Fix: leak of trace_path on error in ust_app_snapshot_record
[lttng-tools.git] / src / bin / lttng-sessiond / ust-app.c
index 96ba2f4bcad0649afe1cfcec93e2f6c1f13002df..66b50eb43df98ea42c0700c5cedd5e9f28e17e8b 100644 (file)
@@ -1,5 +1,6 @@
 /*
  * Copyright (C) 2011 - David Goulet <david.goulet@polymtl.ca>
+ * Copyright (C) 2016 - Jérémie Galarneau <jeremie.galarneau@efficios.com>
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License, version 2 only,
@@ -15,7 +16,6 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
-#define _GNU_SOURCE
 #define _LGPL_SOURCE
 #include <errno.h>
 #include <inttypes.h>
 #include "ust-consumer.h"
 #include "ust-ctl.h"
 #include "utils.h"
+#include "session.h"
+#include "lttng-sessiond.h"
+#include "notification-thread-commands.h"
+#include "rotate.h"
 
 static
 int ust_app_flush_app_session(struct ust_app *app, struct ust_app_session *ua_sess);
@@ -89,6 +93,7 @@ static void copy_channel_attr_to_ustctl(
        attr->switch_timer_interval = uattr->switch_timer_interval;
        attr->read_timer_interval = uattr->read_timer_interval;
        attr->output = uattr->output;
+       attr->blocking_timeout = uattr->u.s.blocking_timeout;
 }
 
 /*
@@ -101,12 +106,14 @@ 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;
+       int ev_loglevel_value;
 
        assert(node);
        assert(_key);
 
        event = caa_container_of(node, struct ust_app_event, node.node);
        key = _key;
+       ev_loglevel_value = event->attr.loglevel;
 
        /* Match the 4 elements of the key: name, filter, loglevel, exclusions */
 
@@ -116,9 +123,10 @@ static int ht_match_ust_app_event(struct cds_lfht_node *node, const void *_key)
        }
 
        /* Event loglevel. */
-       if (event->attr.loglevel != key->loglevel) {
+       if (ev_loglevel_value != key->loglevel_type) {
                if (event->attr.loglevel_type == LTTNG_UST_LOGLEVEL_ALL
-                               && key->loglevel == 0 && event->attr.loglevel == -1) {
+                               && key->loglevel_type == 0 &&
+                               ev_loglevel_value == -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
@@ -184,7 +192,7 @@ static void add_unique_ust_app_event(struct ust_app_channel *ua_chan,
        ht = ua_chan->events;
        key.name = event->attr.name;
        key.filter = event->filter;
-       key.loglevel = event->attr.loglevel;
+       key.loglevel_type = event->attr.loglevel;
        key.exclusion = event->exclusion;
 
        node_ptr = cds_lfht_add_unique(ht->ht,
@@ -242,7 +250,8 @@ static struct ust_registry_session *get_session_registry(
        case LTTNG_BUFFER_PER_UID:
        {
                struct buffer_reg_uid *reg_uid = buffer_reg_uid_find(
-                               ua_sess->tracing_id, ua_sess->bits_per_long, ua_sess->uid);
+                               ua_sess->tracing_id, ua_sess->bits_per_long,
+                               ua_sess->real_credentials.uid);
                if (!reg_uid) {
                        goto error;
                }
@@ -262,14 +271,17 @@ error:
  * this function.
  */
 static
-void delete_ust_app_ctx(int sock, struct ust_app_ctx *ua_ctx)
+void delete_ust_app_ctx(int sock, struct ust_app_ctx *ua_ctx,
+               struct ust_app *app)
 {
        int ret;
 
        assert(ua_ctx);
 
        if (ua_ctx->obj) {
+               pthread_mutex_lock(&app->sock_lock);
                ret = ustctl_release_object(sock, ua_ctx->obj);
+               pthread_mutex_unlock(&app->sock_lock);
                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);
@@ -284,7 +296,8 @@ void delete_ust_app_ctx(int sock, struct ust_app_ctx *ua_ctx)
  * this function.
  */
 static
-void delete_ust_app_event(int sock, struct ust_app_event *ua_event)
+void delete_ust_app_event(int sock, struct ust_app_event *ua_event,
+               struct ust_app *app)
 {
        int ret;
 
@@ -294,7 +307,9 @@ void delete_ust_app_event(int sock, struct ust_app_event *ua_event)
        if (ua_event->exclusion != NULL)
                free(ua_event->exclusion);
        if (ua_event->obj != NULL) {
+               pthread_mutex_lock(&app->sock_lock);
                ret = ustctl_release_object(sock, ua_event->obj);
+               pthread_mutex_unlock(&app->sock_lock);
                if (ret < 0 && ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
                        ERR("UST app sock %d release event obj failed with ret %d",
                                        sock, ret);
@@ -309,14 +324,17 @@ void delete_ust_app_event(int sock, struct ust_app_event *ua_event)
  *
  * Return 0 on success or else a negative value.
  */
-static int release_ust_app_stream(int sock, struct ust_app_stream *stream)
+static int release_ust_app_stream(int sock, struct ust_app_stream *stream,
+               struct ust_app *app)
 {
        int ret = 0;
 
        assert(stream);
 
        if (stream->obj) {
+               pthread_mutex_lock(&app->sock_lock);
                ret = ustctl_release_object(sock, stream->obj);
+               pthread_mutex_unlock(&app->sock_lock);
                if (ret < 0 && ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
                        ERR("UST app sock %d release stream obj failed with ret %d",
                                        sock, ret);
@@ -333,11 +351,12 @@ static int release_ust_app_stream(int sock, struct ust_app_stream *stream)
  * this function.
  */
 static
-void delete_ust_app_stream(int sock, struct ust_app_stream *stream)
+void delete_ust_app_stream(int sock, struct ust_app_stream *stream,
+               struct ust_app *app)
 {
        assert(stream);
 
-       (void) release_ust_app_stream(sock, stream);
+       (void) release_ust_app_stream(sock, stream, app);
        free(stream);
 }
 
@@ -358,9 +377,77 @@ void delete_ust_app_channel_rcu(struct rcu_head *head)
        free(ua_chan);
 }
 
