Merge remote-tracking branch 'cbab-github/tests-cleanup' into cbab
[lttng-tools.git] / src / bin / lttng-sessiond / ust-app.c
index c93f93dc9add8333c07470688ae95f14a8f1ec80..225d3f86a81a3837b4acbf9ed23c5d3cf01a9416 100644 (file)
@@ -25,6 +25,7 @@
 #include <sys/types.h>
 #include <unistd.h>
 #include <urcu/compiler.h>
+#include <lttng/ust-error.h>
 
 #include <common/common.h>
 #include <common/sessiond-comm/sessiond-comm.h>
 #include "ust-consumer.h"
 #include "ust-ctl.h"
 
+/* Next available channel key. */
+static unsigned long next_channel_key;
+
+/*
+ * Return the atomically incremented value of next_channel_key.
+ */
+static inline unsigned long get_next_channel_key(void)
+{
+       return uatomic_add_return(&next_channel_key, 1);
+}
+
+/*
+ * Return the consumer socket from the given consumer output with the right
+ * bitness. On error, returns NULL.
+ *
+ * The caller MUST acquire a rcu read side lock and keep it until the socket
+ * object reference is not needed anymore.
+ */
+static struct consumer_socket *find_consumer_socket_by_bitness(int bits,
+               struct consumer_output *consumer)
+{
+       int consumer_fd;
+       struct consumer_socket *socket = NULL;
+
+       switch (bits) {
+       case 64:
+               consumer_fd = uatomic_read(&ust_consumerd64_fd);
+               break;
+       case 32:
+               consumer_fd = uatomic_read(&ust_consumerd32_fd);
+               break;
+       default:
+               assert(0);
+               goto end;
+       }
+
+       socket = consumer_find_socket(consumer_fd, consumer);
+       if (!socket) {
+               ERR("Consumer socket fd %d not found in consumer obj %p",
+                               consumer_fd, consumer);
+       }
+
+end:
+       return socket;
+}
+
+/*
+ * 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);
+}
+
 /*
  * Delete ust context safely. RCU read lock must be held before calling
  * this function.
 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);
@@ -57,21 +220,17 @@ 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);
@@ -82,10 +241,18 @@ void delete_ust_app_event(int sock, struct ust_app_event *ua_event)
  * this function.
  */
 static
-void delete_ust_app_stream(int sock, struct ltt_ust_stream *stream)
+void delete_ust_app_stream(int sock, struct ust_app_stream *stream)
 {
+       int ret;
+
+       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);
        }
@@ -97,13 +264,18 @@ void delete_ust_app_stream(int sock, struct ltt_ust_stream *stream)
  * 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;
+
+       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) {
@@ -128,49 +300,202 @@ void delete_ust_app_channel(int sock, struct ust_app_channel *ua_chan)
        }
        lttng_ht_destroy(ua_chan->events);
 
+       /* Wipe and free registry. */
+       ust_registry_channel_destroy(&ua_chan->session->registry, &ua_chan->registry);
+
        if (ua_chan->obj != NULL) {
-               ustctl_release_object(sock, ua_chan->obj);
+               /* 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, 2);
                free(ua_chan->obj);
        }
        free(ua_chan);
 }
 
