               struct ust_app_channel *ua_chan)
+{
+       int ret;
 
-       lttng_ht_add_unique_str(ua_chan->events, &ua_event->node);
+       assert(reg_chan);
+       assert(ua_chan);
 
-       DBG2("UST app create event %s for PID %d completed", ua_event->name,
-                       app->pid);
+       /* Need two fds for the channel. */
+       ret = lttng_fd_get(LTTNG_FD_APPS, 1);
+       if (ret < 0) {
+               ERR("Exhausted number of available FD upon duplicate channel");
+               goto error_fd_get;
+       }
 
-end:
-       return ret;
+       /* Duplicate object for stream once the original is in the registry. */
+       ret = ustctl_duplicate_ust_object_data(&ua_chan->obj, reg_chan->obj.ust);
+       if (ret < 0) {
+               ERR("Duplicate channel obj from %p to %p failed with ret: %d",
+                               reg_chan->obj.ust, ua_chan->obj, ret);
+               goto error;
+       }
+       ua_chan->handle = ua_chan->obj->handle;
+
+       return 0;
 
 error:
-       /* Valid. Calling here is already in a read side lock */
-       delete_ust_app_event(-1, ua_event);
+       lttng_fd_put(LTTNG_FD_APPS, 1);
+error_fd_get:
        return ret;
 }
 
 /*
- * Create UST metadata and open it on the tracer side.
+ * For a given channel buffer registry, setup all streams of the given ust
+ * application channel.
+ *
+ * Return 0 on success or else a negative value.
  */
-static int create_ust_app_metadata(struct ust_app_session *ua_sess,
-               char *pathname, struct ust_app *app)
+static int setup_buffer_reg_streams(struct buffer_reg_channel *reg_chan,
+               struct ust_app_channel *ua_chan)
 {
        int ret = 0;
+       struct ust_app_stream *stream, *stmp;
 
-       if (ua_sess->metadata == NULL) {
-               /* Allocate UST metadata */
-               ua_sess->metadata = trace_ust_create_metadata(pathname);
-               if (ua_sess->metadata == NULL) {
-                       /* malloc() failed */
-                       goto error;
-               }
-
-               ret = open_ust_metadata(app, ua_sess);
-               if (ret < 0) {
-                       DBG3("Opening metadata failed. Cleaning up memory");
+       assert(reg_chan);
+       assert(ua_chan);
 
-                       /* Cleanup failed metadata struct */
-                       free(ua_sess->metadata);
-                       /*
-                        * This is very important because delete_ust_app_session check if
-                        * the pointer is null or not in order to delete the metadata.
-                        */
-                       ua_sess->metadata = NULL;
-                       goto error;
-               }
+       DBG2("UST app setup buffer registry stream");
 
-               DBG2("UST metadata opened for app pid %d", app->pid);
-       }
+       /* Send all streams to application. */
+       cds_list_for_each_entry_safe(stream, stmp, &ua_chan->streams.head, list) {
+               struct buffer_reg_stream *reg_stream;
 
-       /* Open UST metadata stream */
-       if (ua_sess->metadata->stream_obj == NULL) {
-               ret = create_ust_stream(app, ua_sess);
+               ret = buffer_reg_stream_create(&reg_stream);
                if (ret < 0) {
                        goto error;
                }
 
-               ret = snprintf(ua_sess->metadata->pathname, PATH_MAX,
-                               "%s/metadata", ua_sess->path);
-               if (ret < 0) {
-                       PERROR("asprintf UST create stream");
-                       goto error;
-               }
+               /*
+                * Keep original pointer and nullify it in the stream so the delete
+                * stream call does not release the object.
+                */
+               reg_stream->obj.ust = stream->obj;
+               stream->obj = NULL;
+               buffer_reg_stream_add(reg_stream, reg_chan);
 
-               DBG2("UST metadata stream object created for app pid %d",
-                               app->pid);
-       } else {
-               ERR("Attempting to create stream without metadata opened");
-               goto error;
+               /* We don't need the streams anymore. */
+               cds_list_del(&stream->list);
+               delete_ust_app_stream(-1, stream);
        }
 
-       return 0;
-
 error:
-       return -1;
+       return ret;
 }
 
 /*
- * Return pointer to traceable apps list.
+ * Create a buffer registry channel for the given session registry and
+ * application channel object. If regp pointer is valid, it's set with the
+ * created object. Important, the created object is NOT added to the session
+ * registry hash table.
+ *
+ * Return 0 on success else a negative value.
  */
-struct lttng_ht *ust_app_get_ht(void)
+static int create_buffer_reg_channel(struct buffer_reg_session *reg_sess,
+               struct ust_app_channel *ua_chan, struct buffer_reg_channel **regp)
 {
-       return ust_app_ht;
-}
+       int ret;
+       struct buffer_reg_channel *reg_chan = NULL;
 
-/*
- * Return ust app pointer or NULL if not found.
- */
-struct ust_app *ust_app_find_by_pid(pid_t pid)
-{
-       struct lttng_ht_node_ulong *node;
-       struct lttng_ht_iter iter;
+       assert(reg_sess);
+       assert(ua_chan);
 
-       rcu_read_lock();
-       lttng_ht_lookup(ust_app_ht, (void *)((unsigned long) pid), &iter);
-       node = lttng_ht_iter_get_node_ulong(&iter);
+       DBG2("UST app creating buffer registry channel for %s", ua_chan->name);
+
+       /* Create buffer registry channel. */
+       ret = buffer_reg_channel_create(ua_chan->tracing_channel_id, &reg_chan);
+       if (ret < 0) {
+               goto error_create;
+       }
+       assert(reg_chan);
+       reg_chan->consumer_key = ua_chan->key;
+       reg_chan->subbuf_size = ua_chan->attr.subbuf_size;
+
+       /* Create and add a channel registry to session. */
+       ret = ust_registry_channel_add(reg_sess->reg.ust,
+                       ua_chan->tracing_channel_id);
+       if (ret < 0) {
+               goto error;
+       }
+       buffer_reg_channel_add(reg_sess, reg_chan);
+
+       if (regp) {
+               *regp = reg_chan;
+       }
+
+       return 0;
+
+error:
+       /* Safe because the registry channel object was not added to any HT. */
+       buffer_reg_channel_destroy(reg_chan, LTTNG_DOMAIN_UST);
+error_create:
+       return ret;
+}
+
+/*
+ * Setup buffer registry channel for the given session registry and application
+ * channel object. If regp pointer is valid, it's set with the created object.
+ *
+ * Return 0 on success else a negative value.
+ */
+static int setup_buffer_reg_channel(struct buffer_reg_session *reg_sess,
+               struct ust_app_channel *ua_chan, struct buffer_reg_channel *reg_chan)
+{
+       int ret;
+
+       assert(reg_sess);
+       assert(reg_chan);
+       assert(ua_chan);
+       assert(ua_chan->obj);
+
+       DBG2("UST app setup buffer registry channel for %s", ua_chan->name);
+
+       /* Setup all streams for the registry. */
+       ret = setup_buffer_reg_streams(reg_chan, ua_chan);
+       if (ret < 0) {
+               goto error;
+       }
+
+       reg_chan->obj.ust = ua_chan->obj;
+       ua_chan->obj = NULL;
+
+       return 0;
+
+error:
+       buffer_reg_channel_remove(reg_sess, reg_chan);
+       buffer_reg_channel_destroy(reg_chan, LTTNG_DOMAIN_UST);
+       return ret;
+}
+
+/*
+ * Send buffer registry channel to the application.
+ *
+ * Return 0 on success else a negative value.
+ */
+static int send_channel_uid_to_ust(struct buffer_reg_channel *reg_chan,
+               struct ust_app *app, struct ust_app_session *ua_sess,
+               struct ust_app_channel *ua_chan)
+{
+       int ret;
+       struct buffer_reg_stream *reg_stream;
+
+       assert(reg_chan);
+       assert(app);
+       assert(ua_sess);
+       assert(ua_chan);
+
+       DBG("UST app sending buffer registry channel to ust sock %d", app->sock);
+
+       ret = duplicate_channel_object(reg_chan, ua_chan);
+       if (ret < 0) {
+               goto error;
+       }
+
+       /* Send channel to the application. */
+       ret = ust_consumer_send_channel_to_ust(app, ua_sess, ua_chan);
+       if (ret < 0) {
+               goto error;
+       }
+
+       health_code_update();
+
+       /* Send all streams to application. */
+       pthread_mutex_lock(&reg_chan->stream_list_lock);
+       cds_list_for_each_entry(reg_stream, &reg_chan->streams, lnode) {
+               struct ust_app_stream stream;
+
+               ret = duplicate_stream_object(reg_stream, &stream);
+               if (ret < 0) {
+                       goto error_stream_unlock;
+               }
+
+               ret = ust_consumer_send_stream_to_ust(app, ua_chan, &stream);
+               if (ret < 0) {
+                       (void) release_ust_app_stream(-1, &stream);
+                       goto error_stream_unlock;
+               }
+
+               /*
+                * The return value is not important here. This function will output an
+                * error if needed.
+                */
+               (void) release_ust_app_stream(-1, &stream);
+       }
+       ua_chan->is_sent = 1;
+
+error_stream_unlock:
+       pthread_mutex_unlock(&reg_chan->stream_list_lock);
+error:
+       return ret;
+}
+
+/*
+ * Create and send to the application the created buffers with per UID buffers.
+ *
+ * Return 0 on success else a negative value.
+ */
+static int create_channel_per_uid(struct ust_app *app,
+               struct ltt_ust_session *usess, struct ust_app_session *ua_sess,
+               struct ust_app_channel *ua_chan)
+{
+       int ret;
+       struct buffer_reg_uid *reg_uid;
+       struct buffer_reg_channel *reg_chan;
+
+       assert(app);
+       assert(usess);
+       assert(ua_sess);
+       assert(ua_chan);
+
+       DBG("UST app creating channel %s with per UID buffers", ua_chan->name);
+
+       reg_uid = buffer_reg_uid_find(usess->id, app->bits_per_long, app->uid);
+       /*
+        * The session creation handles the creation of this global registry
+        * object. If none can be find, there is a code flow problem or a
+        * teardown race.
+        */
+       assert(reg_uid);
+
+       reg_chan = buffer_reg_channel_find(ua_chan->tracing_channel_id,
+                       reg_uid);
+       if (!reg_chan) {
+               /* Create the buffer registry channel object. */
+               ret = create_buffer_reg_channel(reg_uid->registry, ua_chan, &reg_chan);
+               if (ret < 0) {
+                       goto error;
+               }
+               assert(reg_chan);
+
+               /*
+                * Create the buffers on the consumer side. This call populates the
+                * ust app channel object with all streams and data object.
+                */
+               ret = do_consumer_create_channel(usess, ua_sess, ua_chan,
+                               app->bits_per_long, reg_uid->registry->reg.ust);
+               if (ret < 0) {
+                       /*
+                        * Let's remove the previously created buffer registry channel so
+                        * it's not visible anymore in the session registry.
+                        */
+                       ust_registry_channel_del_free(reg_uid->registry->reg.ust,
+                                       ua_chan->tracing_channel_id);
+                       buffer_reg_channel_remove(reg_uid->registry, reg_chan);
+                       buffer_reg_channel_destroy(reg_chan, LTTNG_DOMAIN_UST);
+                       goto error;
+               }
+
+               /*
+                * Setup the streams and add it to the session registry.
+                */
+               ret = setup_buffer_reg_channel(reg_uid->registry, ua_chan, reg_chan);
+               if (ret < 0) {
+                       goto error;
+               }
+
+       }
+
+       /* Send buffers to the application. */
+       ret = send_channel_uid_to_ust(reg_chan, app, ua_sess, ua_chan);
+       if (ret < 0) {
+               goto error;
+       }
+
+error:
+       return ret;
+}
+
+/*
+ * Create and send to the application the created buffers with per PID buffers.
+ *
+ * Return 0 on success else a negative value.
+ */
+static int create_channel_per_pid(struct ust_app *app,
+               struct ltt_ust_session *usess, struct ust_app_session *ua_sess,
+               struct ust_app_channel *ua_chan)
+{
+       int ret;
+       struct ust_registry_session *registry;
+
+       assert(app);
+       assert(usess);
+       assert(ua_sess);
+       assert(ua_chan);
+
+       DBG("UST app creating channel %s with per PID buffers", ua_chan->name);
+
+       rcu_read_lock();
+
+       registry = get_session_registry(ua_sess);
+       assert(registry);
+
+       /* Create and add a new channel registry to session. */
+       ret = ust_registry_channel_add(registry, ua_chan->key);
+       if (ret < 0) {
+               goto error;
+       }
+
+       /* Create and get channel on the consumer side. */
+       ret = do_consumer_create_channel(usess, ua_sess, ua_chan,
+                       app->bits_per_long, registry);
+       if (ret < 0) {
+               goto error;
+       }
+
+       ret = send_channel_pid_to_ust(app, ua_sess, ua_chan);
+       if (ret < 0) {
+               goto error;
+       }
+
+error:
+       rcu_read_unlock();
+       return ret;
+}
+
+/*
+ * From an already allocated ust app channel, create the channel buffers if
+ * need and send it to the application. This MUST be called with a RCU read
+ * side lock acquired.
+ *
+ * Return 0 on success or else a negative value.
+ */
+static int do_create_channel(struct ust_app *app,
+               struct ltt_ust_session *usess, struct ust_app_session *ua_sess,
+               struct ust_app_channel *ua_chan)
+{
+       int ret;
+
+       assert(app);
+       assert(usess);
+       assert(ua_sess);
+       assert(ua_chan);
+
+       /* Handle buffer type before sending the channel to the application. */
+       switch (usess->buffer_type) {
+       case LTTNG_BUFFER_PER_UID:
+       {
+               ret = create_channel_per_uid(app, usess, ua_sess, ua_chan);
+               if (ret < 0) {
+                       goto error;
+               }
+               break;
+       }
+       case LTTNG_BUFFER_PER_PID:
+       {
+               ret = create_channel_per_pid(app, usess, ua_sess, ua_chan);
+               if (ret < 0) {
+                       goto error;
+               }
+               break;
+       }
+       default:
+               assert(0);
+               ret = -EINVAL;
+               goto error;
+       }
+
+       /* Initialize ust objd object using the received handle and add it. */
+       lttng_ht_node_init_ulong(&ua_chan->ust_objd_node, ua_chan->handle);
+       lttng_ht_add_unique_ulong(app->ust_objd, &ua_chan->ust_objd_node);
+
+       /* If channel is not enabled, disable it on the tracer */
+       if (!ua_chan->enabled) {
+               ret = disable_ust_channel(app, ua_sess, ua_chan);
+               if (ret < 0) {
+                       goto error;
+               }
+       }
+
+error:
+       return ret;
+}
+
+/*
+ * Create UST app channel and create it on the tracer. Set ua_chanp of the
+ * newly created channel if not NULL.
+ *
+ * Called with UST app session lock and RCU read-side lock held.
+ *
+ * Return 0 on success or else a negative value.
+ */
+static int create_ust_app_channel(struct ust_app_session *ua_sess,
+               struct ltt_ust_channel *uchan, struct ust_app *app,
+               enum lttng_ust_chan_type type, struct ltt_ust_session *usess,
+               struct ust_app_channel **ua_chanp)
+{
+       int ret = 0;
+       struct lttng_ht_iter iter;
+       struct lttng_ht_node_str *ua_chan_node;
+       struct ust_app_channel *ua_chan;
+
+       /* Lookup channel in the ust app session */
+       lttng_ht_lookup(ua_sess->channels, (void *)uchan->name, &iter);
+       ua_chan_node = lttng_ht_iter_get_node_str(&iter);
+       if (ua_chan_node != NULL) {
+               ua_chan = caa_container_of(ua_chan_node, struct ust_app_channel, node);
+               goto end;
+       }
+
+       ua_chan = alloc_ust_app_channel(uchan->name, ua_sess, &uchan->attr);
+       if (ua_chan == NULL) {
+               /* Only malloc can fail here */
+               ret = -ENOMEM;
+               goto error_alloc;
+       }
+       shadow_copy_channel(ua_chan, uchan);
+
+       /* Set channel type. */
+       ua_chan->attr.type = type;
+
+       ret = do_create_channel(app, usess, ua_sess, ua_chan);
+       if (ret < 0) {
+               goto error;
+       }
+
+       DBG2("UST app create channel %s for PID %d completed", ua_chan->name,
+                       app->pid);
+
+       /* Only add the channel if successful on the tracer side. */
+       lttng_ht_add_unique_str(ua_sess->channels, &ua_chan->node);
+
+end:
+       if (ua_chanp) {
+               *ua_chanp = ua_chan;
+       }
+
+       /* Everything went well. */
+       return 0;
+
+error:
+       delete_ust_app_channel(ua_chan->is_sent ? app->sock : -1, ua_chan, app);
+error_alloc:
+       return ret;
+}
+
+/*
+ * Create UST app event and create it on the tracer side.
+ *
+ * Called with ust app session mutex held.
+ */
+static
+int create_ust_app_event(struct ust_app_session *ua_sess,
+               struct ust_app_channel *ua_chan, struct ltt_ust_event *uevent,
+               struct ust_app *app)
+{
+       int ret = 0;
+       struct ust_app_event *ua_event;
+
+       /* Get event node */
+       ua_event = find_ust_app_event(ua_chan->events, uevent->attr.name,
+                       uevent->filter, uevent->attr.loglevel);
+       if (ua_event != NULL) {
+               ret = -EEXIST;
+               goto end;
+       }
+
+       /* Does not exist so create one */
+       ua_event = alloc_ust_app_event(uevent->attr.name, &uevent->attr);
+       if (ua_event == NULL) {
+               /* Only malloc can failed so something is really wrong */
+               ret = -ENOMEM;
+               goto end;
+       }
+       shadow_copy_event(ua_event, uevent);
+
+       /* Create it on the tracer side */
+       ret = create_ust_event(app, ua_sess, ua_chan, ua_event);
+       if (ret < 0) {
+               /* Not found previously means that it does not exist on the tracer */
+               assert(ret != -LTTNG_UST_ERR_EXIST);
+               goto error;
+       }
+
+       add_unique_ust_app_event(ua_chan, ua_event);
+
+       DBG2("UST app create event %s for PID %d completed", ua_event->name,
+                       app->pid);
+
+end:
+       return ret;
+
+error:
+       /* Valid. Calling here is already in a read side lock */
+       delete_ust_app_event(-1, ua_event);
+       return ret;
+}
+
+/*
+ * Create UST metadata and open it on the tracer side.
+ *
+ * Called with UST app session lock held and RCU read side lock.
+ */
+static int create_ust_app_metadata(struct ust_app_session *ua_sess,
+               struct ust_app *app, struct consumer_output *consumer,
+               struct ustctl_consumer_channel_attr *attr)
+{
+       int ret = 0;
+       struct ust_app_channel *metadata;
+       struct consumer_socket *socket;
+       struct ust_registry_session *registry;
+
+       assert(ua_sess);
+       assert(app);
+       assert(consumer);
+
+       registry = get_session_registry(ua_sess);
+       assert(registry);
+
+       /* Metadata already exists for this registry or it was closed previously */
+       if (registry->metadata_key || registry->metadata_closed) {
+               ret = 0;
+               goto error;
+       }
+
+       /* Allocate UST metadata */
+       metadata = alloc_ust_app_channel(DEFAULT_METADATA_NAME, ua_sess, NULL);
+       if (!metadata) {
+               /* malloc() failed */
+               ret = -ENOMEM;
+               goto error;
+       }
+
+       if (!attr) {
+               /* Set default attributes for metadata. */
+               metadata->attr.overwrite = DEFAULT_CHANNEL_OVERWRITE;
+               metadata->attr.subbuf_size = default_get_metadata_subbuf_size();
+               metadata->attr.num_subbuf = DEFAULT_METADATA_SUBBUF_NUM;
+               metadata->attr.switch_timer_interval = DEFAULT_METADATA_SWITCH_TIMER;
+               metadata->attr.read_timer_interval = DEFAULT_METADATA_READ_TIMER;
+               metadata->attr.output = LTTNG_UST_MMAP;
+               metadata->attr.type = LTTNG_UST_CHAN_METADATA;
+       } else {
+               memcpy(&metadata->attr, attr, sizeof(metadata->attr));
+               metadata->attr.output = LTTNG_UST_MMAP;
+               metadata->attr.type = LTTNG_UST_CHAN_METADATA;
+       }
+
+       /* Need one fd for the channel. */
+       ret = lttng_fd_get(LTTNG_FD_APPS, 1);
+       if (ret < 0) {
+               ERR("Exhausted number of available FD upon create metadata");
+               goto error;
+       }
+
+       /* Get the right consumer socket for the application. */
+       socket = consumer_find_socket_by_bitness(app->bits_per_long, consumer);
+       if (!socket) {
+               ret = -EINVAL;
+               goto error_consumer;
+       }
+
+       /*
+        * Keep metadata key so we can identify it on the consumer side. Assign it
+        * to the registry *before* we ask the consumer so we avoid the race of the
+        * consumer requesting the metadata and the ask_channel call on our side
+        * did not returned yet.
+        */
+       registry->metadata_key = metadata->key;
+
+       /*
+        * Ask the metadata channel creation to the consumer. The metadata object
+        * will be created by the consumer and kept their. However, the stream is
+        * never added or monitored until we do a first push metadata to the
+        * consumer.
+        */
+       ret = ust_consumer_ask_channel(ua_sess, metadata, consumer, socket,
+                       registry);
+       if (ret < 0) {
+               /* Nullify the metadata key so we don't try to close it later on. */
+               registry->metadata_key = 0;
+               goto error_consumer;
+       }
+
+       /*
+        * The setup command will make the metadata stream be sent to the relayd,
+        * if applicable, and the thread managing the metadatas. This is important
+        * because after this point, if an error occurs, the only way the stream
+        * can be deleted is to be monitored in the consumer.
+        */
+       ret = consumer_setup_metadata(socket, metadata->key);
+       if (ret < 0) {
+               /* Nullify the metadata key so we don't try to close it later on. */
+               registry->metadata_key = 0;
+               goto error_consumer;
+       }
+
+       DBG2("UST metadata with key %" PRIu64 " created for app pid %d",
+                       metadata->key, app->pid);
+
+error_consumer:
+       lttng_fd_put(LTTNG_FD_APPS, 1);
+       delete_ust_app_channel(-1, metadata, app);
+error:
+       return ret;
+}
+
+/*
+ * Return pointer to traceable apps list.
+ */
+struct lttng_ht *ust_app_get_ht(void)
+{
+       return ust_app_ht;
+}
+
+/*
+ * Return ust app pointer or NULL if not found. RCU read side lock MUST be
+ * acquired before calling this function.
+ */
+struct ust_app *ust_app_find_by_pid(pid_t pid)
+{
+       struct ust_app *app = NULL;
+       struct lttng_ht_node_ulong *node;
+       struct lttng_ht_iter iter;
+
+       lttng_ht_lookup(ust_app_ht, (void *)((unsigned long) pid), &iter);
+       node = lttng_ht_iter_get_node_ulong(&iter);
        if (node == NULL) {
                DBG2("UST app no found with pid %d", pid);
                goto error;
        }
-       rcu_read_unlock();
 