+/*
+ * Extract the lost packet or discarded events counter when the channel is
+ * being deleted and store the value in the parent channel so we can
+ * access it from lttng list and at stop/destroy.
+ *
+ * The session list lock must be held by the caller.
+ */
+static
+void save_per_pid_lost_discarded_counters(struct ust_app_channel *ua_chan)
+{
+       uint64_t discarded = 0, lost = 0;
+       struct ltt_session *session;
+       struct ltt_ust_channel *uchan;
+
+       if (ua_chan->attr.type != LTTNG_UST_CHAN_PER_CPU) {
+               return;
+       }
+
+       rcu_read_lock();
+       session = session_find_by_id(ua_chan->session->tracing_id);
+       if (!session || !session->ust_session) {
+               /*
+                * Not finding the session is not an error because there are
+                * multiple ways the channels can be torn down.
+                *
+                * 1) The session daemon can initiate the destruction of the
+                *    ust app session after receiving a destroy command or
+                *    during its shutdown/teardown.
+                * 2) The application, since we are in per-pid tracing, is
+                *    unregistering and tearing down its ust app session.
+                *
+                * Both paths are protected by the session list lock which
+                * ensures that the accounting of lost packets and discarded
+                * events is done exactly once. The session is then unpublished
+                * from the session list, resulting in this condition.
+                */
+               goto end;
+       }
+
+       if (ua_chan->attr.overwrite) {
+               consumer_get_lost_packets(ua_chan->session->tracing_id,
+                               ua_chan->key, session->ust_session->consumer,
+                               &lost);
+       } else {
+               consumer_get_discarded_events(ua_chan->session->tracing_id,
+                               ua_chan->key, session->ust_session->consumer,
+                               &discarded);
+       }
+       uchan = trace_ust_find_channel_by_name(
+                       session->ust_session->domain_global.channels,
+                       ua_chan->name);
+       if (!uchan) {
+               ERR("Missing UST channel to store discarded counters");
+               goto end;
+       }
+
+       uchan->per_pid_closed_app_discarded += discarded;
+       uchan->per_pid_closed_app_lost += lost;
+
+end:
+       rcu_read_unlock();
+       if (session) {
+               session_put(session);
+       }
+}
+
 /*
  * Delete ust app channel safely. RCU read lock must be held before calling
  * this function.
+ *
+ * The session list lock must be held by the caller.
  */
 static
 void delete_ust_app_channel(int sock, struct ust_app_channel *ua_chan,
@@ -380,7 +467,7 @@ void delete_ust_app_channel(int sock, struct ust_app_channel *ua_chan,
        /* Wipe stream */
        cds_list_for_each_entry_safe(stream, stmp, &ua_chan->streams.head, list) {
                cds_list_del(&stream->list);
-               delete_ust_app_stream(sock, stream);
+               delete_ust_app_stream(sock, stream, app);
        }
 
        /* Wipe context */
@@ -388,7 +475,7 @@ void delete_ust_app_channel(int sock, struct ust_app_channel *ua_chan,
                cds_list_del(&ua_ctx->list);
                ret = lttng_ht_del(ua_chan->ctx, &iter);
                assert(!ret);
-               delete_ust_app_ctx(sock, ua_ctx);
+               delete_ust_app_ctx(sock, ua_ctx, app);
        }
 
        /* Wipe events */
@@ -396,14 +483,23 @@ void delete_ust_app_channel(int sock, struct ust_app_channel *ua_chan,
                        node.node) {
                ret = lttng_ht_del(ua_chan->events, &iter);
                assert(!ret);
-               delete_ust_app_event(sock, ua_event);
+               delete_ust_app_event(sock, ua_event, app);
        }
 
        if (ua_chan->session->buffer_type == LTTNG_BUFFER_PER_PID) {
                /* Wipe and free registry from session registry. */
                registry = get_session_registry(ua_chan->session);
                if (registry) {
-                       ust_registry_channel_del_free(registry, ua_chan->key);
+                       ust_registry_channel_del_free(registry, ua_chan->key,
+                               sock >= 0);
+               }
+               /*
+                * A negative socket can be used by the caller when
+                * cleaning-up a ua_chan in an error path. Skip the
+                * accounting in this case.
+                */
+               if (sock >= 0) {
+                       save_per_pid_lost_discarded_counters(ua_chan);
                }
        }
 
@@ -412,7 +508,9 @@ void delete_ust_app_channel(int sock, struct ust_app_channel *ua_chan,
                iter.iter.node = &ua_chan->ust_objd_node.node;
                ret = lttng_ht_del(app->ust_objd, &iter);
                assert(!ret);
+               pthread_mutex_lock(&app->sock_lock);
                ret = ustctl_release_object(sock, ua_chan->obj);
+               pthread_mutex_unlock(&app->sock_lock);
                if (ret < 0 && ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
                        ERR("UST app sock %d release channel obj failed with ret %d",
                                        sock, ret);
@@ -423,6 +521,33 @@ void delete_ust_app_channel(int sock, struct ust_app_channel *ua_chan,
        call_rcu(&ua_chan->rcu_head, delete_ust_app_channel_rcu);
 }
 
+int ust_app_register_done(struct ust_app *app)
+{
+       int ret;
+
+       pthread_mutex_lock(&app->sock_lock);
+       ret = ustctl_register_done(app->sock);
+       pthread_mutex_unlock(&app->sock_lock);
+       return ret;
+}
+
+int ust_app_release_object(struct ust_app *app, struct lttng_ust_object_data *data)
+{
+       int ret, sock;
+
+       if (app) {
+               pthread_mutex_lock(&app->sock_lock);
+               sock = app->sock;
+       } else {
+               sock = -1;
+       }
+       ret = ustctl_release_object(sock, data);
+       if (app) {
+               pthread_mutex_unlock(&app->sock_lock);
+       }
+       return ret;
+}
+
 /*
  * Push metadata to consumer socket.
  *
@@ -440,34 +565,27 @@ ssize_t ust_app_push_metadata(struct ust_registry_session *registry,
 {
        int ret;
        char *metadata_str = NULL;
-       size_t len, offset;
+       size_t len, offset, new_metadata_len_sent;
        ssize_t ret_val;
+       uint64_t metadata_key, metadata_version;
 
        assert(registry);
        assert(socket);
 
+       metadata_key = registry->metadata_key;
+
        /*
         * Means that no metadata was assigned to the session. This can
         * happens if no start has been done previously.
         */
-       if (!registry->metadata_key) {
+       if (!metadata_key) {
                return 0;
        }
 