+/*
+ * For a given application and session, push metadata to consumer. The session
+ * lock MUST be acquired here before calling this.
+ *
+ * Return 0 on success else a negative error.
+ */
+static int push_metadata(struct ust_app *app, struct ust_app_session *ua_sess)
+{
+       int ret;
+       char *metadata_str = NULL;
+       size_t len, offset;
+       struct consumer_socket *socket;
+
+       assert(app);
+       assert(ua_sess);
+
+       if (!ua_sess->consumer || !ua_sess->metadata) {
+               /* No consumer means no stream associated so just return gracefully. */
+               ret = 0;
+               goto end;
+       }
+
+       rcu_read_lock();
+
+       /* Get consumer socket to use to push the metadata.*/
+       socket = find_consumer_socket_by_bitness(app->bits_per_long,
+                       ua_sess->consumer);
+       if (!socket) {
+               ret = -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);
+       pthread_mutex_lock(&ua_sess->registry.lock);
+
+       offset = ua_sess->registry.metadata_len_sent;
+       len = ua_sess->registry.metadata_len - ua_sess->registry.metadata_len_sent;
+       if (len == 0) {
+               DBG3("No metadata to push for session id %d", ua_sess->id);
+               ret = 0;
+               goto error_reg_unlock;
+       }
+       assert(len > 0);
+
+       /* Allocate only what we have to send. */
+       metadata_str = zmalloc(len);
+       if (!metadata_str) {
+               PERROR("zmalloc ust app metadata string");
+               ret = -ENOMEM;
+               goto error_reg_unlock;
+       }
+       /* Copy what we haven't send out. */
+       memcpy(metadata_str, ua_sess->registry.metadata + offset, len);
+
+       pthread_mutex_unlock(&ua_sess->registry.lock);
+
+       ret = ust_consumer_push_metadata(socket, ua_sess, metadata_str, len,
+                       offset);
+       if (ret < 0) {
+               pthread_mutex_unlock(socket->lock);
+               goto error_rcu_unlock;
+       }
+
+       /* Update len sent of the registry. */
+       pthread_mutex_lock(&ua_sess->registry.lock);
+       ua_sess->registry.metadata_len_sent += len;
+       pthread_mutex_unlock(&ua_sess->registry.lock);
+       pthread_mutex_unlock(socket->lock);
+
+       rcu_read_unlock();
+       free(metadata_str);
+       return 0;
+
+error_reg_unlock:
+       pthread_mutex_unlock(&ua_sess->registry.lock);
+       pthread_mutex_unlock(socket->lock);
+error_rcu_unlock:
+       rcu_read_unlock();
+       free(metadata_str);
+end:
+       return ret;
+}
+
+/*
+ * 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_app *app, struct ust_app_session *ua_sess)
+{
+       int ret;
+       struct consumer_socket *socket;
+
+       assert(app);
+       assert(ua_sess);
+
+       /* Ignore if no metadata. Valid since it can be called on unregister. */
+       if (!ua_sess->metadata) {
+               ret = 0;
+               goto error;
+       }
+
+       rcu_read_lock();
+
+       /* Get consumer socket to use to push the metadata.*/
+       socket = find_consumer_socket_by_bitness(app->bits_per_long,
+                       ua_sess->consumer);
+       if (!socket) {
+               ret = -1;
+               goto error_rcu_unlock;
+       }
+
+       ret = ust_consumer_close_metadata(socket, ua_sess->metadata);
+       if (ret < 0) {
+               goto error_rcu_unlock;
+       }
+
+error_rcu_unlock:
+       /* Destroy metadata on our side since we must not use it anymore. */
+       delete_ust_app_channel(-1, ua_sess->metadata, app);
+       ua_sess->metadata = NULL;
+
+       rcu_read_unlock();
+error:
+       return ret;
+}
+
 /*
  * Delete ust app session safely. RCU read lock must be held before calling
  * 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;
 
+       assert(ua_sess);
+
        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);
-               }
-               trace_ust_destroy_metadata(ua_sess->metadata);
+               /* Push metadata for application before freeing the application. */
+               (void) push_metadata(app, ua_sess);
+
+               /* And ask to close it for this session. */
+               (void) close_metadata(app, ua_sess);
        }
 
        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);
        }
        lttng_ht_destroy(ua_sess->channels);
 
+       ust_registry_session_destroy(&ua_sess->registry);
+
        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);
 }
@@ -183,8 +508,7 @@ static
 void delete_ust_app(struct ust_app *app)
 {
        int ret, sock;
-       struct lttng_ht_iter iter;
-       struct ust_app_session *ua_sess;
+       struct ust_app_session *ua_sess, *tmp_ua_sess;
 
        rcu_read_lock();
 
@@ -192,14 +516,14 @@ void delete_ust_app(struct ust_app *app)
        sock = app->sock;
        app->sock = -1;
 
+       lttng_ht_destroy(app->sessions);
+
        /* 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. */
+               delete_ust_app_session(sock, ua_sess, app);
        }
-       lttng_ht_destroy(app->sessions);
 
        /*
         * Wait until we have deleted the application from the sock hash table
@@ -241,11 +565,38 @@ void delete_ust_app_rcu(struct rcu_head *head)
        delete_ust_app(app);
 }
 
+/*
+ * Delete the session from the application ht and delete the data structure by
+ * freeing every object inside and releasing them.
+ */
+static void destroy_app_session(struct ust_app *app,
+               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(void)
+struct ust_app_session *alloc_ust_app_session(struct ust_app *app)
 {
        struct ust_app_session *ua_sess;
 
@@ -253,15 +604,28 @@ struct ust_app_session *alloc_ust_app_session(void)
        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);
+       if (ust_registry_session_init(&ua_sess->registry, 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)) {
+               goto error;
+       }
 
        return ua_sess;
 
 error:
+       free(ua_sess);
+error_free:
        return NULL;
 }
 
@@ -270,7 +634,8 @@ 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;
 
@@ -287,16 +652,28 @@ struct ust_app_channel *alloc_ust_app_channel(char *name,
 
        ua_chan->enabled = 1;
        ua_chan->handle = -1;
+       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);
 
        CDS_INIT_LIST_HEAD(&ua_chan->streams.head);
 
+       /* Initialize UST registry. */
+       ust_registry_channel_init(&ua_sess->registry, &ua_chan->registry);
+
        /* 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 +683,28 @@ 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 +724,6 @@ 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 +762,29 @@ 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 +809,96 @@ 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;
 }
 