        DBG2("Found UST app by pid %d", pid);
 
-       return caa_container_of(node, struct ust_app, pid_n);
+       app = caa_container_of(node, struct ust_app, pid_n);
 
 error:
-       rcu_read_unlock();
-       return NULL;
+       return app;
 }
 
 /*
- * Using pid and uid (of the app), allocate a new ust_app struct and
- * add it to the global traceable app list.
+ * Allocate and init an UST app object using the registration information and
+ * the command socket. This is called when the command socket connects to the
+ * session daemon.
  *
- * On success, return 0, else return malloc -ENOMEM, or -EINVAL if app
- * bitness is not supported.
+ * The object is returned on success or else NULL.
  */
-int ust_app_register(struct ust_register_msg *msg, int sock)
+struct ust_app *ust_app_create(struct ust_register_msg *msg, int sock)
 {
-       struct ust_app *lta;
-       int ret;
+       struct ust_app *lta = NULL;
+
+       assert(msg);
+       assert(sock >= 0);
+
+       DBG3("UST app creating application for socket %d", sock);
 
        if ((msg->bits_per_long == 64 &&
                                (uatomic_read(&ust_consumerd64_fd) == -EINVAL))
                        || (msg->bits_per_long == 32 &&
                                (uatomic_read(&ust_consumerd32_fd) == -EINVAL))) {
                ERR("Registration failed: application \"%s\" (pid: %d) has "
-                       "%d-bit long, but no consumerd for this long size is available.\n",
-                       msg->name, msg->pid, msg->bits_per_long);
-               ret = close(sock);
-               if (ret) {
-                       PERROR("close");
-               }
-               lttng_fd_put(LTTNG_FD_APPS, 1);
-               return -EINVAL;
-       }
-       if (msg->major != LTTNG_UST_COMM_MAJOR) {
-               ERR("Registration failed: application \"%s\" (pid: %d) has "
-                       "communication protocol version %u.%u, but sessiond supports 2.x.\n",
-                       msg->name, msg->pid, msg->major, msg->minor);
-               ret = close(sock);
-               if (ret) {
-                       PERROR("close");
-               }
-               lttng_fd_put(LTTNG_FD_APPS, 1);
-               return -EINVAL;
+                               "%d-bit long, but no consumerd for this size is available.\n",
+                               msg->name, msg->pid, msg->bits_per_long);
+               goto error;
        }
+
        lta = zmalloc(sizeof(struct ust_app));
        if (lta == NULL) {
                PERROR("malloc");
-               return -ENOMEM;
+               goto error;
        }
 
        lta->ppid = msg->ppid;
        lta->uid = msg->uid;
        lta->gid = msg->gid;
-       lta->compatible = 0;  /* Not compatible until proven */
+
        lta->bits_per_long = msg->bits_per_long;
+       lta->uint8_t_alignment = msg->uint8_t_alignment;
+       lta->uint16_t_alignment = msg->uint16_t_alignment;
+       lta->uint32_t_alignment = msg->uint32_t_alignment;
+       lta->uint64_t_alignment = msg->uint64_t_alignment;
+       lta->long_alignment = msg->long_alignment;
+       lta->byte_order = msg->byte_order;
+
        lta->v_major = msg->major;
        lta->v_minor = msg->minor;
+       lta->sessions = lttng_ht_new(0, LTTNG_HT_TYPE_U64);
+       lta->ust_objd = lttng_ht_new(0, LTTNG_HT_TYPE_ULONG);
+       lta->notify_sock = -1;
+
+       /* Copy name and make sure it's NULL terminated. */
        strncpy(lta->name, msg->name, sizeof(lta->name));
-       lta->name[16] = '\0';
-       lta->sessions = lttng_ht_new(0, LTTNG_HT_TYPE_ULONG);
+       lta->name[UST_APP_PROCNAME_LEN] = '\0';
+
+       /*
+        * Before this can be called, when receiving the registration information,
+        * the application compatibility is checked. So, at this point, the
+        * application can work with this session daemon.
+        */
+       lta->compatible = 1;
 
        lta->pid = msg->pid;
-       lttng_ht_node_init_ulong(&lta->pid_n, (unsigned long)lta->pid);
+       lttng_ht_node_init_ulong(&lta->pid_n, (unsigned long) lta->pid);
        lta->sock = sock;
-       lttng_ht_node_init_ulong(&lta->sock_n, (unsigned long)lta->sock);
+       lttng_ht_node_init_ulong(&lta->sock_n, (unsigned long) lta->sock);
+
+       CDS_INIT_LIST_HEAD(&lta->teardown_head);
+
+error:
+       return lta;
+}
+
+/*
+ * For a given application object, add it to every hash table.
+ */
+void ust_app_add(struct ust_app *app)
+{
+       assert(app);
+       assert(app->notify_sock >= 0);
 
        rcu_read_lock();
 
@@ -1458,22 +2777,49 @@ int ust_app_register(struct ust_register_msg *msg, int sock)
         * On a re-registration, we want to kick out the previous registration of
         * that pid
         */
-       lttng_ht_add_replace_ulong(ust_app_ht, &lta->pid_n);
+       lttng_ht_add_replace_ulong(ust_app_ht, &app->pid_n);
 