-       /*
-        * On a push metadata error either the consumer is dead or the
-        * metadata channel has been destroyed because its endpoint
-        * might have died (e.g: relayd), or because the application has
-        * exited. If so, the metadata closed flag is set to 1 so we
-        * deny pushing metadata again which is not valid anymore on the
-        * consumer side.
-        */
-       if (registry->metadata_closed) {
-               return -EPIPE;
-       }
-
        offset = registry->metadata_len_sent;
        len = registry->metadata_len - registry->metadata_len_sent;
+       new_metadata_len_sent = registry->metadata_len;
+       metadata_version = registry->metadata_version;
        if (len == 0) {
                DBG3("No metadata to push for metadata key %" PRIu64,
                                registry->metadata_key);
@@ -486,13 +604,26 @@ ssize_t ust_app_push_metadata(struct ust_registry_session *registry,
                ret_val = -ENOMEM;
                goto error;
        }
-       /* Copy what we haven't send out. */
+       /* Copy what we haven't sent out. */
        memcpy(metadata_str, registry->metadata + offset, len);
-       registry->metadata_len_sent += len;
 
 push_data:
-       ret = consumer_push_metadata(socket, registry->metadata_key,
-                       metadata_str, len, offset);
+       pthread_mutex_unlock(&registry->lock);
+       /*
+        * We need to unlock the registry while we push metadata to
+        * break a circular dependency between the consumerd metadata
+        * lock and the sessiond registry lock. Indeed, pushing metadata
+        * to the consumerd awaits that it gets pushed all the way to
+        * relayd, but doing so requires grabbing the metadata lock. If
+        * a concurrent metadata request is being performed by
+        * consumerd, this can try to grab the registry lock on the
+        * sessiond while holding the metadata lock on the consumer
+        * daemon. Those push and pull schemes are performed on two
+        * different bidirectionnal communication sockets.
+        */
+       ret = consumer_push_metadata(socket, metadata_key,
+                       metadata_str, len, offset, metadata_version);
+       pthread_mutex_lock(&registry->lock);
        if (ret < 0) {
                /*
                 * There is an acceptable race here between the registry
@@ -510,17 +641,29 @@ push_data:
                 */
                if (ret == -LTTCOMM_CONSUMERD_CHANNEL_FAIL) {
                        ret = 0;
+               } else {
+                       ERR("Error pushing metadata to consumer");
                }
-
-               /*
-                * Update back the actual metadata len sent since it
-                * failed here.
-                */
-               registry->metadata_len_sent -= len;
                ret_val = ret;
                goto error_push;
+       } else {
+               /*
+                * Metadata may have been concurrently pushed, since
+                * we're not holding the registry lock while pushing to
+                * consumer.  This is handled by the fact that we send
+                * the metadata content, size, and the offset at which
+                * that metadata belongs. This may arrive out of order
+                * on the consumer side, and the consumer is able to
+                * deal with overlapping fragments. The consumer
+                * supports overlapping fragments, which must be
+                * contiguous starting from offset 0. We keep the
+                * largest metadata_len_sent value of the concurrent
+                * send.
+                */
+               registry->metadata_len_sent =
+                       max_t(size_t, registry->metadata_len_sent,
+                               new_metadata_len_sent);
        }
-
        free(metadata_str);
        return len;
 
@@ -663,6 +806,8 @@ void delete_ust_app_session_rcu(struct rcu_head *head)
 /*
  * Delete ust app session safely. RCU read lock must be held before calling
  * this function.
+ *
+ * The session list lock must be held by the caller.
  */
 static
 void delete_ust_app_session(int sock, struct ust_app_session *ua_sess,
@@ -681,6 +826,7 @@ void delete_ust_app_session(int sock, struct ust_app_session *ua_sess,
        ua_sess->deleted = true;
 
        registry = get_session_registry(ua_sess);
+       /* Registry can be null on error path during initialization. */
        if (registry) {
                /* Push metadata for application before freeing the application. */
                (void) push_metadata(registry, ua_sess->consumer);
@@ -708,18 +854,29 @@ void delete_ust_app_session(int sock, struct ust_app_session *ua_sess,
        if (ua_sess->buffer_type == LTTNG_BUFFER_PER_PID) {
                struct buffer_reg_pid *reg_pid = buffer_reg_pid_find(ua_sess->id);
                if (reg_pid) {
+                       /*
+                        * Registry can be null on error path during
+                        * initialization.
+                        */
                        buffer_reg_pid_remove(reg_pid);
                        buffer_reg_pid_destroy(reg_pid);
                }
        }
 
        if (ua_sess->handle != -1) {
+               pthread_mutex_lock(&app->sock_lock);
                ret = ustctl_release_handle(sock, ua_sess->handle);
+               pthread_mutex_unlock(&app->sock_lock);
                if (ret < 0 && ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
                        ERR("UST app sock %d release session handle failed with ret %d",
                                        sock, ret);
                }
+               /* Remove session from application UST object descriptor. */
+               iter.iter.node = &ua_sess->ust_objd_node.node;
+               ret = lttng_ht_del(app->ust_sessions_objd, &iter);
+               assert(!ret);
        }
+
        pthread_mutex_unlock(&ua_sess->lock);
 
        consumer_output_put(ua_sess->consumer);
@@ -739,6 +896,11 @@ void delete_ust_app(struct ust_app *app)
        int ret, sock;
        struct ust_app_session *ua_sess, *tmp_ua_sess;
 
+       /*
+        * The session list lock must be held during this function to guarantee
+        * the existence of ua_sess.
+        */
+       session_lock_list();
        /* Delete ust app sessions info */
        sock = app->sock;
        app->sock = -1;
@@ -753,6 +915,7 @@ void delete_ust_app(struct ust_app *app)
        }
 
        ht_cleanup_push(app->sessions);
+       ht_cleanup_push(app->ust_sessions_objd);
        ht_cleanup_push(app->ust_objd);
 
        /*
@@ -776,6 +939,7 @@ void delete_ust_app(struct ust_app *app)
 
        DBG2("UST app pid %d deleted", app->pid);
        free(app);
+       session_unlock_list();
 }
 
 /*
@@ -796,6 +960,8 @@ void delete_ust_app_rcu(struct rcu_head *head)
 /*
  * Delete the session from the application ht and delete the data structure by
  * freeing every object inside and releasing them.
+ *
+ * The session list lock must be held by the caller.
  */
 static void destroy_app_session(struct ust_app *app,
                struct ust_app_session *ua_sess)
@@ -824,7 +990,7 @@ end:
  * Alloc new UST app session.
  */
 static
-struct ust_app_session *alloc_ust_app_session(struct ust_app *app)
+struct ust_app_session *alloc_ust_app_session(void)
 {
        struct ust_app_session *ua_sess;
 
@@ -887,6 +1053,7 @@ struct ust_app_channel *alloc_ust_app_channel(char *name,
                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;
+               ua_chan->attr.blocking_timeout = attr->u.s.blocking_timeout;
        }
        /* By default, the channel is a per cpu channel. */
        ua_chan->attr.type = LTTNG_UST_CHAN_PER_CPU;
@@ -959,7 +1126,7 @@ error:
  * Alloc new UST app context.
  */
 static
-struct ust_app_ctx *alloc_ust_app_ctx(struct lttng_ust_context *uctx)
+struct ust_app_ctx *alloc_ust_app_ctx(struct lttng_ust_context_attr *uctx)
 {
        struct ust_app_ctx *ua_ctx;
 