 /*
- * Create the event context on the tracer.
+ * 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
-int create_ust_event_context(struct ust_app_event *ua_event,
-               struct ust_app_ctx *ua_ctx, struct ust_app *app)
+static struct ust_app_event *find_ust_app_event(struct lttng_ht *ht,
+               char *name, struct lttng_ust_filter_bytecode *filter, int loglevel)
 {
-       int ret;
+       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;
+       }
 
-       health_code_update(&health_thread_cmd);
+       event = caa_container_of(node, struct ust_app_event, node);
+
+end:
+       return event;
+}
+
+/*
+ * Create the channel context on the tracer.
+ *
+ * Called with UST app session lock held.
+ */
+static
+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();
 
        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 +911,7 @@ 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 +921,19 @@ 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 +945,17 @@ 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 +963,7 @@ 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 +975,17 @@ 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 +993,7 @@ 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 +1005,17 @@ 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 +1025,7 @@ 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 +1037,17 @@ 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;
        }
 
@@ -570,118 +1055,103 @@ static int enable_ust_event(struct ust_app *app,
                        ua_event->attr.name, app->pid);
 
 error:
-       health_code_update(&health_thread_cmd);
+       health_code_update();
        return ret;
 }
 
 /*
- * Open metadata onto the UST tracer for a UST session.
+ * Create the specified channel onto the UST tracer for a UST session. This
+ * MUST be called with UST app session lock held.
+ *
+ * Return 0 on success. On error, a negative value is returned.
  */
-static int open_ust_metadata(struct ust_app *app,
-               struct ust_app_session *ua_sess)
+static int create_ust_channel(struct ust_app *app,
+               struct ust_app_session *ua_sess, struct ust_app_channel *ua_chan,
+               struct consumer_output *consumer)
 {
        int ret;
-       struct lttng_ust_channel_attr uattr;
+       unsigned int nb_fd = 0;
+       struct consumer_socket *socket;
+       struct ust_app_stream *stream, *stmp;
 
-       health_code_update(&health_thread_cmd);
+       assert(app);
+       assert(ua_sess);
+       assert(ua_chan);
+       assert(consumer);
 
-       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;
+       rcu_read_lock();
+       health_code_update();
 
-       /* 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);
+       /* Get the right consumer socket for the application. */
+       socket = find_consumer_socket_by_bitness(app->bits_per_long, consumer);
+       if (!socket) {
+               ret = -1;
                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);
+       health_code_update();
 
-       /* 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);
+       /*
+        * 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, consumer, socket);
        if (ret < 0) {
-               ERR("UST create metadata stream failed");
                goto error;
        }
 
-error:
-       health_code_update(&health_thread_cmd);
-       return ret;
-}
-
-/*
- * Create the specified channel onto the UST tracer for a UST session.
- */
-static int create_ust_channel(struct ust_app *app,
-               struct ust_app_session *ua_sess, struct ust_app_channel *ua_chan)
-{
-       int ret;
-
-       health_code_update(&health_thread_cmd);
-
-       /* TODO: remove cast and use lttng-ust-abi.h */
+       /*
+        * 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;
 
-       /* We are going to receive 2 fds, we need to reserve them. */
-       ret = lttng_fd_get(LTTNG_FD_APPS, 2);
+       /* 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;
+               goto error_fd_get;
        }
 
-       health_code_update(&health_thread_cmd);
+       health_code_update();
 
-       ret = ustctl_create_channel(app->sock, ua_sess->handle,
-                       (struct lttng_ust_channel_attr *)&ua_chan->attr, &ua_chan->obj);
+       /*
+        * Now get the channel from the consumer. This call wil populate the stream
+        * list of that channel and set the ust object.
+        */
+       ret = ust_consumer_get_channel(socket, ua_chan);
+       if (ret < 0) {
+               goto error_destroy;
+       }
+
+       /* 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;
+       health_code_update();
+
+       /* 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);
+       }
 
-       DBG2("UST app channel %s created successfully for pid:%d and sock:%d",
-                       ua_chan->name, app->pid, app->sock);
+       /* Flag the channel that it is sent to the application. */
+       ua_chan->is_sent = 1;
+       /* Assign session to channel. */
+       ua_chan->session = ua_sess;
+       /* 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);
 
-       health_code_update(&health_thread_cmd);
+       health_code_update();
 
        /* If channel is not enabled, disable it on the tracer */
        if (!ua_chan->enabled) {
@@ -691,13 +1161,29 @@ static int create_ust_channel(struct ust_app *app,
                }
        }
 