        /*
         * The socket _should_ be unique until _we_ call close. So, a add_unique
         * for the ust_app_ht_by_sock is used which asserts fail if the entry was
         * already in the table.
         */
-       lttng_ht_add_unique_ulong(ust_app_ht_by_sock, &lta->sock_n);
+       lttng_ht_add_unique_ulong(ust_app_ht_by_sock, &app->sock_n);
+
+       /* Add application to the notify socket hash table. */
+       lttng_ht_node_init_ulong(&app->notify_sock_n, app->notify_sock);
+       lttng_ht_add_unique_ulong(ust_app_ht_by_notify_sock, &app->notify_sock_n);
+
+       DBG("App registered with pid:%d ppid:%d uid:%d gid:%d sock:%d name:%s "
+                       "notify_sock:%d (version %d.%d)", app->pid, app->ppid, app->uid,
+                       app->gid, app->sock, app->name, app->notify_sock, app->v_major,
+                       app->v_minor);
 
        rcu_read_unlock();
+}
 
-       DBG("App registered with pid:%d ppid:%d uid:%d gid:%d sock:%d name:%s"
-                       " (version %d.%d)", lta->pid, lta->ppid, lta->uid, lta->gid,
-                       lta->sock, lta->name, lta->v_major, lta->v_minor);
+/*
+ * Set the application version into the object.
+ *
+ * Return 0 on success else a negative value either an errno code or a
+ * LTTng-UST error code.
+ */
+int ust_app_version(struct ust_app *app)
+{
+       int ret;
 
-       return 0;
+       assert(app);
+
+       ret = ustctl_tracer_version(app->sock, &app->version);
+       if (ret < 0) {
+               if (ret != -LTTNG_UST_ERR_EXITING && ret != -EPIPE) {
+                       ERR("UST app %d verson failed with ret %d", app->sock, ret);
+               } else {
+                       DBG3("UST app %d verion failed. Application is dead", app->sock);
+               }
+       }
+
+       return ret;
 }
 
 /*
@@ -1487,6 +2833,7 @@ void ust_app_unregister(int sock)
        struct ust_app *lta;
        struct lttng_ht_node_ulong *node;
        struct lttng_ht_iter iter;
+       struct ust_app_session *ua_sess;
        int ret;
 
        rcu_read_lock();
@@ -1494,29 +2841,89 @@ void ust_app_unregister(int sock)
        /* Get the node reference for a call_rcu */
        lttng_ht_lookup(ust_app_ht_by_sock, (void *)((unsigned long) sock), &iter);
        node = lttng_ht_iter_get_node_ulong(&iter);
-       if (node == NULL) {
-               ERR("Unable to find app by sock %d", sock);
-               goto error;
-       }
+       assert(node);
 
        lta = caa_container_of(node, struct ust_app, sock_n);
+       DBG("PID %d unregistering with sock %d", lta->pid, sock);
+
+       /* Remove application from PID hash table */
+       ret = lttng_ht_del(ust_app_ht_by_sock, &iter);
+       assert(!ret);
+
+       /*
+        * Remove application from notify hash table. The thread handling the
+        * notify socket could have deleted the node so ignore on error because
+        * either way it's valid. The close of that socket is handled by the other
+        * thread.
+        */
+       iter.iter.node = &lta->notify_sock_n.node;
+       (void) lttng_ht_del(ust_app_ht_by_notify_sock, &iter);
+
+       /*
+        * Ignore return value since the node might have been removed before by an
+        * add replace during app registration because the PID can be reassigned by
+        * the OS.
+        */
+       iter.iter.node = &lta->pid_n.node;
+       ret = lttng_ht_del(ust_app_ht, &iter);
+       if (ret) {
+               DBG3("Unregister app by PID %d failed. This can happen on pid reuse",
+                               lta->pid);
+       }
+
+       /* Remove sessions so they are not visible during deletion.*/
+       cds_lfht_for_each_entry(lta->sessions->ht, &iter.iter, ua_sess,
+                       node.node) {
+               struct ust_registry_session *registry;
+
+               ret = lttng_ht_del(lta->sessions, &iter);
+               if (ret) {
+                       /* The session was already removed so scheduled for teardown. */
+                       continue;
+               }
 
-       DBG("PID %d unregistering with sock %d", lta->pid, sock);
+               /*
+                * Add session to list for teardown. This is safe since at this point we
+                * are the only one using this list.
+                */
+               pthread_mutex_lock(&ua_sess->lock);
 
-       /* Remove application from PID hash table */
-       ret = lttng_ht_del(ust_app_ht_by_sock, &iter);
-       assert(!ret);
+               /*
+                * Normally, this is done in the delete session process which is
+                * executed in the call rcu below. However, upon registration we can't
+                * afford to wait for the grace period before pushing data or else the
+                * data pending feature can race between the unregistration and stop
+                * command where the data pending command is sent *before* the grace
+                * period ended.
+                *
+                * The close metadata below nullifies the metadata pointer in the
+                * session so the delete session will NOT push/close a second time.
+                */
+               registry = get_session_registry(ua_sess);
+               if (registry && !registry->metadata_closed) {
+                       /* Push metadata for application before freeing the application. */
+                       (void) push_metadata(registry, ua_sess->consumer);
 
-       /* Assign second node for deletion */
-       iter.iter.node = &lta->pid_n.node;
+                       /*
+                        * Don't ask to close metadata for global per UID buffers. Close
+                        * metadata only on destroy trace session in this case. Also, the
+                        * previous push metadata could have flag the metadata registry to
+                        * close so don't send a close command if closed.
+                        */
+                       if (ua_sess->buffer_type != LTTNG_BUFFER_PER_UID &&
+                                       !registry->metadata_closed) {
+                               /* And ask to close it for this session registry. */
+                               (void) close_metadata(registry, ua_sess->consumer);
+                       }
+               }
 
-       ret = lttng_ht_del(ust_app_ht, &iter);
-       assert(!ret);
+               cds_list_add(&ua_sess->teardown_node, &lta->teardown_head);
+               pthread_mutex_unlock(&ua_sess->lock);
+       }
 
        /* Free memory */
        call_rcu(&lta->pid_n.head, delete_ust_app_rcu);
 
-error:
        rcu_read_unlock();
        return;
 }
@@ -1544,11 +2951,11 @@ int ust_app_list_events(struct lttng_event **events)
        size_t nbmem, count = 0;
        struct lttng_ht_iter iter;
        struct ust_app *app;
-       struct lttng_event *tmp;
+       struct lttng_event *tmp_event;
 
        nbmem = UST_APP_EVENT_LIST_SIZE;
-       tmp = zmalloc(nbmem * sizeof(struct lttng_event));
-       if (tmp == NULL) {
+       tmp_event = zmalloc(nbmem * sizeof(struct lttng_event));
+       if (tmp_event == NULL) {
                PERROR("zmalloc ust app events");
                ret = -ENOMEM;
                goto error;
@@ -1559,7 +2966,7 @@ int ust_app_list_events(struct lttng_event **events)
        cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                struct lttng_ust_tracepoint_iter uiter;
 
-               health_code_update(&health_thread_cmd);
+               health_code_update();
 
                if (!app->compatible) {
                        /*
@@ -1570,43 +2977,62 @@ int ust_app_list_events(struct lttng_event **events)
                }
                handle = ustctl_tracepoint_list(app->sock);
                if (handle < 0) {
-                       ERR("UST app list events getting handle failed for app pid %d",
-                                       app->pid);
+                       if (handle != -EPIPE && handle != -LTTNG_UST_ERR_EXITING) {
+                               ERR("UST app list events getting handle failed for app pid %d",
+                                               app->pid);
+                       }
                        continue;
                }
 
                while ((ret = ustctl_tracepoint_list_get(app->sock, handle,
-                                               &uiter)) != -ENOENT) {
-                       health_code_update(&health_thread_cmd);
+                                       &uiter)) != -LTTNG_UST_ERR_NOENT) {
+                       /* Handle ustctl error. */
+                       if (ret < 0) {
+                               free(tmp_event);
+                               if (ret != -LTTNG_UST_ERR_EXITING || ret != -EPIPE) {
+                                       ERR("UST app tp list get failed for app %d with ret %d",
+                                                       app->sock, ret);
+                               } else {
+                                       DBG3("UST app tp list get failed. Application is dead");
+                               }
+                               goto rcu_error;
+                       }
+
+                       health_code_update();
                        if (count >= nbmem) {
+                               /* In case the realloc fails, we free the memory */
+                               void *ptr;
+
                                DBG2("Reallocating event list from %zu to %zu entries", nbmem,
                                                2 * nbmem);
                                nbmem *= 2;
-                               tmp = realloc(tmp, nbmem * sizeof(struct lttng_event));
-                               if (tmp == NULL) {
+                               ptr = realloc(tmp_event, nbmem * sizeof(struct lttng_event));
+                               if (ptr == NULL) {
                                        PERROR("realloc ust app events");
+                                       free(tmp_event);
                                        ret = -ENOMEM;
                                        goto rcu_error;
                                }
+                               tmp_event = ptr;
                        }
-                       memcpy(tmp[count].name, uiter.name, LTTNG_UST_SYM_NAME_LEN);
-                       tmp[count].loglevel = uiter.loglevel;
-                       tmp[count].type = (enum lttng_event_type) LTTNG_UST_TRACEPOINT;
-                       tmp[count].pid = app->pid;
-                       tmp[count].enabled = -1;
+                       memcpy(tmp_event[count].name, uiter.name, LTTNG_UST_SYM_NAME_LEN);
+                       tmp_event[count].loglevel = uiter.loglevel;
+                       tmp_event[count].type = (enum lttng_event_type) LTTNG_UST_TRACEPOINT;
+                       tmp_event[count].pid = app->pid;
+                       tmp_event[count].enabled = -1;
                        count++;
                }
        }
 
        ret = count;
-       *events = tmp;
+       *events = tmp_event;
 
        DBG2("UST app list events done (%zu events)", count);
 
 rcu_error:
        rcu_read_unlock();
 error:
-       health_code_update(&health_thread_cmd);
+       health_code_update();
        return ret;
 }
 
@@ -1619,11 +3045,11 @@ int ust_app_list_event_fields(struct lttng_event_field **fields)
        size_t nbmem, count = 0;
        struct lttng_ht_iter iter;
        struct ust_app *app;
-       struct lttng_event_field *tmp;
+       struct lttng_event_field *tmp_event;
 
        nbmem = UST_APP_EVENT_LIST_SIZE;
-       tmp = zmalloc(nbmem * sizeof(struct lttng_event_field));
-       if (tmp == NULL) {
+       tmp_event = zmalloc(nbmem * sizeof(struct lttng_event_field));
+       if (tmp_event == NULL) {
                PERROR("zmalloc ust app event fields");
                ret = -ENOMEM;
                goto error;
@@ -1634,7 +3060,7 @@ int ust_app_list_event_fields(struct lttng_event_field **fields)
        cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                struct lttng_ust_field_iter uiter;
 
-               health_code_update(&health_thread_cmd);
+               health_code_update();
 
                if (!app->compatible) {
                        /*
@@ -1645,80 +3071,110 @@ int ust_app_list_event_fields(struct lttng_event_field **fields)
                }
                handle = ustctl_tracepoint_field_list(app->sock);
                if (handle < 0) {
-                       ERR("UST app list event fields getting handle failed for app pid %d",
-                                       app->pid);
+                       if (handle != -EPIPE && handle != -LTTNG_UST_ERR_EXITING) {
+                               ERR("UST app list field getting handle failed for app pid %d",
+                                               app->pid);
+                       }
                        continue;
                }
 
                while ((ret = ustctl_tracepoint_field_list_get(app->sock, handle,
-                                               &uiter)) != -ENOENT) {
-                       health_code_update(&health_thread_cmd);
+                                       &uiter)) != -LTTNG_UST_ERR_NOENT) {
+                       /* Handle ustctl error. */
+                       if (ret < 0) {
+                               free(tmp_event);
+                               if (ret != -LTTNG_UST_ERR_EXITING || ret != -EPIPE) {
+                                       ERR("UST app tp list field failed for app %d with ret %d",
+                                                       app->sock, ret);
+                               } else {
+                                       DBG3("UST app tp list field failed. Application is dead");
+                               }
+                               goto rcu_error;
+                       }
+
+                       health_code_update();
                        if (count >= nbmem) {
+                               /* In case the realloc fails, we free the memory */
+                               void *ptr;
+
                                DBG2("Reallocating event field list from %zu to %zu entries", nbmem,
                                                2 * nbmem);
                                nbmem *= 2;
-                               tmp = realloc(tmp, nbmem * sizeof(struct lttng_event_field));
-                               if (tmp == NULL) {
+                               ptr = realloc(tmp_event, nbmem * sizeof(struct lttng_event_field));
+                               if (ptr == NULL) {
                                        PERROR("realloc ust app event fields");
+                                       free(tmp_event);
                                        ret = -ENOMEM;
                                        goto rcu_error;
                                }
+                               tmp_event = ptr;
                        }
 
-                       memcpy(tmp[count].field_name, uiter.field_name, LTTNG_UST_SYM_NAME_LEN);
-                       tmp[count].type = uiter.type;
+                       memcpy(tmp_event[count].field_name, uiter.field_name, LTTNG_UST_SYM_NAME_LEN);
+                       tmp_event[count].type = uiter.type;
+                       tmp_event[count].nowrite = uiter.nowrite;
 
-                       memcpy(tmp[count].event.name, uiter.event_name, LTTNG_UST_SYM_NAME_LEN);
-                       tmp[count].event.loglevel = uiter.loglevel;
-                       tmp[count].event.type = LTTNG_UST_TRACEPOINT;
-                       tmp[count].event.pid = app->pid;
-                       tmp[count].event.enabled = -1;
+                       memcpy(tmp_event[count].event.name, uiter.event_name, LTTNG_UST_SYM_NAME_LEN);
+                       tmp_event[count].event.loglevel = uiter.loglevel;
+                       tmp_event[count].event.type = LTTNG_UST_TRACEPOINT;
+                       tmp_event[count].event.pid = app->pid;
+                       tmp_event[count].event.enabled = -1;
                        count++;
                }
        }
 
        ret = count;
-       *fields = tmp;
+       *fields = tmp_event;
 