@@ -972,12 +1139,27 @@ struct ust_app_ctx *alloc_ust_app_ctx(struct lttng_ust_context *uctx)
 
        if (uctx) {
                memcpy(&ua_ctx->ctx, uctx, sizeof(ua_ctx->ctx));
+               if (uctx->ctx == LTTNG_UST_CONTEXT_APP_CONTEXT) {
+                       char *provider_name = NULL, *ctx_name = NULL;
+
+                       provider_name = strdup(uctx->u.app_ctx.provider_name);
+                       ctx_name = strdup(uctx->u.app_ctx.ctx_name);
+                       if (!provider_name || !ctx_name) {
+                               free(provider_name);
+                               free(ctx_name);
+                               goto error;
+                       }
+
+                       ua_ctx->ctx.u.app_ctx.provider_name = provider_name;
+                       ua_ctx->ctx.u.app_ctx.ctx_name = ctx_name;
+               }
        }
 
        DBG3("UST app context %d allocated", ua_ctx->ctx.ctx);
-
-error:
        return ua_ctx;
+error:
+       free(ua_ctx);
+       return NULL;
 }
 
 /*
@@ -1079,7 +1261,8 @@ error:
  * Return an ust_app_event object or NULL on error.
  */
 static struct ust_app_event *find_ust_app_event(struct lttng_ht *ht,
-               char *name, struct lttng_filter_bytecode *filter, int loglevel,
+               const char *name, const struct lttng_filter_bytecode *filter,
+               int loglevel_value,
                const struct lttng_event_exclusion *exclusion)
 {
        struct lttng_ht_iter iter;
@@ -1093,7 +1276,7 @@ static struct ust_app_event *find_ust_app_event(struct lttng_ht *ht,
        /* Setup key for event lookup. */
        key.name = name;
        key.filter = filter;
-       key.loglevel = loglevel;
+       key.loglevel_type = loglevel_value;
        /* lttng_event_exclusion and lttng_ust_event_exclusion structures are similar */
        key.exclusion = exclusion;
 
@@ -1124,8 +1307,10 @@ int create_ust_channel_context(struct ust_app_channel *ua_chan,
 
        health_code_update();
 
+       pthread_mutex_lock(&app->sock_lock);
        ret = ustctl_add_context(app->sock, &ua_ctx->ctx,
                        ua_chan->obj, &ua_ctx->obj);
+       pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
                        ERR("UST app create channel context failed for app (pid: %d) "
@@ -1137,7 +1322,7 @@ int create_ust_channel_context(struct ust_app_channel *ua_chan,
                         * continue normally.
                         */
                        ret = 0;
-                       DBG3("UST app disable event failed. Application is dead.");
+                       DBG3("UST app add context failed. Application is dead.");
                }
                goto error;
        }
@@ -1174,8 +1359,10 @@ int set_ust_event_filter(struct ust_app_event *ua_event,
                ret = -LTTNG_ERR_NOMEM;
                goto error;
        }
+       pthread_mutex_lock(&app->sock_lock);
        ret = ustctl_set_filter(app->sock, ust_bytecode,
                        ua_event->obj);
+       pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
                        ERR("UST app event %s filter failed for app (pid: %d) "
@@ -1244,7 +1431,9 @@ int set_ust_event_exclusion(struct ust_app_event *ua_event,
                ret = -LTTNG_ERR_NOMEM;
                goto error;
        }
+       pthread_mutex_lock(&app->sock_lock);
        ret = ustctl_set_exclusion(app->sock, ust_exclusion, ua_event->obj);
+       pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
                        ERR("UST app event %s exclusions failed for app (pid: %d) "
@@ -1279,7 +1468,9 @@ static int disable_ust_event(struct ust_app *app,
 
        health_code_update();
 
+       pthread_mutex_lock(&app->sock_lock);
        ret = ustctl_disable(app->sock, ua_event->obj);
+       pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
                        ERR("UST app event %s disable failed for app (pid: %d) "
@@ -1315,7 +1506,9 @@ static int disable_ust_channel(struct ust_app *app,
 
        health_code_update();
 
+       pthread_mutex_lock(&app->sock_lock);
        ret = ustctl_disable(app->sock, ua_chan->obj);
+       pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
                        ERR("UST app channel %s disable failed for app (pid: %d) "
@@ -1351,7 +1544,9 @@ static int enable_ust_channel(struct ust_app *app,
 
        health_code_update();
 
+       pthread_mutex_lock(&app->sock_lock);
        ret = ustctl_enable(app->sock, ua_chan->obj);
+       pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
                        ERR("UST app channel %s enable failed for app (pid: %d) "
@@ -1389,7 +1584,9 @@ static int enable_ust_event(struct ust_app *app,
 
        health_code_update();
 
+       pthread_mutex_lock(&app->sock_lock);
        ret = ustctl_enable(app->sock, ua_event->obj);
+       pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
                        ERR("UST app event %s enable failed for app (pid: %d) "
@@ -1457,7 +1654,7 @@ static int send_channel_pid_to_ust(struct ust_app *app,
                }
                /* We don't need the stream anymore once sent to the tracer. */
                cds_list_del(&stream->list);
-               delete_ust_app_stream(-1, stream);
+               delete_ust_app_stream(-1, stream, app);
        }
        /* Flag the channel that it is sent to the application. */
        ua_chan->is_sent = 1;
@@ -1481,10 +1678,13 @@ int create_ust_event(struct ust_app *app, struct ust_app_session *ua_sess,
        health_code_update();
 
        /* Create UST event on tracer */
+       pthread_mutex_lock(&app->sock_lock);
        ret = ustctl_create_event(app->sock, &ua_event->attr, ua_chan->obj,
                        &ua_event->obj);
+       pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
+                       abort();
                        ERR("Error ustctl create event %s for app pid: %d with ret %d",
                                        ua_event->attr.name, app->pid, ret);
                } else {
@@ -1597,12 +1797,6 @@ static void shadow_copy_event(struct ust_app_event *ua_event,
 static void shadow_copy_channel(struct ust_app_channel *ua_chan,
                struct ltt_ust_channel *uchan)
 {
-       struct lttng_ht_iter iter;
-       struct ltt_ust_event *uevent;
-       struct ltt_ust_context *uctx;
-       struct ust_app_event *ua_event;
-       struct ust_app_ctx *ua_ctx;
-
        DBG2("UST app shadow copy of channel %s started", ua_chan->name);
 
        strncpy(ua_chan->name, uchan->name, sizeof(ua_chan->name));
@@ -1617,7 +1811,10 @@ static void shadow_copy_channel(struct ust_app_channel *ua_chan,
        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->monitor_timer_interval = uchan->monitor_timer_interval;
        ua_chan->attr.output = uchan->attr.output;
+       ua_chan->attr.blocking_timeout = uchan->attr.u.s.blocking_timeout;
+
        /*
         * Note that the attribute channel type is not set since the channel on the
         * tracing registry side does not have this information.
@@ -1626,33 +1823,6 @@ static void shadow_copy_channel(struct ust_app_channel *ua_chan,
        ua_chan->enabled = uchan->enabled;
        ua_chan->tracing_channel_id = uchan->id;
 