+       rcu_read_unlock();
+       return 0;
+
+error_destroy:
+       lttng_fd_put(LTTNG_FD_APPS, nb_fd);
+error_fd_get:
+       /*
+        * 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:
-       health_code_update(&health_thread_cmd);
+       health_code_update();
+       rcu_read_unlock();
        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 +1191,18 @@ 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 +1211,15 @@ 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 +1231,10 @@ 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 +1246,7 @@ 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 +1256,6 @@ 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 +1266,8 @@ 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 +1278,6 @@ 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;
@@ -814,8 +1287,18 @@ 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));
+
+       /* 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;
 
@@ -831,11 +1314,9 @@ 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 +1324,7 @@ 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);
                }
        }
 
@@ -900,13 +1381,19 @@ 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);
        }
 }
@@ -945,39 +1432,62 @@ error:
 }
 
 /*
- * Create a UST session onto the tracer of app and add it the session
- * hashtable.
+ * 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.
  *
- * Return ust app session or NULL on error.
+ * 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 struct ust_app_session *create_ust_app_session(
-               struct ltt_ust_session *usess, 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;
+       int ret, created = 0;
        struct ust_app_session *ua_sess;
 
-       health_code_update(&health_thread_cmd);
+       assert(usess);
+       assert(app);
+       assert(ua_sess_ptr);
+
+       health_code_update();
 
        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();
+               ua_sess = alloc_ust_app_session(app);
                if (ua_sess == NULL) {
                        /* Only malloc can failed so something is really wrong */
-                       goto end;
+                       ret = -ENOMEM;
+                       goto error;
                }
                shadow_copy_session(ua_sess, usess, app);
+               created = 1;
        }
 
-       health_code_update(&health_thread_cmd);
+       health_code_update();
 
        if (ua_sess->handle == -1) {
                ret = ustctl_create_session(app->sock);
                if (ret < 0) {
-                       ERR("Creating session for app pid %d", app->pid);
-                       /* This means that the tracer is gone... */
-                       ua_sess = (void*) -1UL;
+                       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;
                }
 
@@ -990,18 +1500,31 @@ static struct ust_app_session *create_ust_app_session(
                DBG2("UST app session created successfully with handle %d", ret);
        }
 
-end:
-       health_code_update(&health_thread_cmd);
-       return ua_sess;
+       /*
+        * Assign consumer if not already set. For one application, there is only
+        * one possible consumer has of now.
+        */
+       if (!ua_sess->consumer) {
+               ua_sess->consumer = usess->consumer;
+       }
+
+       *ua_sess_ptr = ua_sess;
+       if (is_created) {
+               *is_created = created;
+       }
+
+       /* Everything went well. */
+       ret = 0;
 
 error:
-       delete_ust_app_session(-1, ua_sess);
-       health_code_update(&health_thread_cmd);
-       return NULL;
+       health_code_update();
+       return ret;
 }
 
 /*
  * Create a context for the channel on the tracer.
+ *
+ * Called with UST app session lock held.
  */
 static
 int create_ust_app_channel_context(struct ust_app_session *ua_sess,
@@ -1041,78 +1564,10 @@ error:
        return ret;
 }
 
-/*
- * Create an UST context and enable it for the event on the tracer.
- */
-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)
-{
-       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);
-
-       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;
-       }
-
-       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_event->ctx, &ua_ctx->node);
-
-       ret = create_ust_event_context(ua_event, ua_ctx, app);
-       if (ret < 0) {
-               goto error;
-       }
-
-error:
-       return ret;
-}
-
-/*
- * Set UST filter for the event on the tracer.
- */
-static
-int set_ust_app_event_filter(struct ust_app_session *ua_sess,
-               struct ust_app_event *ua_event,
-               struct lttng_filter_bytecode *bytecode,
-               struct ust_app *app)
-{
-       int ret = 0;
-
-       DBG2("UST app adding context to event %s", ua_event->name);
-
-       /* Copy filter bytecode */
-       ua_event->filter = zmalloc(sizeof(*ua_event->filter) + bytecode->len);
-       if (!ua_event->filter) {
-               return -ENOMEM;
-       }
-       memcpy(ua_event->filter, bytecode,
-               sizeof(*ua_event->filter) + bytecode->len);
-       ret = set_ust_event_filter(ua_event, app);
-       if (ret < 0) {
-               goto error;
-       }
-
-error:
-       return ret;
-}
-
 /*
  * 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,
@@ -1201,11 +1656,15 @@ error:
 }
 
 /*
- * Create UST app channel and create it on the tracer.
+ * 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 held.
  */
-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 create_ust_app_channel(struct ust_app_session *ua_sess,
+               struct ltt_ust_channel *uchan, struct ust_app *app,
+               struct consumer_output *consumer, enum lttng_ust_chan_type type,
+               struct ust_app_channel **ua_chanp)
 {
        int ret = 0;
        struct lttng_ht_iter iter;
@@ -1220,35 +1679,45 @@ static struct ust_app_channel *create_ust_app_channel(
                goto end;
        }
 