        DBG2("UST app list event fields done (%zu events)", count);
 
 rcu_error:
        rcu_read_unlock();
 error:
-       health_code_update(&health_thread_cmd);
+       health_code_update();
        return ret;
 }
 
 /*
  * Free and clean all traceable apps of the global list.
+ *
+ * Should _NOT_ be called with RCU read-side lock held.
  */
 void ust_app_clean_list(void)
 {
        int ret;
+       struct ust_app *app;
        struct lttng_ht_iter iter;
-       struct lttng_ht_node_ulong *node;
 
        DBG2("UST app cleaning registered apps hash table");
 
        rcu_read_lock();
 
-       cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, node, node) {
+       cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                ret = lttng_ht_del(ust_app_ht, &iter);
                assert(!ret);
-               call_rcu(&node->head, delete_ust_app_rcu);
+               call_rcu(&app->pid_n.head, delete_ust_app_rcu);
        }
 
        /* Cleanup socket hash table */
-       cds_lfht_for_each_entry(ust_app_ht_by_sock->ht, &iter.iter, node, node) {
+       cds_lfht_for_each_entry(ust_app_ht_by_sock->ht, &iter.iter, app,
+                       sock_n.node) {
                ret = lttng_ht_del(ust_app_ht_by_sock, &iter);
                assert(!ret);
        }
 
-       /* Destroy is done only when the ht is empty */
-       lttng_ht_destroy(ust_app_ht);
-       lttng_ht_destroy(ust_app_ht_by_sock);
-
+       /* Cleanup notify socket hash table */
+       cds_lfht_for_each_entry(ust_app_ht_by_notify_sock->ht, &iter.iter, app,
+                       notify_sock_n.node) {
+               ret = lttng_ht_del(ust_app_ht_by_notify_sock, &iter);
+               assert(!ret);
+       }
        rcu_read_unlock();
+
+       /* Destroy is done only when the ht is empty */
+       ht_cleanup_push(ust_app_ht);
+       ht_cleanup_push(ust_app_ht_by_sock);
+       ht_cleanup_push(ust_app_ht_by_notify_sock);
 }
 
 /*
@@ -1728,6 +3184,7 @@ void ust_app_ht_alloc(void)
 {
        ust_app_ht = lttng_ht_new(0, LTTNG_HT_TYPE_ULONG);
        ust_app_ht_by_sock = lttng_ht_new(0, LTTNG_HT_TYPE_ULONG);
+       ust_app_ht_by_notify_sock = lttng_ht_new(0, LTTNG_HT_TYPE_ULONG);
 }
 
 /*
@@ -1749,7 +3206,7 @@ int ust_app_disable_channel_glb(struct ltt_ust_session *usess,
                goto error;
        }
 
-       DBG2("UST app disabling channel %s from global domain for session id %d",
+       DBG2("UST app disabling channel %s from global domain for session id %" PRIu64,
                        uchan->name, usess->id);
 
        rcu_read_lock();
@@ -1810,7 +3267,7 @@ int ust_app_enable_channel_glb(struct ltt_ust_session *usess,
                goto error;
        }
 
-       DBG2("UST app enabling channel %s to global domain for session id %d",
+       DBG2("UST app enabling channel %s to global domain for session id %" PRIu64,
                        uchan->name, usess->id);
 
        rcu_read_lock();
@@ -1858,7 +3315,8 @@ int ust_app_disable_event_glb(struct ltt_ust_session *usess,
        struct ust_app_event *ua_event;
 
        DBG("UST app disabling event %s for all apps in channel "
-                       "%s for session id %d", uevent->attr.name, uchan->name, usess->id);
+                       "%s for session id %" PRIu64,
+                       uevent->attr.name, uchan->name, usess->id);
 
        rcu_read_lock();
 
@@ -1881,7 +3339,7 @@ int ust_app_disable_event_glb(struct ltt_ust_session *usess,
                lttng_ht_lookup(ua_sess->channels, (void *)uchan->name, &uiter);
                ua_chan_node = lttng_ht_iter_get_node_str(&uiter);
                if (ua_chan_node == NULL) {
-                       DBG2("Channel %s not found in session id %d for app pid %d."
+                       DBG2("Channel %s not found in session id %" PRIu64 " for app pid %d."
                                        "Skipping", uchan->name, usess->id, app->pid);
                        continue;
                }
@@ -1924,7 +3382,7 @@ int ust_app_disable_all_event_glb(struct ltt_ust_session *usess,
        struct ust_app_event *ua_event;
 
        DBG("UST app disabling all event for all apps in channel "
-                       "%s for session id %d", uchan->name, usess->id);
+                       "%s for session id %" PRIu64, uchan->name, usess->id);
 
        rcu_read_lock();
 
@@ -1938,8 +3396,10 @@ int ust_app_disable_all_event_glb(struct ltt_ust_session *usess,
                        continue;
                }
                ua_sess = lookup_session_by_app(usess, app);
-               /* If ua_sess is NULL, there is a code flow error */
-               assert(ua_sess);
+               if (!ua_sess) {
+                       /* The application has problem or is probably dead. */
+                       continue;
+               }
 
                /* Lookup channel in the ust app session */
                lttng_ht_lookup(ua_sess->channels, (void *)uchan->name, &uiter);
@@ -1971,16 +3431,16 @@ int ust_app_disable_all_event_glb(struct ltt_ust_session *usess,
 int ust_app_create_channel_glb(struct ltt_ust_session *usess,
                struct ltt_ust_channel *uchan)
 {
+       int ret = 0, created;
        struct lttng_ht_iter iter;
        struct ust_app *app;
-       struct ust_app_session *ua_sess;
-       struct ust_app_channel *ua_chan;
+       struct ust_app_session *ua_sess = NULL;
 
        /* Very wrong code flow */
        assert(usess);
        assert(uchan);
 
-       DBG2("UST app adding channel %s to global domain for session id %d",
+       DBG2("UST app adding channel %s to UST domain for session id %" PRIu64,
                        uchan->name, usess->id);
 
        rcu_read_lock();
@@ -1999,29 +3459,51 @@ int ust_app_create_channel_glb(struct ltt_ust_session *usess,
                 * that if session exist, it will simply return a pointer to the ust
                 * app session.
                 */
-               ua_sess = create_ust_app_session(usess, app);
-               if (ua_sess == NULL) {
-                       /* The malloc() failed. */
-                       goto error;
-               } else if (ua_sess == (void *) -1UL) {
-                       /* The application's socket is not valid. Contiuing */
-                       continue;
+               ret = create_ust_app_session(usess, app, &ua_sess, &created);
+               if (ret < 0) {
+                       switch (ret) {
+                       case -ENOTCONN:
+                               /*
+                                * The application's socket is not valid. Either a bad socket
+                                * or a timeout on it. We can't inform the caller that for a
+                                * specific app, the session failed so lets continue here.
+                                */
+                               continue;
+                       case -ENOMEM:
+                       default:
+                               goto error_rcu_unlock;
+                       }
                }
+               assert(ua_sess);
 
-               /* Create channel onto application */
-               ua_chan = create_ust_app_channel(ua_sess, uchan, app);
-               if (ua_chan == NULL) {
-                       /* Major problem here and it's maybe the tracer or malloc() */
-                       goto error;
+               pthread_mutex_lock(&ua_sess->lock);
+               if (!strncmp(uchan->name, DEFAULT_METADATA_NAME,
+                                       sizeof(uchan->name))) {
+                       struct ustctl_consumer_channel_attr attr;
+                       copy_channel_attr_to_ustctl(&attr, &uchan->attr);
+                       ret = create_ust_app_metadata(ua_sess, app, usess->consumer,
+                                       &attr);
+               } else {
+                       /* Create channel onto application. We don't need the chan ref. */
+                       ret = create_ust_app_channel(ua_sess, uchan, app,
+                                       LTTNG_UST_CHAN_PER_CPU, usess, NULL);
+               }
+               pthread_mutex_unlock(&ua_sess->lock);
+               if (ret < 0) {
+                       if (ret == -ENOMEM) {
+                               /* No more memory is a fatal error. Stop right now. */
+                               goto error_rcu_unlock;
+                       }
+                       /* Cleanup the created session if it's the case. */
+                       if (created) {
+                               destroy_app_session(app, ua_sess);
+                       }
                }
        }
 
+error_rcu_unlock:
        rcu_read_unlock();
-
-       return 0;
-
-error:
-       return -1;
+       return ret;
 }
 
 /*
@@ -2032,13 +3514,13 @@ int ust_app_enable_event_glb(struct ltt_ust_session *usess,
 {
        int ret = 0;
        struct lttng_ht_iter iter, uiter;
-       struct lttng_ht_node_str *ua_chan_node, *ua_event_node;
+       struct lttng_ht_node_str *ua_chan_node;
        struct ust_app *app;
        struct ust_app_session *ua_sess;
        struct ust_app_channel *ua_chan;
        struct ust_app_event *ua_event;
 
-       DBG("UST app enabling event %s for all apps for session id %d",
+       DBG("UST app enabling event %s for all apps for session id %" PRIu64,
                        uevent->attr.name, usess->id);
 
        /*
@@ -2059,8 +3541,12 @@ int ust_app_enable_event_glb(struct ltt_ust_session *usess,
                        continue;
                }
                ua_sess = lookup_session_by_app(usess, app);
-               /* If ua_sess is NULL, there is a code flow error */
-               assert(ua_sess);
+               if (!ua_sess) {
+                       /* The application has problem or is probably dead. */
+                       continue;
+               }
+
+               pthread_mutex_lock(&ua_sess->lock);
 
                /* Lookup channel in the ust app session */
                lttng_ht_lookup(ua_sess->channels, (void *)uchan->name, &uiter);
@@ -2070,19 +3556,22 @@ int ust_app_enable_event_glb(struct ltt_ust_session *usess,
 
                ua_chan = caa_container_of(ua_chan_node, struct ust_app_channel, node);
 
-               lttng_ht_lookup(ua_chan->events, (void*)uevent->attr.name, &uiter);
-               ua_event_node = lttng_ht_iter_get_node_str(&uiter);
-               if (ua_event_node == NULL) {
+               /* Get event node */
+               ua_event = find_ust_app_event(ua_chan->events, uevent->attr.name,
+                               uevent->filter, uevent->attr.loglevel);
+               if (ua_event == NULL) {
                        DBG3("UST app enable event %s not found for app PID %d."
                                        "Skipping app", uevent->attr.name, app->pid);
-                       continue;
+                       goto next_app;
                }
-               ua_event = caa_container_of(ua_event_node, struct ust_app_event, node);
 
                ret = enable_ust_app_event(ua_sess, ua_event, app);
                if (ret < 0) {
+                       pthread_mutex_unlock(&ua_sess->lock);
                        goto error;
                }
+       next_app:
+               pthread_mutex_unlock(&ua_sess->lock);
        }
 
 error:
@@ -2104,7 +3593,7 @@ int ust_app_create_event_glb(struct ltt_ust_session *usess,
        struct ust_app_session *ua_sess;
        struct ust_app_channel *ua_chan;
 
-       DBG("UST app creating event %s for all apps for session id %d",
+       DBG("UST app creating event %s for all apps for session id %" PRIu64,
                        uevent->attr.name, usess->id);
 
        rcu_read_lock();
@@ -2119,9 +3608,12 @@ int ust_app_create_event_glb(struct ltt_ust_session *usess,
                        continue;
                }
                ua_sess = lookup_session_by_app(usess, app);
-               /* If ua_sess is NULL, there is a code flow error */
-               assert(ua_sess);
+               if (!ua_sess) {
+                       /* The application has problem or is probably dead. */
+                       continue;
+               }
 
+               pthread_mutex_lock(&ua_sess->lock);
                /* Lookup channel in the ust app session */
                lttng_ht_lookup(ua_sess->channels, (void *)uchan->name, &uiter);
                ua_chan_node = lttng_ht_iter_get_node_str(&uiter);
@@ -2131,8 +3623,9 @@ int ust_app_create_event_glb(struct ltt_ust_session *usess,
                ua_chan = caa_container_of(ua_chan_node, struct ust_app_channel, node);
 
                ret = create_ust_app_event(ua_sess, ua_chan, uevent, app);
+               pthread_mutex_unlock(&ua_sess->lock);
                if (ret < 0) {
-                       if (ret != -EEXIST) {
+                       if (ret != -LTTNG_UST_ERR_EXIST) {
                                /* Possible value at this point: -ENOMEM. If so, we stop! */
                                break;
                        }
@@ -2150,14 +3643,11 @@ int ust_app_create_event_glb(struct ltt_ust_session *usess,
 /*
  * Start tracing for a specific UST session and app.
  */
+static
 int ust_app_start_trace(struct ltt_ust_session *usess, struct ust_app *app)
 {
        int ret = 0;
-       struct lttng_ht_iter iter;
        struct ust_app_session *ua_sess;
-       struct ust_app_channel *ua_chan;
-       struct ltt_ust_stream *ustream;
-       struct consumer_socket *socket;
 
        DBG("Starting tracing for ust app pid %d", app->pid);
 
@@ -2169,9 +3659,12 @@ int ust_app_start_trace(struct ltt_ust_session *usess, struct ust_app *app)
 
        ua_sess = lookup_session_by_app(usess, app);
        if (ua_sess == NULL) {
-               goto error_rcu_unlock;
+               /* The session is in teardown process. Ignore and continue. */
+               goto end;
        }
 