-       cds_list_for_each_entry(uctx, &uchan->ctx_list, list) {
-               ua_ctx = alloc_ust_app_ctx(&uctx->ctx);
-               if (ua_ctx == NULL) {
-                       continue;
-               }
-               lttng_ht_node_init_ulong(&ua_ctx->node,
-                               (unsigned long) ua_ctx->ctx.ctx);
-               lttng_ht_add_ulong(ua_chan->ctx, &ua_ctx->node);
-               cds_list_add_tail(&ua_ctx->list, &ua_chan->ctx_list);
-       }
-
-       /* Copy all events from ltt ust channel to ust app channel */
-       cds_lfht_for_each_entry(uchan->events->ht, &iter.iter, uevent, node.node) {
-               ua_event = find_ust_app_event(ua_chan->events, uevent->attr.name,
-                               uevent->filter, uevent->attr.loglevel, uevent->exclusion);
-               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);
-                       if (ua_event == NULL) {
-                               continue;
-                       }
-                       shadow_copy_event(ua_event, uevent);
-                       add_unique_ust_app_event(ua_chan, ua_event);
-               }
-       }
-
        DBG3("UST app shadow copy of channel %s done", ua_chan->name);
 }
 
@@ -1662,29 +1832,22 @@ static void shadow_copy_channel(struct ust_app_channel *ua_chan,
 static void shadow_copy_session(struct ust_app_session *ua_sess,
                struct ltt_ust_session *usess, struct ust_app *app)
 {
-       struct lttng_ht_node_str *ua_chan_node;
-       struct lttng_ht_iter iter;
-       struct ltt_ust_channel *uchan;
-       struct ust_app_channel *ua_chan;
-       time_t rawtime;
        struct tm *timeinfo;
        char datetime[16];
        int ret;
        char tmp_shm_path[PATH_MAX];
 
-       /* Get date and time for unique app path */
-       time(&rawtime);
-       timeinfo = localtime(&rawtime);
+       timeinfo = localtime(&app->registration_time);
        strftime(datetime, sizeof(datetime), "%Y%m%d-%H%M%S", timeinfo);
 
        DBG2("Shadow copy of session handle %d", ua_sess->handle);
 
        ua_sess->tracing_id = usess->id;
        ua_sess->id = get_next_session_id();
-       ua_sess->uid = app->uid;
-       ua_sess->gid = app->gid;
-       ua_sess->euid = usess->uid;
-       ua_sess->egid = usess->gid;
+       ua_sess->real_credentials.uid = app->uid;
+       ua_sess->real_credentials.gid = app->gid;
+       ua_sess->effective_credentials.uid = usess->uid;
+       ua_sess->effective_credentials.gid = usess->gid;
        ua_sess->buffer_type = usess->buffer_type;
        ua_sess->bits_per_long = app->bits_per_long;
 
@@ -1705,7 +1868,9 @@ static void shadow_copy_session(struct ust_app_session *ua_sess,
                break;
        case LTTNG_BUFFER_PER_UID:
                ret = snprintf(ua_sess->path, sizeof(ua_sess->path),
-                               DEFAULT_UST_TRACE_UID_PATH, ua_sess->uid, app->bits_per_long);
+                               DEFAULT_UST_TRACE_UID_PATH,
+                               ua_sess->real_credentials.uid,
+                               app->bits_per_long);
                break;
        default:
                assert(0);
@@ -1748,36 +1913,6 @@ static void shadow_copy_session(struct ust_app_session *ua_sess,
                        sizeof(ua_sess->shm_path) - strlen(ua_sess->shm_path) - 1);
                ua_sess->shm_path[sizeof(ua_sess->shm_path) - 1] = '\0';
        }
-
-       /* Iterate over all channels in global domain. */
-       cds_lfht_for_each_entry(usess->domain_global.channels->ht, &iter.iter,
-                       uchan, node.node) {
-               struct lttng_ht_iter uiter;
-
-               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) {
-                       /* Session exist. Contiuing. */
-                       continue;
-               }
-
-               DBG2("Channel %s not found on shadow session copy, creating it",
-                               uchan->name);
-               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);
-       }
        return;
 
 error:
@@ -1788,7 +1923,7 @@ error:
  * Lookup sesison wrapper.
  */
 static
-void __lookup_session_by_app(struct ltt_ust_session *usess,
+void __lookup_session_by_app(const struct ltt_ust_session *usess,
                        struct ust_app *app, struct lttng_ht_iter *iter)
 {
        /* Get right UST app session from app */
@@ -1800,7 +1935,7 @@ void __lookup_session_by_app(struct ltt_ust_session *usess,
  * id.
  */
 static struct ust_app_session *lookup_session_by_app(
-               struct ltt_ust_session *usess, struct ust_app *app)
+               const struct ltt_ust_session *usess, struct ust_app *app)
 {
        struct lttng_ht_iter iter;
        struct lttng_ht_node_u64 *node;
@@ -1855,10 +1990,11 @@ static int setup_buffer_reg_pid(struct ust_app_session *ua_sess,
                        app->bits_per_long, app->uint8_t_alignment,
                        app->uint16_t_alignment, app->uint32_t_alignment,
                        app->uint64_t_alignment, app->long_alignment,
-                       app->byte_order, app->version.major,
-                       app->version.minor, reg_pid->root_shm_path,
-                       reg_pid->shm_path,
-                       ua_sess->euid, ua_sess->egid);
+                       app->byte_order, app->version.major, app->version.minor,
+                       reg_pid->root_shm_path, reg_pid->shm_path,
+                       ua_sess->effective_credentials.uid,
+                       ua_sess->effective_credentials.gid, ua_sess->tracing_id,
+                       app->uid);
        if (ret < 0) {
                /*
                 * reg_pid->registry->reg.ust is NULL upon error, so we need to
@@ -1925,7 +2061,8 @@ static int setup_buffer_reg_uid(struct ltt_ust_session *usess,
                        app->uint64_t_alignment, app->long_alignment,
                        app->byte_order, app->version.major,
                        app->version.minor, reg_uid->root_shm_path,
-                       reg_uid->shm_path, usess->uid, usess->gid);
+                       reg_uid->shm_path, usess->uid, usess->gid,
+                       ua_sess->tracing_id, app->uid);
        if (ret < 0) {
                /*
                 * reg_uid->registry->reg.ust is NULL upon error, so we need to
@@ -1962,7 +2099,7 @@ 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 int create_ust_app_session(struct ltt_ust_session *usess,
+static int find_or_create_ust_app_session(struct ltt_ust_session *usess,
                struct ust_app *app, struct ust_app_session **ua_sess_ptr,
                int *is_created)
 {
@@ -1979,7 +2116,7 @@ static int create_ust_app_session(struct ltt_ust_session *usess,
        if (ua_sess == NULL) {
                DBG2("UST app pid: %d session id %" PRIu64 " not found, creating it",
                                app->pid, usess->id);
-               ua_sess = alloc_ust_app_session(app);
+               ua_sess = alloc_ust_app_session();
                if (ua_sess == NULL) {
                        /* Only malloc can failed so something is really wrong */
                        ret = -ENOMEM;
@@ -2015,7 +2152,9 @@ static int create_ust_app_session(struct ltt_ust_session *usess,
        health_code_update();
 