-       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) {
                /* Only malloc can fail here */
+               ret = -ENOMEM;
                goto error;
        }
        shadow_copy_channel(ua_chan, uchan);
 
-       ret = create_ust_channel(app, ua_sess, ua_chan);
+       /* Set channel type. */
+       ua_chan->attr.type = type;
+
+       ret = create_ust_channel(app, ua_sess, ua_chan, consumer);
        if (ret < 0) {
-               /* Not found previously means that it does not exist on the tracer */
-               assert(ret != -EEXIST);
                goto error;
        }
 
-       lttng_ht_add_unique_str(ua_sess->channels, &ua_chan->node);
-
        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:
-       return ua_chan;
+       if (ua_chanp) {
+               *ua_chanp = ua_chan;
+       }
+
+       /* Everything went well. */
+       return 0;
 
 error:
-       delete_ust_app_channel(-1, ua_chan);
-       return NULL;
+       delete_ust_app_channel(ua_chan->is_sent ? app->sock : -1, ua_chan, app);
+       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,
@@ -1256,14 +1725,12 @@ int create_ust_app_event(struct ust_app_session *ua_sess,
                struct ust_app *app)
 {
        int ret = 0;
-       struct lttng_ht_iter iter;
-       struct lttng_ht_node_str *ua_event_node;
        struct ust_app_event *ua_event;
 
        /* 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) {
+       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;
        }
@@ -1281,11 +1748,11 @@ int create_ust_app_event(struct ust_app_session *ua_sess,
        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 != -EEXIST);
+               assert(ret != -LTTNG_UST_ERR_EXIST);
                goto error;
        }
 
-       lttng_ht_add_unique_str(ua_chan->events, &ua_event->node);
+       add_unique_ust_app_event(ua_chan, ua_event);
 
        DBG2("UST app create event %s for PID %d completed", ua_event->name,
                        app->pid);
@@ -1301,62 +1768,81 @@ error:
 
 /*
  * Create UST metadata and open it on the tracer side.
+ *
+ * Called with UST app session lock held.
  */
 static int create_ust_app_metadata(struct ust_app_session *ua_sess,
-               char *pathname, struct ust_app *app)
+               struct ust_app *app, struct consumer_output *consumer)
 {
        int ret = 0;
+       struct ust_app_channel *metadata;
+       struct consumer_socket *socket;
 
-       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(ua_sess);
+       assert(app);
+       assert(consumer);
 
-                       /* 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;
-               }
+       if (ua_sess->metadata) {
+               /* Already exist. Return success. */
+               goto end;
+       }
 
-               DBG2("UST metadata opened for app pid %d", app->pid);
+       /* Allocate UST metadata */
+       metadata = alloc_ust_app_channel(DEFAULT_METADATA_NAME, ua_sess, NULL);
+       if (!metadata) {
+               /* malloc() failed */
+               ret = -ENOMEM;
+               goto error;
        }
 
-       /* Open UST metadata stream */
-       if (ua_sess->metadata->stream_obj == NULL) {
-               ret = create_ust_stream(app, ua_sess);
-               if (ret < 0) {
-                       goto error;
-               }
+       /* 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_UST_CHANNEL_SWITCH_TIMER;
+       metadata->attr.read_timer_interval = DEFAULT_UST_CHANNEL_READ_TIMER;
+       metadata->attr.output = LTTNG_UST_MMAP;
+       metadata->attr.type = LTTNG_UST_CHAN_METADATA;
 
-               ret = snprintf(ua_sess->metadata->pathname, PATH_MAX,
-                               "%s/metadata", ua_sess->path);
-               if (ret < 0) {
-                       PERROR("asprintf UST create stream");
-                       goto error;
-               }
+       /* Get the right consumer socket for the application. */
+       socket = find_consumer_socket_by_bitness(app->bits_per_long, consumer);
+       if (!socket) {
+               ret = -EINVAL;
+               goto error_consumer;
+       }
 
-               DBG2("UST metadata stream object created for app pid %d",
-                               app->pid);
-       } else {
-               ERR("Attempting to create stream without metadata opened");
-               goto error;
+       /*
+        * 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);
+       if (ret < 0) {
+               goto error_consumer;
        }
 
-       return 0;
+       /*
+        * 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 = ust_consumer_setup_metadata(socket, metadata);
+       if (ret < 0) {
+               goto error_consumer;
+       }
+
+       ua_sess->metadata = metadata;
+
+       DBG2("UST metadata created for app pid %d", app->pid);
 
+end:
+       return 0;
+error_consumer:
+       delete_ust_app_channel(-1, metadata, app);
 error:
-       return -1;
+       return ret;
 }
 
 /*
@@ -1368,89 +1854,109 @@ struct lttng_ht *ust_app_get_ht(void)
 }
 
 /*
- * Return ust app pointer or NULL if not found.
+ * 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;
 