+       pthread_mutex_lock(&ua_sess->lock);
+
        /* Upon restart, we skip the setup, already done */
        if (ua_sess->started) {
                goto skip_setup;
@@ -2181,219 +3674,213 @@ int ust_app_start_trace(struct ltt_ust_session *usess, struct ust_app *app)
        if (usess->consumer->type == CONSUMER_DST_LOCAL &&
                        strlen(usess->consumer->dst.trace_path) > 0) {
                ret = run_as_mkdir_recursive(usess->consumer->dst.trace_path,
-                               S_IRWXU | S_IRWXG, usess->uid, usess->gid);
+                               S_IRWXU | S_IRWXG, ua_sess->euid, ua_sess->egid);
                if (ret < 0) {
                        if (ret != -EEXIST) {
                                ERR("Trace directory creation error");
-                               ret = -1;
-                               goto error_rcu_unlock;
-                       }
-               }
-       }
-
-       /* Indicate that the session has been started once */
-       ua_sess->started = 1;
-
-       ret = create_ust_app_metadata(ua_sess, usess->pathname, app);
-       if (ret < 0) {
-               goto error_rcu_unlock;
-       }
-
-       /* For each channel */
-       cds_lfht_for_each_entry(ua_sess->channels->ht, &iter.iter, ua_chan,
-                       node.node) {
-               /* Create all streams */
-               while (1) {
-                       /* Create UST stream */
-                       ustream = zmalloc(sizeof(*ustream));
-                       if (ustream == NULL) {
-                               PERROR("zmalloc ust stream");
-                               goto error_rcu_unlock;
-                       }
-
-                       /* We are going to receive 2 fds, we need to reserve them. */
-                       ret = lttng_fd_get(LTTNG_FD_APPS, 2);
-                       if (ret < 0) {
-                               ERR("Exhausted number of available FD upon stream create");
-                               free(ustream);
-                               goto error_rcu_unlock;
-                       }
-
-                       health_code_update(&health_thread_cmd);
-
-                       ret = ustctl_create_stream(app->sock, ua_chan->obj,
-                                       &ustream->obj);
-                       if (ret < 0) {
-                               /* Got all streams */
-                               lttng_fd_put(LTTNG_FD_APPS, 2);
-                               free(ustream);
-                               break;
-                       }
-                       ustream->handle = ustream->obj->handle;
-
-                       health_code_update(&health_thread_cmd);
-
-                       /* Order is important */
-                       cds_list_add_tail(&ustream->list, &ua_chan->streams.head);
-                       ret = snprintf(ustream->name, sizeof(ustream->name), "%s_%u",
-                                       ua_chan->name, ua_chan->streams.count++);
-                       if (ret < 0) {
-                               PERROR("asprintf UST create stream");
-                               /*
-                                * XXX what should we do here with the
-                                * stream ?
-                                */
-                               continue;
+                               goto error_unlock;
                        }
-                       DBG2("UST stream %d ready (handle: %d)", ua_chan->streams.count,
-                                       ustream->handle);
-               }
-
-               health_code_update(&health_thread_cmd);
-       }
-
-       switch (app->bits_per_long) {
-       case 64:
-               socket = consumer_find_socket(uatomic_read(&ust_consumerd64_fd),
-                               usess->consumer);
-               if (socket == NULL) {
-                       goto skip_setup;
-               }
-               break;
-       case 32:
-               socket = consumer_find_socket(uatomic_read(&ust_consumerd32_fd),
-                               usess->consumer);
-               if (socket == NULL) {
-                       goto skip_setup;
                }
-               break;
-       default:
-               ret = -EINVAL;
-               goto error_rcu_unlock;
        }
 
-       /* Setup UST consumer socket and send fds to it */
-       ret = ust_consumer_send_session(ua_sess, usess->consumer, socket);
+       /*
+        * Create the metadata for the application. This returns gracefully if a
+        * metadata was already set for the session.
+        */
+       ret = create_ust_app_metadata(ua_sess, app, usess->consumer, NULL);
        if (ret < 0) {
-               goto error_rcu_unlock;
+               goto error_unlock;
        }
 
-       health_code_update(&health_thread_cmd);
+       health_code_update();
 
 skip_setup:
        /* This start the UST tracing */
        ret = ustctl_start_session(app->sock, ua_sess->handle);
        if (ret < 0) {
-               ERR("Error starting tracing for app pid: %d", app->pid);
-               goto error_rcu_unlock;
+               if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
+                       ERR("Error starting tracing for app pid: %d (ret: %d)",
+                                       app->pid, ret);
+               } else {
+                       DBG("UST app start session failed. Application is dead.");
+               }
+               goto error_unlock;
        }
 
-       health_code_update(&health_thread_cmd);
+       /* Indicate that the session has been started once */
+       ua_sess->started = 1;
+
+       pthread_mutex_unlock(&ua_sess->lock);
+
+       health_code_update();
 
        /* Quiescent wait after starting trace */
-       ustctl_wait_quiescent(app->sock);
+       ret = ustctl_wait_quiescent(app->sock);
+       if (ret < 0 && ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
+               ERR("UST app wait quiescent failed for app pid %d ret %d",
+                               app->pid, ret);
+       }
 
 end:
        rcu_read_unlock();
-       health_code_update(&health_thread_cmd);
+       health_code_update();
        return 0;
 
-error_rcu_unlock:
+error_unlock:
+       pthread_mutex_unlock(&ua_sess->lock);
        rcu_read_unlock();
-       health_code_update(&health_thread_cmd);
+       health_code_update();
        return -1;
 }
 
 /*
  * Stop tracing for a specific UST session and app.
  */
+static
 int ust_app_stop_trace(struct ltt_ust_session *usess, struct ust_app *app)
 {
        int ret = 0;
-       struct lttng_ht_iter iter;
        struct ust_app_session *ua_sess;
-       struct ust_app_channel *ua_chan;
+       struct ust_registry_session *registry;
 
        DBG("Stopping tracing for ust app pid %d", app->pid);
 
        rcu_read_lock();
 
        if (!app->compatible) {
-               goto end;
+               goto end_no_session;
        }
 
        ua_sess = lookup_session_by_app(usess, app);
        if (ua_sess == NULL) {
-               /* Only malloc can failed so something is really wrong */
-               goto error_rcu_unlock;
+               goto end_no_session;
        }
 
+       pthread_mutex_lock(&ua_sess->lock);
+
        /*
         * If started = 0, it means that stop trace has been called for a session
-        * that was never started. This is a code flow error and should never
-        * happen.
+        * that was never started. It's possible since we can have a fail start
+        * from either the application manager thread or the command thread. Simply
+        * indicate that this is a stop error.
         */
-       assert(ua_sess->started == 1);
+       if (!ua_sess->started) {
+               goto error_rcu_unlock;
+       }
 
-       health_code_update(&health_thread_cmd);
+       health_code_update();
 
        /* This inhibits UST tracing */
        ret = ustctl_stop_session(app->sock, ua_sess->handle);
        if (ret < 0) {
-               ERR("Error stopping tracing for app pid: %d", app->pid);
+               if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
+                       ERR("Error stopping tracing for app pid: %d (ret: %d)",
+                                       app->pid, ret);
+               } else {
+                       DBG("UST app stop session failed. Application is dead.");
+               }
                goto error_rcu_unlock;
        }
 
-       health_code_update(&health_thread_cmd);
+       health_code_update();
 
        /* Quiescent wait after stopping trace */
-       ustctl_wait_quiescent(app->sock);
+       ret = ustctl_wait_quiescent(app->sock);
+       if (ret < 0 && ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
+               ERR("UST app wait quiescent failed for app pid %d ret %d",
+                               app->pid, ret);
+       }
+
+       health_code_update();
 
-       health_code_update(&health_thread_cmd);
+       registry = get_session_registry(ua_sess);
+       assert(registry);
+
+       if (!registry->metadata_closed) {
+               /* Push metadata for application before freeing the application. */
+               (void) push_metadata(registry, ua_sess->consumer);
+       }
+
+       pthread_mutex_unlock(&ua_sess->lock);
+end_no_session:
+       rcu_read_unlock();
+       health_code_update();
+       return 0;
+
+error_rcu_unlock:
+       pthread_mutex_unlock(&ua_sess->lock);
+       rcu_read_unlock();
+       health_code_update();
+       return -1;
+}
+
+/*
+ * Flush buffers for a specific UST session and app.
+ */
+static
+int ust_app_flush_trace(struct ltt_ust_session *usess, struct ust_app *app)
+{
+       int ret = 0;
+       struct lttng_ht_iter iter;
+       struct ust_app_session *ua_sess;
+       struct ust_app_channel *ua_chan;
+
+       DBG("Flushing buffers for ust app pid %d", app->pid);
+
+       rcu_read_lock();
+
+       if (!app->compatible) {
+               goto end_no_session;
+       }
+
+       ua_sess = lookup_session_by_app(usess, app);
+       if (ua_sess == NULL) {
+               goto end_no_session;
+       }
+
+       pthread_mutex_lock(&ua_sess->lock);
+
+       health_code_update();
 
        /* Flushing buffers */
        cds_lfht_for_each_entry(ua_sess->channels->ht, &iter.iter, ua_chan,
                        node.node) {
-               health_code_update(&health_thread_cmd);
+               health_code_update();
+               assert(ua_chan->is_sent);
                ret = ustctl_sock_flush_buffer(app->sock, ua_chan->obj);
                if (ret < 0) {
-                       ERR("UST app PID %d channel %s flush failed with ret %d",
-                                       app->pid, ua_chan->name, ret);
+                       if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
+                               ERR("UST app PID %d channel %s flush failed with ret %d",
+                                               app->pid, ua_chan->name, ret);
+                       } else {
+                               DBG3("UST app failed to flush %s. Application is dead.",
+                                               ua_chan->name);
+                               /* No need to continue. */
+                               break;
+                       }
                        /* Continuing flushing all buffers */
                        continue;
                }
        }
 
-       health_code_update(&health_thread_cmd);
-
-       /* Flush all buffers before stopping */
-       ret = ustctl_sock_flush_buffer(app->sock, ua_sess->metadata->obj);
-       if (ret < 0) {
-               ERR("UST app PID %d metadata flush failed with ret %d", app->pid,
-                               ret);
-       }
+       health_code_update();
 
-end:
+       pthread_mutex_unlock(&ua_sess->lock);
+end_no_session:
        rcu_read_unlock();
-       health_code_update(&health_thread_cmd);
+       health_code_update();
        return 0;
-
-error_rcu_unlock:
-       rcu_read_unlock();
-       health_code_update(&health_thread_cmd);
-       return -1;
 }
 
 /*
  * Destroy a specific UST session in apps.
  */
-int ust_app_destroy_trace(struct ltt_ust_session *usess, struct ust_app *app)
+static int destroy_trace(struct ltt_ust_session *usess, struct ust_app *app)
 {
+       int ret;
        struct ust_app_session *ua_sess;
-       struct lttng_ust_object_data obj;
        struct lttng_ht_iter iter;
-       struct lttng_ht_node_ulong *node;
-       int ret;
+       struct lttng_ht_node_u64 *node;
 
        DBG("Destroy tracing for ust app pid %d", app->pid);
 
@@ -2404,36 +3891,28 @@ int ust_app_destroy_trace(struct ltt_ust_session *usess, struct ust_app *app)
        }
 
        __lookup_session_by_app(usess, app, &iter);
-       node = lttng_ht_iter_get_node_ulong(&iter);
+       node = lttng_ht_iter_get_node_u64(&iter);
        if (node == NULL) {
-               /* Only malloc can failed so something is really wrong */
-               goto error_rcu_unlock;
+               /* Session is being or is deleted. */
+               goto end;
        }
        ua_sess = caa_container_of(node, struct ust_app_session, node);
-       ret = lttng_ht_del(app->sessions, &iter);
-       assert(!ret);
-       obj.handle = ua_sess->handle;
-       obj.shm_fd = -1;
-       obj.wait_fd = -1;
-       obj.memory_map_size = 0;
-       health_code_update(&health_thread_cmd);
-       ustctl_release_object(app->sock, &obj);
 
-       health_code_update(&health_thread_cmd);
-       delete_ust_app_session(app->sock, ua_sess);
+       health_code_update();
+       destroy_app_session(app, ua_sess);
 
-       /* Quiescent wait after stopping trace */
-       ustctl_wait_quiescent(app->sock);
+       health_code_update();
 
+       /* Quiescent wait after stopping trace */
+       ret = ustctl_wait_quiescent(app->sock);
+       if (ret < 0 && ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
+               ERR("UST app wait quiescent failed for app pid %d ret %d",
+                               app->pid, ret);
+       }
 end:
        rcu_read_unlock();
-       health_code_update(&health_thread_cmd);
+       health_code_update();
        return 0;
-
-error_rcu_unlock:
-       rcu_read_unlock();
-       health_code_update(&health_thread_cmd);
-       return -1;
 }
 
 /*
@@ -2483,6 +3962,59 @@ int ust_app_stop_trace_all(struct ltt_ust_session *usess)
                }
        }
 
+       /* Flush buffers and push metadata (for UID buffers). */
+       switch (usess->buffer_type) {
+       case LTTNG_BUFFER_PER_UID:
+       {
+               struct buffer_reg_uid *reg;
+
+               /* Flush all per UID buffers associated to that session. */
+               cds_list_for_each_entry(reg, &usess->buffer_reg_uid_list, lnode) {
+                       struct ust_registry_session *ust_session_reg;
+                       struct buffer_reg_channel *reg_chan;
+                       struct consumer_socket *socket;
+
+                       /* Get consumer socket to use to push the metadata.*/
+                       socket = consumer_find_socket_by_bitness(reg->bits_per_long,
+                                       usess->consumer);
+                       if (!socket) {
+                               /* Ignore request if no consumer is found for the session. */
+                               continue;
+                       }
+
+                       cds_lfht_for_each_entry(reg->registry->channels->ht, &iter.iter,
+                                       reg_chan, node.node) {
+                               /*
+                                * The following call will print error values so the return
+                                * code is of little importance because whatever happens, we
+                                * have to try them all.
+                                */
+                               (void) consumer_flush_channel(socket, reg_chan->consumer_key);
+                       }
+
+                       ust_session_reg = reg->registry->reg.ust;
+                       if (!ust_session_reg->metadata_closed) {
+                               /* Push metadata. */
+                               (void) push_metadata(ust_session_reg, usess->consumer);
+                       }
+               }
+
+               break;
+       }
+       case LTTNG_BUFFER_PER_PID:
+               cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app, pid_n.node) {
+                       ret = ust_app_flush_trace(usess, app);
+                       if (ret < 0) {
+                               /* Continue to next apps even on error */
+                               continue;
+                       }
+               }
+               break;
+       default:
+               assert(0);
+               break;
+       }
+
        rcu_read_unlock();
 
        return 0;
@@ -2502,7 +4034,7 @@ int ust_app_destroy_trace_all(struct ltt_ust_session *usess)
        rcu_read_lock();
 
        cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app, pid_n.node) {
-               ret = ust_app_destroy_trace(usess, app);
+               ret = destroy_trace(usess, app);
                if (ret < 0) {
                        /* Continue to next apps even on error */
                        continue;
@@ -2522,24 +4054,26 @@ void ust_app_global_update(struct ltt_ust_session *usess, int sock)
        int ret = 0;
        struct lttng_ht_iter iter, uiter, iter_ctx;
        struct ust_app *app;
-       struct ust_app_session *ua_sess;
+       struct ust_app_session *ua_sess = NULL;
        struct ust_app_channel *ua_chan;
        struct ust_app_event *ua_event;
        struct ust_app_ctx *ua_ctx;
 