        if (ua_sess->handle == -1) {
+               pthread_mutex_lock(&app->sock_lock);
                ret = ustctl_create_session(app->sock);
+               pthread_mutex_unlock(&app->sock_lock);
                if (ret < 0) {
                        if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
                                ERR("Creating session for app pid %d with ret %d",
@@ -2047,6 +2186,9 @@ static int create_ust_app_session(struct ltt_ust_session *usess,
                lttng_ht_node_init_u64(&ua_sess->node,
                                ua_sess->tracing_id);
                lttng_ht_add_unique_u64(app->sessions, &ua_sess->node);
+               lttng_ht_node_init_ulong(&ua_sess->ust_objd_node, ua_sess->handle);
+               lttng_ht_add_unique_ulong(app->ust_sessions_objd,
+                               &ua_sess->ust_objd_node);
 
                DBG2("UST app session created successfully with handle %d", ret);
        }
@@ -2073,7 +2215,7 @@ error:
 static int ht_match_ust_app_ctx(struct cds_lfht_node *node, const void *_key)
 {
        struct ust_app_ctx *ctx;
-       const struct lttng_ust_context *key;
+       const struct lttng_ust_context_attr *key;
 
        assert(node);
        assert(_key);
@@ -2086,13 +2228,24 @@ static int ht_match_ust_app_ctx(struct cds_lfht_node *node, const void *_key)
                goto no_match;
        }
 
-       /* Check the name in the case of perf thread counters. */
-       if (key->ctx == LTTNG_UST_CONTEXT_PERF_THREAD_COUNTER) {
+       switch(key->ctx) {
+       case LTTNG_UST_CONTEXT_PERF_THREAD_COUNTER:
                if (strncmp(key->u.perf_counter.name,
-                       ctx->ctx.u.perf_counter.name,
-                       sizeof(key->u.perf_counter.name))) {
+                               ctx->ctx.u.perf_counter.name,
+                               sizeof(key->u.perf_counter.name))) {
+                       goto no_match;
+               }
+               break;
+       case LTTNG_UST_CONTEXT_APP_CONTEXT:
+               if (strcmp(key->u.app_ctx.provider_name,
+                               ctx->ctx.u.app_ctx.provider_name) ||
+                               strcmp(key->u.app_ctx.ctx_name,
+                               ctx->ctx.u.app_ctx.ctx_name)) {
                        goto no_match;
                }
+               break;
+       default:
+               break;
        }
 
        /* Match. */
@@ -2110,7 +2263,7 @@ no_match:
  */
 static
 struct ust_app_ctx *find_ust_app_context(struct lttng_ht *ht,
-               struct lttng_ust_context *uctx)
+               struct lttng_ust_context_attr *uctx)
 {
        struct lttng_ht_iter iter;
        struct lttng_ht_node_ulong *node;
@@ -2139,8 +2292,8 @@ end:
  * Called with UST app session lock held and a RCU read side lock.
  */
 static
-int create_ust_app_channel_context(struct ust_app_session *ua_sess,
-               struct ust_app_channel *ua_chan, struct lttng_ust_context *uctx,
+int create_ust_app_channel_context(struct ust_app_channel *ua_chan,
+               struct lttng_ust_context_attr *uctx,
                struct ust_app *app)
 {
        int ret = 0;
@@ -2157,7 +2310,7 @@ int create_ust_app_channel_context(struct ust_app_session *ua_sess,
        ua_ctx = alloc_ust_app_ctx(uctx);
        if (ua_ctx == NULL) {
                /* malloc failed */
-               ret = -1;
+               ret = -ENOMEM;
                goto error;
        }
 
@@ -2269,11 +2422,14 @@ error:
 /*
  * Ask the consumer to create a channel and get it if successful.
  *
+ * Called with UST app session lock held.
+ *
  * Return 0 on success or else a negative value.
  */
 static int do_consumer_create_channel(struct ltt_ust_session *usess,
                struct ust_app_session *ua_sess, struct ust_app_channel *ua_chan,
-               int bitness, struct ust_registry_session *registry)
+               int bitness, struct ust_registry_session *registry,
+               uint64_t trace_archive_id)
 {
        int ret;
        unsigned int nb_fd = 0;
@@ -2308,7 +2464,7 @@ static int do_consumer_create_channel(struct ltt_ust_session *usess,
         * stream we have to expect.
         */
        ret = ust_consumer_ask_channel(ua_sess, ua_chan, usess->consumer, socket,
-                       registry);
+                       registry, usess->current_trace_chunk);
        if (ret < 0) {
                goto error_ask;
        }
@@ -2441,7 +2597,8 @@ error_fd_get:
  * Return 0 on success or else a negative value.
  */
 static int setup_buffer_reg_streams(struct buffer_reg_channel *reg_chan,
-               struct ust_app_channel *ua_chan)
+               struct ust_app_channel *ua_chan,
+               struct ust_app *app)
 {
        int ret = 0;
        struct ust_app_stream *stream, *stmp;
@@ -2470,7 +2627,7 @@ static int setup_buffer_reg_streams(struct buffer_reg_channel *reg_chan,
 
                /* We don't need the streams anymore. */
                cds_list_del(&stream->list);
-               delete_ust_app_stream(-1, stream);
+               delete_ust_app_stream(-1, stream, app);
        }
 
 error:
@@ -2534,7 +2691,8 @@ error_create:
  * Return 0 on success else a negative value.
  */
 static int setup_buffer_reg_channel(struct buffer_reg_session *reg_sess,
-               struct ust_app_channel *ua_chan, struct buffer_reg_channel *reg_chan)
+               struct ust_app_channel *ua_chan, struct buffer_reg_channel *reg_chan,
+               struct ust_app *app)
 {
        int ret;
 
@@ -2546,7 +2704,7 @@ static int setup_buffer_reg_channel(struct buffer_reg_session *reg_sess,
        DBG2("UST app setup buffer registry channel for %s", ua_chan->name);
 