-       rcu_read_lock();
        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;
-       strncpy(lta->name, msg->name, sizeof(lta->name));
-       lta->name[16] = '\0';
        lta->sessions = lttng_ht_new(0, LTTNG_HT_TYPE_ULONG);
+       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[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 +1964,49 @@ 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 +2020,7 @@ 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 +2028,72 @@ 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);
 
-       /* Assign second node for deletion */
-       iter.iter.node = &lta->pid_n.node;
+       /*
+        * 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);
-       assert(!ret);
+       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) {
+               ret = lttng_ht_del(lta->sessions, &iter);
+               if (ret) {
+                       /* The session was already removed so scheduled for teardown. */
+                       continue;
+               }
+
+               /*
+                * 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);
+
+               /*
+                * 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.
+                */
+               (void) push_metadata(lta, ua_sess);
+               (void) close_metadata(lta, ua_sess);
+
+               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 +2121,11 @@ 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 +2136,7 @@ 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) {
                        /*
@@ -1570,46 +2147,62 @@ 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 *tmp_ptr = (void *) tmp;
+                               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_ptr);
+                                       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;
 }
 
@@ -1622,11 +2215,11 @@ 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;
@@ -1637,7 +2230,7 @@ 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) {
                        /*
@@ -1648,51 +2241,67 @@ 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 *tmp_ptr = (void *) tmp;
+                               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_ptr);
+                                       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;
-                       tmp[count].nowrite = uiter.nowrite;
+                       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;
 }
 
@@ -1702,28 +2311,37 @@ error:
 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);
        }
 
+       /* 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);
+       }
+
        /* Destroy is done only when the ht is empty */
        lttng_ht_destroy(ust_app_ht);
        lttng_ht_destroy(ust_app_ht_by_sock);
+       lttng_ht_destroy(ust_app_ht_by_notify_sock);
 
        rcu_read_unlock();
 }
@@ -1735,6 +2353,7 @@ 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);
 }
 
 /*
@@ -1945,8 +2564,10 @@ 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);
@@ -1978,10 +2599,10 @@ 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);
@@ -2006,29 +2627,43 @@ 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);
+               /* Create channel onto application. We don't need the chan ref. */
+               ret = create_ust_app_channel(ua_sess, uchan, app, usess->consumer,
+                               LTTNG_UST_CHAN_PER_CPU, 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;
 }
 
 /*
@@ -2039,7 +2674,7 @@ 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;
@@ -2066,8 +2701,12 @@ 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);
@@ -2077,19 +2716,22 @@ 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:
@@ -2126,9 +2768,12 @@ 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);
@@ -2138,8 +2783,9 @@ 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;
                        }
@@ -2160,11 +2806,7 @@ int ust_app_create_event_glb(struct ltt_ust_session *usess,
 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);
 
@@ -2176,9 +2818,12 @@ 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;
@@ -2192,125 +2837,55 @@ int ust_app_start_trace(struct ltt_ust_session *usess, struct ust_app *app)
                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) {
-               ret = LTTNG_ERR_UST_META_FAIL;
-               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);
-                               ret = LTTNG_ERR_UST_STREAM_FAIL;
-                               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);
-                       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. */
+       ret = create_ust_app_metadata(ua_sess, app, usess->consumer);
        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;
 }
 
@@ -2329,81 +2904,101 @@ int ust_app_stop_trace(struct ltt_ust_session *usess, struct ust_app *app)
        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_thread_cmd);
+       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);
+       health_code_update();
 
-       /* Flush all buffers before stopping */
-       ret = ustctl_sock_flush_buffer(app->sock, ua_sess->metadata->obj);
+       ret = push_metadata(app, ua_sess);
        if (ret < 0) {
-               ERR("UST app PID %d metadata flush failed with ret %d", app->pid,
-                               ret);
+               goto error_rcu_unlock;
        }
 
-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:
+       pthread_mutex_unlock(&ua_sess->lock);
        rcu_read_unlock();
-       health_code_update(&health_thread_cmd);
+       health_code_update();
        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;
 
        DBG("Destroy tracing for ust app pid %d", app->pid);
 
@@ -2416,34 +3011,26 @@ 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);
        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;
 }
 
 /*
@@ -2488,6 +3075,7 @@ int ust_app_stop_trace_all(struct ltt_ust_session *usess)
        cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                ret = ust_app_stop_trace(usess, app);
                if (ret < 0) {
+                       ERR("UST app stop trace failed with ret %d", ret);
                        /* Continue to next apps even on error */
                        continue;
                }
@@ -2512,7 +3100,7 @@ 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;
@@ -2532,15 +3120,13 @@ 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,
                        usess->id);
@@ -2549,7 +3135,11 @@ void ust_app_global_update(struct ltt_ust_session *usess, int sock)
 