-       if (usess == NULL) {
-               ERR("No UST session on global update. Returning");
-               goto error;
-       }
+       assert(usess);
+       assert(sock >= 0);
 
-       DBG2("UST app global update for app sock %d for session id %d", sock,
+       DBG2("UST app global update for app sock %d for session id %" PRIu64, sock,
                        usess->id);
 
        rcu_read_lock();
 
        app = find_app_by_sock(sock);
        if (app == NULL) {
-               ERR("Failed to update app sock %d", sock);
+               /*
+                * Application can be unregistered before so this is possible hence
+                * simply stopping the update.
+                */
+               DBG3("UST app update failed to find app sock %d", sock);
                goto error;
        }
 
@@ -2547,30 +4081,54 @@ void ust_app_global_update(struct ltt_ust_session *usess, int sock)
                goto error;
        }
 
-       ua_sess = create_ust_app_session(usess, app);
-       if (ua_sess == NULL) {
+       ret = create_ust_app_session(usess, app, &ua_sess, NULL);
+       if (ret < 0) {
+               /* Tracer is probably gone or ENOMEM. */
                goto error;
        }
+       assert(ua_sess);
+
+       pthread_mutex_lock(&ua_sess->lock);
 
        /*
-        * We can iterate safely here over all UST app session sicne the create ust
+        * We can iterate safely here over all UST app session since the create ust
         * app session above made a shadow copy of the UST global domain from the
         * ltt ust session.
         */
        cds_lfht_for_each_entry(ua_sess->channels->ht, &iter.iter, ua_chan,
                        node.node) {
-               ret = create_ust_channel(app, ua_sess, ua_chan);
-               if (ret < 0) {
-                       /* FIXME: Should we quit here or continue... */
+               /*
+                * For a metadata channel, handle it differently.
+                */
+               if (!strncmp(ua_chan->name, DEFAULT_METADATA_NAME,
+                                       sizeof(ua_chan->name))) {
+                       ret = create_ust_app_metadata(ua_sess, app, usess->consumer,
+                                       &ua_chan->attr);
+                       if (ret < 0) {
+                               goto error_unlock;
+                       }
+                       /* Remove it from the hash table and continue!. */
+                       ret = lttng_ht_del(ua_sess->channels, &iter);
+                       assert(!ret);
+                       delete_ust_app_channel(-1, ua_chan, app);
                        continue;
+               } else {
+                       ret = do_create_channel(app, usess, ua_sess, ua_chan);
+                       if (ret < 0) {
+                               /*
+                                * Stop everything. On error, the application failed, no more
+                                * file descriptor are available or ENOMEM so stopping here is
+                                * the only thing we can do for now.
+                                */
+                               goto error_unlock;
+                       }
                }
 
                cds_lfht_for_each_entry(ua_chan->ctx->ht, &iter_ctx.iter, ua_ctx,
                                node.node) {
                        ret = create_ust_channel_context(ua_chan, ua_ctx, app);
                        if (ret < 0) {
-                               /* FIXME: Should we quit here or continue... */
-                               continue;
+                               goto error_unlock;
                        }
                }
 
@@ -2580,27 +4138,13 @@ void ust_app_global_update(struct ltt_ust_session *usess, int sock)
                                node.node) {
                        ret = create_ust_event(app, ua_sess, ua_chan, ua_event);
                        if (ret < 0) {
-                               /* FIXME: Should we quit here or continue... */
-                               continue;
-                       }
-
-                       /* Add context on events. */
-                       cds_lfht_for_each_entry(ua_event->ctx->ht, &iter_ctx.iter,
-                                       ua_ctx, node.node) {
-                               ret = create_ust_event_context(ua_event, ua_ctx, app);
-                               if (ret < 0) {
-                                       /* FIXME: Should we quit here or continue... */
-                                       continue;
-                               }
-                       }
-                       ret = set_ust_event_filter(ua_event, app);
-                       if (ret < 0) {
-                               /* FIXME: Should we quit here or continue... */
-                               continue;
+                               goto error_unlock;
                        }
                }
        }
 
+       pthread_mutex_unlock(&ua_sess->lock);
+
        if (usess->start_trace) {
                ret = ust_app_start_trace(usess, app);
                if (ret < 0) {
@@ -2610,7 +4154,16 @@ void ust_app_global_update(struct ltt_ust_session *usess, int sock)
                DBG2("UST trace started for app pid %d", app->pid);
        }
 
+       /* Everything went well at this point. */
+       rcu_read_unlock();
+       return;
+
+error_unlock:
+       pthread_mutex_unlock(&ua_sess->lock);
 error:
+       if (ua_sess) {
+               destroy_app_session(app, ua_sess);
+       }
        rcu_read_unlock();
        return;
 }
@@ -2643,19 +4196,21 @@ int ust_app_add_ctx_channel_glb(struct ltt_ust_session *usess,
                        continue;
                }
 
+               pthread_mutex_lock(&ua_sess->lock);
                /* Lookup channel in the ust app session */
                lttng_ht_lookup(ua_sess->channels, (void *)uchan->name, &uiter);
                ua_chan_node = lttng_ht_iter_get_node_str(&uiter);
                if (ua_chan_node == NULL) {
-                       continue;
+                       goto next_app;
                }
                ua_chan = caa_container_of(ua_chan_node, struct ust_app_channel,
                                node);
-
                ret = create_ust_app_channel_context(ua_sess, ua_chan, &uctx->ctx, app);
                if (ret < 0) {
-                       continue;
+                       goto next_app;
                }
+       next_app:
+               pthread_mutex_unlock(&ua_sess->lock);
        }
 
        rcu_read_unlock();
@@ -2663,75 +4218,142 @@ int ust_app_add_ctx_channel_glb(struct ltt_ust_session *usess,
 }
 
 /*
- * Add context to a specific event in a channel for global UST domain.
+ * Enable event for a channel from a UST session for a specific PID.
  */
-int ust_app_add_ctx_event_glb(struct ltt_ust_session *usess,
-               struct ltt_ust_channel *uchan, struct ltt_ust_event *uevent,
-               struct ltt_ust_context *uctx)
+int ust_app_enable_event_pid(struct ltt_ust_session *usess,
+               struct ltt_ust_channel *uchan, struct ltt_ust_event *uevent, pid_t pid)
 {
        int ret = 0;
-       struct lttng_ht_node_str *ua_chan_node, *ua_event_node;
-       struct lttng_ht_iter iter, uiter;
+       struct lttng_ht_iter iter;
+       struct lttng_ht_node_str *ua_chan_node;
+       struct ust_app *app;
        struct ust_app_session *ua_sess;
+       struct ust_app_channel *ua_chan;
        struct ust_app_event *ua_event;
-       struct ust_app_channel *ua_chan = NULL;
-       struct ust_app *app;
+
+       DBG("UST app enabling event %s for PID %d", uevent->attr.name, pid);
 
        rcu_read_lock();
 
-       cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app, pid_n.node) {
-               if (!app->compatible) {
-                       /*
-                        * TODO: In time, we should notice the caller of this error by
-                        * telling him that this is a version error.
-                        */
-                       continue;
-               }
-               ua_sess = lookup_session_by_app(usess, app);
-               if (ua_sess == NULL) {
-                       continue;
-               }
+       app = ust_app_find_by_pid(pid);
+       if (app == NULL) {
+               ERR("UST app enable event per PID %d not found", pid);
+               ret = -1;
+               goto end;
+       }
 
-               /* Lookup channel in the ust app session */
-               lttng_ht_lookup(ua_sess->channels, (void *)uchan->name, &uiter);
-               ua_chan_node = lttng_ht_iter_get_node_str(&uiter);
-               if (ua_chan_node == NULL) {
-                       continue;
-               }
-               ua_chan = caa_container_of(ua_chan_node, struct ust_app_channel,
-                               node);
+       if (!app->compatible) {
+               ret = 0;
+               goto end;
+       }
 
-               lttng_ht_lookup(ua_chan->events, (void *)uevent->attr.name, &uiter);
-               ua_event_node = lttng_ht_iter_get_node_str(&uiter);
-               if (ua_event_node == NULL) {
-                       continue;
-               }
-               ua_event = caa_container_of(ua_event_node, struct ust_app_event,
-                               node);
+       ua_sess = lookup_session_by_app(usess, app);
+       if (!ua_sess) {
+               /* The application has problem or is probably dead. */
+               ret = 0;
+               goto end;
+       }
+
+       pthread_mutex_lock(&ua_sess->lock);
+       /* Lookup channel in the ust app session */
+       lttng_ht_lookup(ua_sess->channels, (void *)uchan->name, &iter);
+       ua_chan_node = lttng_ht_iter_get_node_str(&iter);
+       /* If the channel is not found, there is a code flow error */
+       assert(ua_chan_node);
+
+       ua_chan = caa_container_of(ua_chan_node, struct ust_app_channel, node);
 
-               ret = create_ust_app_event_context(ua_sess, ua_event, &uctx->ctx, app);
+       ua_event = find_ust_app_event(ua_chan->events, uevent->attr.name,
+                       uevent->filter, uevent->attr.loglevel);
+       if (ua_event == NULL) {
+               ret = create_ust_app_event(ua_sess, ua_chan, uevent, app);
                if (ret < 0) {
-                       continue;
+                       goto end_unlock;
+               }
+       } else {
+               ret = enable_ust_app_event(ua_sess, ua_event, app);
+               if (ret < 0) {
+                       goto end_unlock;
                }
        }
 
+end_unlock:
+       pthread_mutex_unlock(&ua_sess->lock);
+end:
        rcu_read_unlock();
        return ret;
 }
 
 /*
- * Add context to a specific event in a channel for global UST domain.
+ * Disable event for a channel from a UST session for a specific PID.
  */
-int ust_app_set_filter_event_glb(struct ltt_ust_session *usess,
-               struct ltt_ust_channel *uchan, struct ltt_ust_event *uevent,
-               struct lttng_filter_bytecode *bytecode)
+int ust_app_disable_event_pid(struct ltt_ust_session *usess,
+               struct ltt_ust_channel *uchan, struct ltt_ust_event *uevent, pid_t pid)
 {
        int ret = 0;
+       struct lttng_ht_iter iter;
        struct lttng_ht_node_str *ua_chan_node, *ua_event_node;
-       struct lttng_ht_iter iter, uiter;
+       struct ust_app *app;
        struct ust_app_session *ua_sess;
+       struct ust_app_channel *ua_chan;
        struct ust_app_event *ua_event;
-       struct ust_app_channel *ua_chan = NULL;
+
+       DBG("UST app disabling event %s for PID %d", uevent->attr.name, pid);
+
+       rcu_read_lock();
+
+       app = ust_app_find_by_pid(pid);
+       if (app == NULL) {
+               ERR("UST app disable event per PID %d not found", pid);
+               ret = -1;
+               goto error;
+       }
+
+       if (!app->compatible) {
+               ret = 0;
+               goto error;
+       }
+
+       ua_sess = lookup_session_by_app(usess, app);
+       if (!ua_sess) {
+               /* The application has problem or is probably dead. */
+               goto error;
+       }
+
+       /* Lookup channel in the ust app session */
+       lttng_ht_lookup(ua_sess->channels, (void *)uchan->name, &iter);
+       ua_chan_node = lttng_ht_iter_get_node_str(&iter);
+       if (ua_chan_node == NULL) {
+               /* Channel does not exist, skip disabling */
+               goto error;
+       }
+       ua_chan = caa_container_of(ua_chan_node, struct ust_app_channel, node);
+
+       lttng_ht_lookup(ua_chan->events, (void *)uevent->attr.name, &iter);
+       ua_event_node = lttng_ht_iter_get_node_str(&iter);
+       if (ua_event_node == NULL) {
+               /* Event does not exist, skip disabling */
+               goto error;
+       }
+       ua_event = caa_container_of(ua_event_node, struct ust_app_event, node);
+
+       ret = disable_ust_app_event(ua_sess, ua_event, app);
+       if (ret < 0) {
+               goto error;
+       }
+
+error:
+       rcu_read_unlock();
+       return ret;
+}
+
+/*
+ * Calibrate registered applications.
+ */
+int ust_app_calibrate_glb(struct lttng_ust_calibrate *calibrate)
+{
+       int ret = 0;
+       struct lttng_ht_iter iter;
        struct ust_app *app;
 
        rcu_read_lock();
@@ -2744,248 +4366,729 @@ int ust_app_set_filter_event_glb(struct ltt_ust_session *usess,
                         */
                        continue;
                }
-               ua_sess = lookup_session_by_app(usess, app);
-               if (ua_sess == NULL) {
-                       continue;
+
+               health_code_update();
+
+               ret = ustctl_calibrate(app->sock, calibrate);
+               if (ret < 0) {
+                       switch (ret) {
+                       case -ENOSYS:
+                               /* Means that it's not implemented on the tracer side. */
+                               ret = 0;
+                               break;
+                       default:
+                               DBG2("Calibrate app PID %d returned with error %d",
+                                               app->pid, ret);
+                               break;
+                       }
                }
+       }
 