        /* Setup all streams for the registry. */
-       ret = setup_buffer_reg_streams(reg_chan, ua_chan);
+       ret = setup_buffer_reg_streams(reg_chan, ua_chan, app);
        if (ret < 0) {
                goto error;
        }
@@ -2609,12 +2767,9 @@ static int send_channel_uid_to_ust(struct buffer_reg_channel *reg_chan,
 
                ret = ust_consumer_send_stream_to_ust(app, ua_chan, &stream);
                if (ret < 0) {
-                       (void) release_ust_app_stream(-1, &stream);
+                       (void) release_ust_app_stream(-1, &stream, app);
                        if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                                ret = -ENOTCONN; /* Caused by app exiting. */
-                               goto error_stream_unlock;
-                       } else if (ret < 0) {
-                               goto error_stream_unlock;
                        }
                        goto error_stream_unlock;
                }
@@ -2623,7 +2778,7 @@ static int send_channel_uid_to_ust(struct buffer_reg_channel *reg_chan,
                 * The return value is not important here. This function will output an
                 * error if needed.
                 */
-               (void) release_ust_app_stream(-1, &stream);
+               (void) release_ust_app_stream(-1, &stream, app);
        }
        ua_chan->is_sent = 1;
 
@@ -2636,6 +2791,9 @@ error:
 /*
  * Create and send to the application the created buffers with per UID buffers.
  *
+ * This MUST be called with a RCU read side lock acquired.
+ * The session list lock and the session's lock must be acquired.
+ *
  * Return 0 on success else a negative value.
  */
 static int create_channel_per_uid(struct ust_app *app,
@@ -2645,6 +2803,9 @@ static int create_channel_per_uid(struct ust_app *app,
        int ret;
        struct buffer_reg_uid *reg_uid;
        struct buffer_reg_channel *reg_chan;
+       struct ltt_session *session = NULL;
+       enum lttng_error_code notification_ret;
+       struct ust_registry_channel *chan_reg;
 
        assert(app);
        assert(usess);
@@ -2663,49 +2824,77 @@ static int create_channel_per_uid(struct ust_app *app,
 
        reg_chan = buffer_reg_channel_find(ua_chan->tracing_channel_id,
                        reg_uid);
-       if (!reg_chan) {
-               /* Create the buffer registry channel object. */
-               ret = create_buffer_reg_channel(reg_uid->registry, ua_chan, &reg_chan);
-               if (ret < 0) {
-                       ERR("Error creating the UST channel \"%s\" registry instance",
-                               ua_chan->name);
-                       goto error;
-               }
-               assert(reg_chan);
+       if (reg_chan) {
+               goto send_channel;
+       }
 
-               /*
-                * Create the buffers on the consumer side. This call populates the
-                * ust app channel object with all streams and data object.
-                */
-               ret = do_consumer_create_channel(usess, ua_sess, ua_chan,
-                               app->bits_per_long, reg_uid->registry->reg.ust);
-               if (ret < 0) {
-                       ERR("Error creating UST channel \"%s\" on the consumer daemon",
+       /* Create the buffer registry channel object. */
+       ret = create_buffer_reg_channel(reg_uid->registry, ua_chan, &reg_chan);
+       if (ret < 0) {
+               ERR("Error creating the UST channel \"%s\" registry instance",
                                ua_chan->name);
+               goto error;
+       }
 
-                       /*
-                        * Let's remove the previously created buffer registry channel so
-                        * it's not visible anymore in the session registry.
-                        */
-                       ust_registry_channel_del_free(reg_uid->registry->reg.ust,
-                                       ua_chan->tracing_channel_id);
-                       buffer_reg_channel_remove(reg_uid->registry, reg_chan);
-                       buffer_reg_channel_destroy(reg_chan, LTTNG_DOMAIN_UST);
-                       goto error;
-               }
+       session = session_find_by_id(ua_sess->tracing_id);
+       assert(session);
+       assert(pthread_mutex_trylock(&session->lock));
+       assert(session_trylock_list());
+
+       /*
+        * Create the buffers on the consumer side. This call populates the
+        * ust app channel object with all streams and data object.
+        */
+       ret = do_consumer_create_channel(usess, ua_sess, ua_chan,
+                       app->bits_per_long, reg_uid->registry->reg.ust,
+                       session->most_recent_chunk_id.value);
+       if (ret < 0) {
+               ERR("Error creating UST channel \"%s\" on the consumer daemon",
+                               ua_chan->name);
 
                /*
-                * Setup the streams and add it to the session registry.
+                * Let's remove the previously created buffer registry channel so
+                * it's not visible anymore in the session registry.
                 */
-               ret = setup_buffer_reg_channel(reg_uid->registry, ua_chan, reg_chan);
-               if (ret < 0) {
-                       ERR("Error setting up UST channel \"%s\"",
-                               ua_chan->name);
-                       goto error;
-               }
+               ust_registry_channel_del_free(reg_uid->registry->reg.ust,
+                               ua_chan->tracing_channel_id, false);
+               buffer_reg_channel_remove(reg_uid->registry, reg_chan);
+               buffer_reg_channel_destroy(reg_chan, LTTNG_DOMAIN_UST);
+               goto error;
+       }
+
+       /*
+        * Setup the streams and add it to the session registry.
+        */
+       ret = setup_buffer_reg_channel(reg_uid->registry,
+                       ua_chan, reg_chan, app);
+       if (ret < 0) {
+               ERR("Error setting up UST channel \"%s\"", ua_chan->name);
+               goto error;
+       }
 
+       /* Notify the notification subsystem of the channel's creation. */
+       pthread_mutex_lock(&reg_uid->registry->reg.ust->lock);
+       chan_reg = ust_registry_channel_find(reg_uid->registry->reg.ust,
+                       ua_chan->tracing_channel_id);
+       assert(chan_reg);
+       chan_reg->consumer_key = ua_chan->key;
+       chan_reg = NULL;
+       pthread_mutex_unlock(&reg_uid->registry->reg.ust->lock);
+
+       notification_ret = notification_thread_command_add_channel(
+                       notification_thread_handle, session->name,
+                       ua_sess->effective_credentials.uid,
+                       ua_sess->effective_credentials.gid, ua_chan->name,
+                       ua_chan->key, LTTNG_DOMAIN_UST,
+                       ua_chan->attr.subbuf_size * ua_chan->attr.num_subbuf);
+       if (notification_ret != LTTNG_OK) {
+               ret = - (int) notification_ret;
+               ERR("Failed to add channel to notification thread");
+               goto error;
        }
 