        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;
        }
 
@@ -2557,30 +3147,37 @@ 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);
+               ret = create_ust_channel(app, ua_sess, ua_chan, usess->consumer);
                if (ret < 0) {
-                       /* FIXME: Should we quit here or continue... */
-                       continue;
+                       /*
+                        * 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;
                        }
                }
 
@@ -2590,27 +3187,13 @@ 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) {
@@ -2620,7 +3203,16 @@ 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;
 }
@@ -2653,19 +3245,21 @@ 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();
@@ -2673,128 +3267,14 @@ 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 ret = 0;
-       struct lttng_ht_node_str *ua_chan_node, *ua_event_node;
-       struct lttng_ht_iter iter, uiter;
-       struct ust_app_session *ua_sess;
-       struct ust_app_event *ua_event;
-       struct ust_app_channel *ua_chan = NULL;
-       struct ust_app *app;
-
-       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;
-               }
-
-               /* 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);
-
-               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);
-
-               ret = create_ust_app_event_context(ua_sess, ua_event, &uctx->ctx, app);
-               if (ret < 0) {
-                       continue;
-               }
-       }
-
-       rcu_read_unlock();
-       return ret;
-}
-
-/*
- * Add context to a specific event in a channel for global UST domain.
- */
-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 ret = 0;
-       struct lttng_ht_node_str *ua_chan_node, *ua_event_node;
-       struct lttng_ht_iter iter, uiter;
-       struct ust_app_session *ua_sess;
-       struct ust_app_event *ua_event;
-       struct ust_app_channel *ua_chan = NULL;
-       struct ust_app *app;
-
-       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;
-               }
-
-               /* 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);
-
-               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);
-
-               ret = set_ust_app_event_filter(ua_sess, ua_event, bytecode, app);
-               if (ret < 0) {
-                       continue;
-               }
-       }
-
-       rcu_read_unlock();
-       return ret;
-}
-
-/*
- * Enable event for a channel from a UST session for a specific PID.
- */
-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 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_iter iter;
-       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;
@@ -2808,18 +3288,22 @@ int ust_app_enable_event_pid(struct ltt_ust_session *usess,
        if (app == NULL) {
                ERR("UST app enable event per PID %d not found", pid);
                ret = -1;
-               goto error;
+               goto end;
        }
 
        if (!app->compatible) {
                ret = 0;
-               goto error;
+               goto end;
        }
 
        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. */
+               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);
@@ -2828,23 +3312,23 @@ int ust_app_enable_event_pid(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, &iter);
-       ua_event_node = lttng_ht_iter_get_node_str(&iter);
-       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) {
                ret = create_ust_app_event(ua_sess, ua_chan, uevent, app);
                if (ret < 0) {
-                       goto error;
+                       goto end_unlock;
                }
        } else {
-               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) {
-                       goto error;
+                       goto end_unlock;
                }
        }
 
-error:
+end_unlock:
+       pthread_mutex_unlock(&ua_sess->lock);
+end:
        rcu_read_unlock();
        return ret;
 }
@@ -2880,8 +3364,10 @@ int ust_app_disable_event_pid(struct ltt_ust_session *usess,
        }
 
        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. */
+               goto error;
+       }
 
        /* Lookup channel in the ust app session */
        lttng_ht_lookup(ua_sess->channels, (void *)uchan->name, &iter);
@@ -2910,49 +3396,6 @@ error:
        return ret;
 }
 
-/*
- * Validate version of UST apps and set the compatible bit.
- */
-int ust_app_validate_version(int sock)
-{
-       int ret;
-       struct ust_app *app;
-
-       rcu_read_lock();
-
-       app = find_app_by_sock(sock);
-       assert(app);
-
-       health_code_update(&health_thread_cmd);
-
-       ret = ustctl_tracer_version(sock, &app->version);
-       if (ret < 0) {
-               goto error;
-       }
-
-       /* Validate version */
-       if (app->version.major != UST_APP_MAJOR_VERSION) {
-               goto error;
-       }
-
-       DBG2("UST app PID %d is compatible with internal 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;
-
-error:
-       DBG2("UST app PID %d is not compatible with internal 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;
-}
-
 /*
  * Calibrate registered applications.
  */
@@ -2973,7 +3416,7 @@ int ust_app_calibrate_glb(struct lttng_ust_calibrate *calibrate)
                        continue;
                }
 
-               health_code_update(&health_thread_cmd);
+               health_code_update();
 
                ret = ustctl_calibrate(app->sock, calibrate);
                if (ret < 0) {
@@ -2983,7 +3426,6 @@ int ust_app_calibrate_glb(struct lttng_ust_calibrate *calibrate)
                                ret = 0;
                                break;
                        default:
-                               /* TODO: Report error to user */
                                DBG2("Calibrate app PID %d returned with error %d",
                                                app->pid, ret);
                                break;
@@ -2995,7 +3437,413 @@ int ust_app_calibrate_glb(struct lttng_ust_calibrate *calibrate)
 
        rcu_read_unlock();
 