-               /* Lookup channel in the ust app session */
-               lttng_ht_lookup(ua_sess->channels, (void *)uchan->name, &uiter);
-               ua_chan_node = lttng_ht_iter_get_node_str(&uiter);
-               if (ua_chan_node == NULL) {
-                       continue;
+       DBG("UST app global domain calibration finished");
+
+       rcu_read_unlock();
+
+       health_code_update();
+
+       return ret;
+}
+
+/*
+ * Receive registration and populate the given msg structure.
+ *
+ * On success return 0 else a negative value returned by the ustctl call.
+ */
+int ust_app_recv_registration(int sock, struct ust_register_msg *msg)
+{
+       int ret;
+       uint32_t pid, ppid, uid, gid;
+
+       assert(msg);
+
+       ret = ustctl_recv_reg_msg(sock, &msg->type, &msg->major, &msg->minor,
+                       &pid, &ppid, &uid, &gid,
+                       &msg->bits_per_long,
+                       &msg->uint8_t_alignment,
+                       &msg->uint16_t_alignment,
+                       &msg->uint32_t_alignment,
+                       &msg->uint64_t_alignment,
+                       &msg->long_alignment,
+                       &msg->byte_order,
+                       msg->name);
+       if (ret < 0) {
+               switch (-ret) {
+               case EPIPE:
+               case ECONNRESET:
+               case LTTNG_UST_ERR_EXITING:
+                       DBG3("UST app recv reg message failed. Application died");
+                       break;
+               case LTTNG_UST_ERR_UNSUP_MAJOR:
+                       ERR("UST app recv reg unsupported version %d.%d. Supporting %d.%d",
+                                       msg->major, msg->minor, LTTNG_UST_ABI_MAJOR_VERSION,
+                                       LTTNG_UST_ABI_MINOR_VERSION);
+                       break;
+               default:
+                       ERR("UST app recv reg message failed with ret %d", ret);
+                       break;
                }
-               ua_chan = caa_container_of(ua_chan_node, struct ust_app_channel,
-                               node);
+               goto error;
+       }
+       msg->pid = (pid_t) pid;
+       msg->ppid = (pid_t) ppid;
+       msg->uid = (uid_t) uid;
+       msg->gid = (gid_t) gid;
 
-               lttng_ht_lookup(ua_chan->events, (void *)uevent->attr.name, &uiter);
-               ua_event_node = lttng_ht_iter_get_node_str(&uiter);
-               if (ua_event_node == NULL) {
-                       continue;
+error:
+       return ret;
+}
+
+/*
+ * Return a ust app channel object using the application object and the channel
+ * object descriptor has a key. If not found, NULL is returned. A RCU read side
+ * lock MUST be acquired before calling this function.
+ */
+static struct ust_app_channel *find_channel_by_objd(struct ust_app *app,
+               int objd)
+{
+       struct lttng_ht_node_ulong *node;
+       struct lttng_ht_iter iter;
+       struct ust_app_channel *ua_chan = NULL;
+
+       assert(app);
+
+       lttng_ht_lookup(app->ust_objd, (void *)((unsigned long) objd), &iter);
+       node = lttng_ht_iter_get_node_ulong(&iter);
+       if (node == NULL) {
+               DBG2("UST app channel find by objd %d not found", objd);
+               goto error;
+       }
+
+       ua_chan = caa_container_of(node, struct ust_app_channel, ust_objd_node);
+
+error:
+       return ua_chan;
+}
+
+/*
+ * Reply to a register channel notification from an application on the notify
+ * socket. The channel metadata is also created.
+ *
+ * The session UST registry lock is acquired in this function.
+ *
+ * On success 0 is returned else a negative value.
+ */
+static int reply_ust_register_channel(int sock, int sobjd, int cobjd,
+               size_t nr_fields, struct ustctl_field *fields)
+{
+       int ret, ret_code = 0;
+       uint32_t chan_id, reg_count;
+       uint64_t chan_reg_key;
+       enum ustctl_channel_header type;
+       struct ust_app *app;
+       struct ust_app_channel *ua_chan;
+       struct ust_app_session *ua_sess;
+       struct ust_registry_session *registry;
+       struct ust_registry_channel *chan_reg;
+
+       rcu_read_lock();
+
+       /* Lookup application. If not found, there is a code flow error. */
+       app = find_app_by_notify_sock(sock);
+       if (!app) {
+               DBG("Application socket %d is being teardown. Abort event notify",
+                               sock);
+               ret = 0;
+               free(fields);
+               goto error_rcu_unlock;
+       }
+
+       /* Lookup channel by UST object descriptor. */
+       ua_chan = find_channel_by_objd(app, cobjd);
+       if (!ua_chan) {
+               DBG("Application channel is being teardown. Abort event notify");
+               ret = 0;
+               free(fields);
+               goto error_rcu_unlock;
+       }
+
+       assert(ua_chan->session);
+       ua_sess = ua_chan->session;
+
+       /* Get right session registry depending on the session buffer type. */
+       registry = get_session_registry(ua_sess);
+       assert(registry);
+
+       /* Depending on the buffer type, a different channel key is used. */
+       if (ua_sess->buffer_type == LTTNG_BUFFER_PER_UID) {
+               chan_reg_key = ua_chan->tracing_channel_id;
+       } else {
+               chan_reg_key = ua_chan->key;
+       }
+
+       pthread_mutex_lock(&registry->lock);
+
+       chan_reg = ust_registry_channel_find(registry, chan_reg_key);
+       assert(chan_reg);
+
+       if (!chan_reg->register_done) {
+               reg_count = ust_registry_get_event_count(chan_reg);
+               if (reg_count < 31) {
+                       type = USTCTL_CHANNEL_HEADER_COMPACT;
+               } else {
+                       type = USTCTL_CHANNEL_HEADER_LARGE;
                }
-               ua_event = caa_container_of(ua_event_node, struct ust_app_event,
-                               node);
 
-               ret = set_ust_app_event_filter(ua_sess, ua_event, bytecode, app);
-               if (ret < 0) {
-                       continue;
+               chan_reg->nr_ctx_fields = nr_fields;
+               chan_reg->ctx_fields = fields;
+               chan_reg->header_type = type;
+       } else {
+               /* Get current already assigned values. */
+               type = chan_reg->header_type;
+               free(fields);
+               /* Set to NULL so the error path does not do a double free. */
+               fields = NULL;
+       }
+       /* Channel id is set during the object creation. */
+       chan_id = chan_reg->chan_id;
+
+       /* Append to metadata */
+       if (!chan_reg->metadata_dumped) {
+               ret_code = ust_metadata_channel_statedump(registry, chan_reg);
+               if (ret_code) {
+                       ERR("Error appending channel metadata (errno = %d)", ret_code);
+                       goto reply;
+               }
+       }
+
+reply:
+       DBG3("UST app replying to register channel key %" PRIu64
+                       " with id %u, type: %d, ret: %d", chan_reg_key, chan_id, type,
+                       ret_code);
+
+       ret = ustctl_reply_register_channel(sock, chan_id, type, ret_code);
+       if (ret < 0) {
+               if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
+                       ERR("UST app reply channel failed with ret %d", ret);
+               } else {
+                       DBG3("UST app reply channel failed. Application died");
                }
+               goto error;
        }
 
+       /* This channel registry registration is completed. */
+       chan_reg->register_done = 1;
+
+error:
+       pthread_mutex_unlock(&registry->lock);
+error_rcu_unlock:
        rcu_read_unlock();
+       if (ret) {
+               free(fields);
+       }
        return ret;
 }
 
 /*
- * Enable event for a channel from a UST session for a specific PID.
+ * Add event to the UST channel registry. When the event is added to the
+ * registry, the metadata is also created. Once done, this replies to the
+ * application with the appropriate error code.
+ *
+ * The session UST registry lock is acquired in the function.
+ *
+ * On success 0 is returned else a negative value.
  */
-int ust_app_enable_event_pid(struct ltt_ust_session *usess,
-               struct ltt_ust_channel *uchan, struct ltt_ust_event *uevent, pid_t pid)
+static int add_event_ust_registry(int sock, int sobjd, int cobjd, char *name,
+               char *sig, size_t nr_fields, struct ustctl_field *fields, int loglevel,
+               char *model_emf_uri)
 {
-       int ret = 0;
-       struct lttng_ht_iter iter;
-       struct lttng_ht_node_str *ua_chan_node, *ua_event_node;
+       int ret, ret_code;
+       uint32_t event_id = 0;
+       uint64_t chan_reg_key;
        struct ust_app *app;
-       struct ust_app_session *ua_sess;
        struct ust_app_channel *ua_chan;
-       struct ust_app_event *ua_event;
-
-       DBG("UST app enabling event %s for PID %d", uevent->attr.name, pid);
+       struct ust_app_session *ua_sess;
+       struct ust_registry_session *registry;
 
        rcu_read_lock();
 
-       app = ust_app_find_by_pid(pid);
-       if (app == NULL) {
-               ERR("UST app enable event per PID %d not found", pid);
-               ret = -1;
-               goto error;
+       /* Lookup application. If not found, there is a code flow error. */
+       app = find_app_by_notify_sock(sock);
+       if (!app) {
+               DBG("Application socket %d is being teardown. Abort event notify",
+                               sock);
+               ret = 0;
+               free(sig);
+               free(fields);
+               free(model_emf_uri);
+               goto error_rcu_unlock;
        }
 
-       if (!app->compatible) {
+       /* Lookup channel by UST object descriptor. */
+       ua_chan = find_channel_by_objd(app, cobjd);
+       if (!ua_chan) {
+               DBG("Application channel is being teardown. Abort event notify");
                ret = 0;
+               free(sig);
+               free(fields);
+               free(model_emf_uri);
+               goto error_rcu_unlock;
+       }
+
+       assert(ua_chan->session);
+       ua_sess = ua_chan->session;
+
+       registry = get_session_registry(ua_sess);
+       assert(registry);
+
+       if (ua_sess->buffer_type == LTTNG_BUFFER_PER_UID) {
+               chan_reg_key = ua_chan->tracing_channel_id;
+       } else {
+               chan_reg_key = ua_chan->key;
+       }
+
+       pthread_mutex_lock(&registry->lock);
+
+       /*
+        * From this point on, this call acquires the ownership of the sig, fields
+        * and model_emf_uri meaning any free are done inside it if needed. These
+        * three variables MUST NOT be read/write after this.
+        */
+       ret_code = ust_registry_create_event(registry, chan_reg_key,
+                       sobjd, cobjd, name, sig, nr_fields, fields, loglevel,
+                       model_emf_uri, ua_sess->buffer_type, &event_id);
+
+       /*
+        * The return value is returned to ustctl so in case of an error, the
+        * application can be notified. In case of an error, it's important not to
+        * return a negative error or else the application will get closed.
+        */
+       ret = ustctl_reply_register_event(sock, event_id, ret_code);
+       if (ret < 0) {
+               if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
+                       ERR("UST app reply event failed with ret %d", ret);
+               } else {
+                       DBG3("UST app reply event failed. Application died");
+               }
+               /*
+                * No need to wipe the create event since the application socket will
+                * get close on error hence cleaning up everything by itself.
+                */
                goto error;
        }
 
-       ua_sess = lookup_session_by_app(usess, app);
-       /* If ua_sess is NULL, there is a code flow error */
-       assert(ua_sess);
+       DBG3("UST registry event %s with id %" PRId32 " added successfully",
+                       name, event_id);
 
-       /* Lookup channel in the ust app session */
-       lttng_ht_lookup(ua_sess->channels, (void *)uchan->name, &iter);
-       ua_chan_node = lttng_ht_iter_get_node_str(&iter);
-       /* If the channel is not found, there is a code flow error */
-       assert(ua_chan_node);
+error:
+       pthread_mutex_unlock(&registry->lock);
+error_rcu_unlock:
+       rcu_read_unlock();
+       return ret;
+}
 
-       ua_chan = caa_container_of(ua_chan_node, struct ust_app_channel, node);
+/*
+ * Handle application notification through the given notify socket.
+ *
+ * Return 0 on success or else a negative value.
+ */
+int ust_app_recv_notify(int sock)
+{
+       int ret;
+       enum ustctl_notify_cmd cmd;
 
-       lttng_ht_lookup(ua_chan->events, (void *)uevent->attr.name, &iter);
-       ua_event_node = lttng_ht_iter_get_node_str(&iter);
-       if (ua_event_node == NULL) {
-               ret = create_ust_app_event(ua_sess, ua_chan, uevent, app);
+       DBG3("UST app receiving notify from sock %d", sock);
+
+       ret = ustctl_recv_notify(sock, &cmd);
+       if (ret < 0) {
+               if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
+                       ERR("UST app recv notify failed with ret %d", ret);
+               } else {
+                       DBG3("UST app recv notify failed. Application died");
+               }
+               goto error;
+       }
+
+       switch (cmd) {
+       case USTCTL_NOTIFY_CMD_EVENT:
+       {
+               int sobjd, cobjd, loglevel;
+               char name[LTTNG_UST_SYM_NAME_LEN], *sig, *model_emf_uri;
+               size_t nr_fields;
+               struct ustctl_field *fields;
+
+               DBG2("UST app ustctl register event received");
+
+               ret = ustctl_recv_register_event(sock, &sobjd, &cobjd, name, &loglevel,
+                               &sig, &nr_fields, &fields, &model_emf_uri);
                if (ret < 0) {
+                       if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
+                               ERR("UST app recv event failed with ret %d", ret);
+                       } else {
+                               DBG3("UST app recv event failed. Application died");
+                       }
                        goto error;
                }
-       } else {
-               ua_event = caa_container_of(ua_event_node, struct ust_app_event, node);
 
-               ret = enable_ust_app_event(ua_sess, ua_event, app);
+               /*
+                * Add event to the UST registry coming from the notify socket. This
+                * call will free if needed the sig, fields and model_emf_uri. This
+                * code path loses the ownsership of these variables and transfer them
+                * to the this function.
+                */
+               ret = add_event_ust_registry(sock, sobjd, cobjd, name, sig, nr_fields,
+                               fields, loglevel, model_emf_uri);
+               if (ret < 0) {
+                       goto error;
+               }
+
+               break;
+       }
+       case USTCTL_NOTIFY_CMD_CHANNEL:
+       {
+               int sobjd, cobjd;
+               size_t nr_fields;
+               struct ustctl_field *fields;
+
+               DBG2("UST app ustctl register channel received");
+
+               ret = ustctl_recv_register_channel(sock, &sobjd, &cobjd, &nr_fields,
+                               &fields);
+               if (ret < 0) {
+                       if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
+                               ERR("UST app recv channel failed with ret %d", ret);
+                       } else {
+                               DBG3("UST app recv channel failed. Application died");
+                       }
+                       goto error;
+               }
+
+               /*
+                * The fields ownership are transfered to this function call meaning
+                * that if needed it will be freed. After this, it's invalid to access
+                * fields or clean it up.
+                */
+               ret = reply_ust_register_channel(sock, sobjd, cobjd, nr_fields,
+                               fields);
                if (ret < 0) {
                        goto error;
                }
+
+               break;
+       }
+       default:
+               /* Should NEVER happen. */
+               assert(0);
        }
 
 error:
-       rcu_read_unlock();
        return ret;
 }
 
 /*
- * Disable event for a channel from a UST session for a specific PID.
+ * Once the notify socket hangs up, this is called. First, it tries to find the
+ * corresponding application. On failure, the call_rcu to close the socket is
+ * executed. If an application is found, it tries to delete it from the notify
+ * socket hash table. Whathever the result, it proceeds to the call_rcu.
+ *
+ * Note that an object needs to be allocated here so on ENOMEM failure, the
+ * call RCU is not done but the rest of the cleanup is.
  */
-int ust_app_disable_event_pid(struct ltt_ust_session *usess,
-               struct ltt_ust_channel *uchan, struct ltt_ust_event *uevent, pid_t pid)
+void ust_app_notify_sock_unregister(int sock)
 {
-       int ret = 0;
+       int err_enomem = 0;
        struct lttng_ht_iter iter;
-       struct lttng_ht_node_str *ua_chan_node, *ua_event_node;
        struct ust_app *app;
-       struct ust_app_session *ua_sess;
-       struct ust_app_channel *ua_chan;
-       struct ust_app_event *ua_event;
+       struct ust_app_notify_sock_obj *obj;
 