-       health_code_update(&health_thread_cmd);
+       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;
+               }
+               goto error;
+       }
+       msg->pid = (pid_t) pid;
+       msg->ppid = (pid_t) ppid;
+       msg->uid = (uid_t) uid;
+       msg->gid = (gid_t) gid;
+
+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;
+       enum ustctl_channel_header type;
+       struct ust_app *app;
+       struct ust_app_channel *ua_chan;
+       struct ust_app_session *ua_sess;
+
+       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;
+               goto error_rcu_unlock;
+       }
+
+       /* Lookup channel by UST object descriptor. Should always be found. */
+       ua_chan = find_channel_by_objd(app, cobjd);
+       assert(ua_chan);
+       assert(ua_chan->session);
+       ua_sess = ua_chan->session;
+       assert(ua_sess);
+
+       pthread_mutex_lock(&ua_sess->registry.lock);
+
+       if (ust_registry_is_max_id(ua_chan->session->registry.used_channel_id)) {
+               ret_code = -1;
+               chan_id = -1U;
+               type = -1;
+               goto reply;
+       }
+
+       /* Don't assign ID to metadata. */
+       if (ua_chan->attr.type == LTTNG_UST_CHAN_METADATA) {
+               chan_id = -1U;
+       } else {
+               chan_id = ust_registry_get_next_chan_id(&ua_chan->session->registry);
+       }
+
+       reg_count = ust_registry_get_event_count(&ua_chan->registry);
+       if (reg_count < 31) {
+               type = USTCTL_CHANNEL_HEADER_COMPACT;
+       } else {
+               type = USTCTL_CHANNEL_HEADER_LARGE;
+       }
+
+       ua_chan->registry.nr_ctx_fields = nr_fields;
+       ua_chan->registry.ctx_fields = fields;
+       ua_chan->registry.chan_id = chan_id;
+       ua_chan->registry.header_type = type;
+
+       /* Append to metadata */
+       if (!ret_code) {
+               ret_code = ust_metadata_channel_statedump(&ua_chan->session->registry,
+                               &ua_chan->registry);
+               if (ret_code) {
+                       ERR("Error appending channel metadata (errno = %d)", ret_code);
+                       goto reply;
+               }
+       }
+
+reply:
+       DBG3("UST app replying to register channel with id %u, type: %d, ret: %d",
+                       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;
+       }
+
+error:
+       pthread_mutex_unlock(&ua_sess->registry.lock);
+error_rcu_unlock:
+       rcu_read_unlock();
+       return ret;
+}
+
+/*
+ * 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.
+ */
+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, ret_code;
+       uint32_t event_id = 0;
+       struct ust_app *app;
+       struct ust_app_channel *ua_chan;
+       struct ust_app_session *ua_sess;
+
+       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;
+               goto error_rcu_unlock;
+       }
+
+       /* Lookup channel by UST object descriptor. Should always be found. */
+       ua_chan = find_channel_by_objd(app, cobjd);
+       assert(ua_chan);
+       assert(ua_chan->session);
+       ua_sess = ua_chan->session;
+
+       pthread_mutex_lock(&ua_sess->registry.lock);
+
+       ret_code = ust_registry_create_event(&ua_sess->registry,
+                       &ua_chan->registry, sobjd, cobjd, name, sig, nr_fields, fields,
+                       loglevel, model_emf_uri, &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;
+       }
+
+       DBG3("UST registry event %s has been added successfully", name);
+
+error:
+       pthread_mutex_unlock(&ua_sess->registry.lock);
+error_rcu_unlock:
+       rcu_read_unlock();
+       return ret;
+}
+
+/*
+ * 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;
+
+       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;
+               }
+
+               /* Add event to the UST registry coming from the notify socket. */
+               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;
+               }
 
+               ret = reply_ust_register_channel(sock, sobjd, cobjd, nr_fields,
+                               fields);
+               if (ret < 0) {
+                       goto error;
+               }
+
+               break;
+       }
+       default:
+               /* Should NEVER happen. */
+               assert(0);
+       }
+
+error:
        return ret;
 }
+
+/*
+ * 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.
+ */
+void ust_app_notify_sock_unregister(int sock)
+{
+       int err_enomem = 0;
+       struct lttng_ht_iter iter;
+       struct ust_app *app;
+       struct ust_app_notify_sock_obj *obj;
+
+       assert(sock >= 0);
+
+       rcu_read_lock();
+
+       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;
+       }
+
+       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;
+       }
+
+       iter.iter.node = &app->notify_sock_n.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);
+
+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);
+       }
+}
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