-       DBG("UST app disabling event %s for PID %d", uevent->attr.name, pid);
+       assert(sock >= 0);
 
        rcu_read_lock();
 
-       app = ust_app_find_by_pid(pid);
-       if (app == NULL) {
-               ERR("UST app disable event per PID %d not found", pid);
-               ret = -1;
-               goto error;
+       obj = zmalloc(sizeof(*obj));
+       if (!obj) {
+               /*
+                * An ENOMEM is kind of uncool. If this strikes we continue the
+                * procedure but the call_rcu will not be called. In this case, we
+                * accept the fd leak rather than possibly creating an unsynchronized
+                * state between threads.
+                *
+                * TODO: The notify object should be created once the notify socket is
+                * registered and stored independantely from the ust app object. The
+                * tricky part is to synchronize the teardown of the application and
+                * this notify object. Let's keep that in mind so we can avoid this
+                * kind of shenanigans with ENOMEM in the teardown path.
+                */
+               err_enomem = 1;
+       } else {
+               obj->fd = sock;
        }
 
-       if (!app->compatible) {
-               ret = 0;
-               goto error;
+       DBG("UST app notify socket unregister %d", sock);
+
+       /*
+        * Lookup application by notify socket. If this fails, this means that the
+        * hash table delete has already been done by the application
+        * unregistration process so we can safely close the notify socket in a
+        * call RCU.
+        */
+       app = find_app_by_notify_sock(sock);
+       if (!app) {
+               goto close_socket;
        }
 
-       ua_sess = lookup_session_by_app(usess, app);
-       /* If ua_sess is NULL, there is a code flow error */
-       assert(ua_sess);
+       iter.iter.node = &app->notify_sock_n.node;
 
-       /* Lookup channel in the ust app session */
-       lttng_ht_lookup(ua_sess->channels, (void *)uchan->name, &iter);
-       ua_chan_node = lttng_ht_iter_get_node_str(&iter);
-       if (ua_chan_node == NULL) {
-               /* Channel does not exist, skip disabling */
-               goto error;
-       }
-       ua_chan = caa_container_of(ua_chan_node, struct ust_app_channel, node);
+       /*
+        * Whatever happens here either we fail or succeed, in both cases we have
+        * to close the socket after a grace period to continue to the call RCU
+        * here. If the deletion is successful, the application is not visible
+        * anymore by other threads and is it fails it means that it was already
+        * deleted from the hash table so either way we just have to close the
+        * socket.
+        */
+       (void) lttng_ht_del(ust_app_ht_by_notify_sock, &iter);
 
-       lttng_ht_lookup(ua_chan->events, (void *)uevent->attr.name, &iter);
-       ua_event_node = lttng_ht_iter_get_node_str(&iter);
-       if (ua_event_node == NULL) {
-               /* Event does not exist, skip disabling */
-               goto error;
+close_socket:
+       rcu_read_unlock();
+
+       /*
+        * Close socket after a grace period to avoid for the socket to be reused
+        * before the application object is freed creating potential race between
+        * threads trying to add unique in the global hash table.
+        */
+       if (!err_enomem) {
+               call_rcu(&obj->head, close_notify_sock_rcu);
        }
-       ua_event = caa_container_of(ua_event_node, struct ust_app_event, node);
+}
 
-       ret = disable_ust_app_event(ua_sess, ua_event, app);
-       if (ret < 0) {
-               goto error;
+/*
+ * Destroy a ust app data structure and free its memory.
+ */
+void ust_app_destroy(struct ust_app *app)
+{
+       if (!app) {
+               return;
        }
 
-error:
-       rcu_read_unlock();
-       return ret;
+       call_rcu(&app->pid_n.head, delete_ust_app_rcu);
 }
 
 /*
- * Validate version of UST apps and set the compatible bit.
+ * Take a snapshot for a given UST session. The snapshot is sent to the given
+ * output.
+ *
+ * Return 0 on success or else a negative value.
  */
-int ust_app_validate_version(int sock)
+int ust_app_snapshot_record(struct ltt_ust_session *usess,
+               struct snapshot_output *output, int wait, unsigned int nb_streams)
 {
-       int ret;
+       int ret = 0;
+       struct lttng_ht_iter iter;
        struct ust_app *app;
+       char pathname[PATH_MAX];
+       uint64_t max_stream_size = 0;
+
+       assert(usess);
+       assert(output);
 
        rcu_read_lock();
 
-       app = find_app_by_sock(sock);
-       assert(app);
+       /*
+        * Compute the maximum size of a single stream if a max size is asked by
+        * the caller.
+        */
+       if (output->max_size > 0 && nb_streams > 0) {
+               max_stream_size = output->max_size / nb_streams;
+       }
 
-       health_code_update(&health_thread_cmd);
+       switch (usess->buffer_type) {
+       case LTTNG_BUFFER_PER_UID:
+       {
+               struct buffer_reg_uid *reg;
+
+               cds_list_for_each_entry(reg, &usess->buffer_reg_uid_list, lnode) {
+                       struct buffer_reg_channel *reg_chan;
+                       struct consumer_socket *socket;
+
+                       /* Get consumer socket to use to push the metadata.*/
+                       socket = consumer_find_socket_by_bitness(reg->bits_per_long,
+                                       usess->consumer);
+                       if (!socket) {
+                               ret = -EINVAL;
+                               goto error;
+                       }
 
-       ret = ustctl_tracer_version(sock, &app->version);
-       if (ret < 0) {
-               goto error;
-       }
+                       memset(pathname, 0, sizeof(pathname));
+                       ret = snprintf(pathname, sizeof(pathname),
+                                       DEFAULT_UST_TRACE_DIR "/" DEFAULT_UST_TRACE_UID_PATH,
+                                       reg->uid, reg->bits_per_long);
+                       if (ret < 0) {
+                               PERROR("snprintf snapshot path");
+                               goto error;
+                       }
 
-       /* Validate version */
-       if (app->version.major != UST_APP_MAJOR_VERSION) {
-               goto error;
+                       /* Add the UST default trace dir to path. */
+                       cds_lfht_for_each_entry(reg->registry->channels->ht, &iter.iter,
+                                       reg_chan, node.node) {
+
+                               /*
+                                * Make sure the maximum stream size is not lower than the
+                                * subbuffer size or else it's an error since we won't be able to
+                                * snapshot anything.
+                                */
+                               if (max_stream_size &&
+                                               reg_chan->subbuf_size > max_stream_size) {
+                                       ret = -EINVAL;
+                                       DBG3("UST app snapshot record maximum stream size %" PRIu64
+                                                       " is smaller than subbuffer size of %zu",
+                                                       max_stream_size, reg_chan->subbuf_size);
+                                       goto error;
+                               }
+                               ret = consumer_snapshot_channel(socket, reg_chan->consumer_key, output, 0,
+                                               usess->uid, usess->gid, pathname, wait,
+                                               max_stream_size);
+                               if (ret < 0) {
+                                       goto error;
+                               }
+                       }
+                       ret = consumer_snapshot_channel(socket, reg->registry->reg.ust->metadata_key, output,
+                                       1, usess->uid, usess->gid, pathname, wait,
+                                       max_stream_size);
+                       if (ret < 0) {
+                               goto error;
+                       }
+               }
+               break;
        }
+       case LTTNG_BUFFER_PER_PID:
+       {
+               cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app, pid_n.node) {
+                       struct consumer_socket *socket;
+                       struct lttng_ht_iter chan_iter;
+                       struct ust_app_channel *ua_chan;
+                       struct ust_app_session *ua_sess;
+                       struct ust_registry_session *registry;
+
+                       ua_sess = lookup_session_by_app(usess, app);
+                       if (!ua_sess) {
+                               /* Session not associated with this app. */
+                               continue;
+                       }
 
-       DBG2("UST app PID %d is compatible with major version %d "
-                       "(supporting == %d)", app->pid, app->version.major,
-                       UST_APP_MAJOR_VERSION);
-       app->compatible = 1;
-       rcu_read_unlock();
-       health_code_update(&health_thread_cmd);
-       return 0;
+                       /* Get the right consumer socket for the application. */
+                       socket = consumer_find_socket_by_bitness(app->bits_per_long,
+                                       output->consumer);
+                       if (!socket) {
+                               ret = -EINVAL;
+                               goto error;
+                       }
+
+                       /* Add the UST default trace dir to path. */
+                       memset(pathname, 0, sizeof(pathname));
+                       ret = snprintf(pathname, sizeof(pathname), DEFAULT_UST_TRACE_DIR "/%s",
+                                       ua_sess->path);
+                       if (ret < 0) {
+                               PERROR("snprintf snapshot path");
+                               goto error;
+                       }
+
+                       cds_lfht_for_each_entry(ua_sess->channels->ht, &chan_iter.iter,
+                                       ua_chan, node.node) {
+                               /*
+                                * Make sure the maximum stream size is not lower than the
+                                * subbuffer size or else it's an error since we won't be able to
+                                * snapshot anything.
+                                */
+                               if (max_stream_size &&
+                                               ua_chan->attr.subbuf_size > max_stream_size) {
+                                       ret = -EINVAL;
+                                       DBG3("UST app snapshot record maximum stream size %" PRIu64
+                                                       " is smaller than subbuffer size of %" PRIu64,
+                                                       max_stream_size, ua_chan->attr.subbuf_size);
+                                       goto error;
+                               }
+
+                               ret = consumer_snapshot_channel(socket, ua_chan->key, output, 0,
+                                               ua_sess->euid, ua_sess->egid, pathname, wait,
+                                               max_stream_size);
+                               if (ret < 0) {
+                                       goto error;
+                               }
+                       }
+
+                       registry = get_session_registry(ua_sess);
+                       assert(registry);
+                       ret = consumer_snapshot_channel(socket, registry->metadata_key, output,
+                                       1, ua_sess->euid, ua_sess->egid, pathname, wait,
+                                       max_stream_size);
+                       if (ret < 0) {
+                               goto error;
+                       }
+               }
+               break;
+       }
+       default:
+               assert(0);
+               break;
+       }
 
 error:
-       DBG2("UST app PID %d is not compatible with major version %d "
-                       "(supporting == %d)", app->pid, app->version.major,
-                       UST_APP_MAJOR_VERSION);
-       app->compatible = 0;
        rcu_read_unlock();
-       health_code_update(&health_thread_cmd);
-       return -1;
+       return ret;
 }
 
 /*
- * Calibrate registered applications.
+ * Return the number of streams for a UST session.
  */
-int ust_app_calibrate_glb(struct lttng_ust_calibrate *calibrate)
+unsigned int ust_app_get_nb_stream(struct ltt_ust_session *usess)
 {
-       int ret = 0;
-       struct lttng_ht_iter iter;
+       unsigned int ret = 0;
        struct ust_app *app;
+       struct lttng_ht_iter iter;
 
-       rcu_read_lock();
+       assert(usess);
 
-       cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app, pid_n.node) {
-               if (!app->compatible) {
-                       /*
-                        * TODO: In time, we should notice the caller of this error by
-                        * telling him that this is a version error.
-                        */
-                       continue;
-               }
+       switch (usess->buffer_type) {
+       case LTTNG_BUFFER_PER_UID:
+       {
+               struct buffer_reg_uid *reg;
 
-               health_code_update(&health_thread_cmd);
+               cds_list_for_each_entry(reg, &usess->buffer_reg_uid_list, lnode) {
+                       struct buffer_reg_channel *reg_chan;
 
-               ret = ustctl_calibrate(app->sock, calibrate);
-               if (ret < 0) {
-                       switch (ret) {
-                       case -ENOSYS:
-                               /* Means that it's not implemented on the tracer side. */
-                               ret = 0;
-                               break;
-                       default:
-                               /* TODO: Report error to user */
-                               DBG2("Calibrate app PID %d returned with error %d",
-                                               app->pid, ret);
-                               break;
+                       cds_lfht_for_each_entry(reg->registry->channels->ht, &iter.iter,
+                                       reg_chan, node.node) {
+                               ret += reg_chan->stream_count;
                        }
                }
+               break;
        }
+       case LTTNG_BUFFER_PER_PID:
+       {
+               rcu_read_lock();
+               cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app, pid_n.node) {
+                       struct ust_app_channel *ua_chan;
+                       struct ust_app_session *ua_sess;
+                       struct lttng_ht_iter chan_iter;
+
+                       ua_sess = lookup_session_by_app(usess, app);
+                       if (!ua_sess) {
+                               /* Session not associated with this app. */
+                               continue;
+                       }
 
-       DBG("UST app global domain calibration finished");
-
-       rcu_read_unlock();
-
-       health_code_update(&health_thread_cmd);
+                       cds_lfht_for_each_entry(ua_sess->channels->ht, &chan_iter.iter,
+                                       ua_chan, node.node) {
+                               ret += ua_chan->streams.count;
+                       }
+               }
+               rcu_read_unlock();
+               break;
+       }
+       default:
+               assert(0);
+               break;
+       }
 
        return ret;
 }
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