+send_channel:
        /* Send buffers to the application. */
        ret = send_channel_uid_to_ust(reg_chan, app, ua_sess, ua_chan);
        if (ret < 0) {
@@ -2716,12 +2905,18 @@ static int create_channel_per_uid(struct ust_app *app,
        }
 
 error:
+       if (session) {
+               session_put(session);
+       }
        return ret;
 }
 
 /*
  * Create and send to the application the created buffers with per PID buffers.
  *
+ * Called with UST app session lock held.
+ * The session list lock and the session's lock must be acquired.
+ *
  * Return 0 on success else a negative value.
  */
 static int create_channel_per_pid(struct ust_app *app,
@@ -2730,6 +2925,10 @@ static int create_channel_per_pid(struct ust_app *app,
 {
        int ret;
        struct ust_registry_session *registry;
+       enum lttng_error_code cmd_ret;
+       struct ltt_session *session = NULL;
+       uint64_t chan_reg_key;
+       struct ust_registry_channel *chan_reg;
 
        assert(app);
        assert(usess);
@@ -2741,6 +2940,7 @@ static int create_channel_per_pid(struct ust_app *app,
        rcu_read_lock();
 
        registry = get_session_registry(ua_sess);
+       /* The UST app session lock is held, registry shall not be null. */
        assert(registry);
 
        /* Create and add a new channel registry to session. */
@@ -2751,13 +2951,20 @@ static int create_channel_per_pid(struct ust_app *app,
                goto error;
        }
 
+       session = session_find_by_id(ua_sess->tracing_id);
+       assert(session);
+
+       assert(pthread_mutex_trylock(&session->lock));
+       assert(session_trylock_list());
+
        /* Create and get channel on the consumer side. */
        ret = do_consumer_create_channel(usess, ua_sess, ua_chan,
-                       app->bits_per_long, registry);
+                       app->bits_per_long, registry,
+                       session->most_recent_chunk_id.value);
        if (ret < 0) {
                ERR("Error creating UST channel \"%s\" on the consumer daemon",
                        ua_chan->name);
-               goto error;
+               goto error_remove_from_registry;
        }
 
        ret = send_channel_pid_to_ust(app, ua_sess, ua_chan);
@@ -2765,23 +2972,51 @@ static int create_channel_per_pid(struct ust_app *app,
                if (ret != -ENOTCONN) {
                        ERR("Error sending channel to application");
                }
-               goto error;
+               goto error_remove_from_registry;
+       }
+
+       chan_reg_key = ua_chan->key;
+       pthread_mutex_lock(&registry->lock);
+       chan_reg = ust_registry_channel_find(registry, chan_reg_key);
+       assert(chan_reg);
+       chan_reg->consumer_key = ua_chan->key;
+       pthread_mutex_unlock(&registry->lock);
+
+       cmd_ret = notification_thread_command_add_channel(
+                       notification_thread_handle, session->name,
+                       ua_sess->effective_credentials.uid,
+                       ua_sess->effective_credentials.gid, ua_chan->name,
+                       ua_chan->key, LTTNG_DOMAIN_UST,
+                       ua_chan->attr.subbuf_size * ua_chan->attr.num_subbuf);
+       if (cmd_ret != LTTNG_OK) {
+               ret = - (int) cmd_ret;
+               ERR("Failed to add channel to notification thread");
+               goto error_remove_from_registry;
        }
 
+error_remove_from_registry:
+       if (ret) {
+               ust_registry_channel_del_free(registry, ua_chan->key, false);
+       }
 error:
        rcu_read_unlock();
+       if (session) {
+               session_put(session);
+       }
        return ret;
 }
 
 /*
  * From an already allocated ust app channel, create the channel buffers if
- * need and send it to the application. This MUST be called with a RCU read
+ * needed and send them to the application. This MUST be called with a RCU read
  * side lock acquired.
  *
+ * Called with UST app session lock held.
+ *
  * Return 0 on success or else a negative value. Returns -ENOTCONN if
  * the application exited concurrently.
  */
-static int do_create_channel(struct ust_app *app,
+static int ust_app_channel_send(struct ust_app *app,
                struct ltt_ust_session *usess, struct ust_app_session *ua_sess,
                struct ust_app_channel *ua_chan)
 {
@@ -2789,6 +3024,7 @@ static int do_create_channel(struct ust_app *app,
 
        assert(app);
        assert(usess);
+       assert(usess->active);
        assert(ua_sess);
        assert(ua_chan);
 
@@ -2833,16 +3069,14 @@ error:
 }
 
 /*
- * Create UST app channel and create it on the tracer. Set ua_chanp of the
- * newly created channel if not NULL.
+ * Create UST app channel and return it through ua_chanp if not NULL.
  *
  * Called with UST app session lock and RCU read-side lock held.
  *
- * Return 0 on success or else a negative value. Returns -ENOTCONN if
- * the application exited concurrently.
+ * Return 0 on success or else a negative value.
  */
-static int create_ust_app_channel(struct ust_app_session *ua_sess,
-               struct ltt_ust_channel *uchan, struct ust_app *app,
+static int ust_app_channel_allocate(struct ust_app_session *ua_sess,
+               struct ltt_ust_channel *uchan,
                enum lttng_ust_chan_type type, struct ltt_ust_session *usess,
                struct ust_app_channel **ua_chanp)
 {
@@ -2863,24 +3097,15 @@ static int create_ust_app_channel(struct ust_app_session *ua_sess,
        if (ua_chan == NULL) {
                /* Only malloc can fail here */
                ret = -ENOMEM;
-               goto error_alloc;
+               goto error;
        }
        shadow_copy_channel(ua_chan, uchan);
 
        /* Set channel type. */
        ua_chan->attr.type = type;
 
-       ret = do_create_channel(app, usess, ua_sess, ua_chan);
-       if (ret < 0) {
-               goto error;
-       }
-
-       DBG2("UST app create channel %s for PID %d completed", ua_chan->name,
-                       app->pid);
-
        /* Only add the channel if successful on the tracer side. */
        lttng_ht_add_unique_str(ua_sess->channels, &ua_chan->node);
-
 end:
        if (ua_chanp) {
                *ua_chanp = ua_chan;
@@ -2890,8 +3115,6 @@ end:
        return 0;
 
 error:
-       delete_ust_app_channel(ua_chan->is_sent ? app->sock : -1, ua_chan, app);
-error_alloc:
        return ret;
 }
 
@@ -2908,15 +3131,6 @@ int create_ust_app_event(struct ust_app_session *ua_sess,
        int ret = 0;
        struct ust_app_event *ua_event;
 
-       /* Get event node */
-       ua_event = find_ust_app_event(ua_chan->events, uevent->attr.name,
-                       uevent->filter, uevent->attr.loglevel, uevent->exclusion);
-       if (ua_event != NULL) {
-               ret = -EEXIST;
-               goto end;
-       }
-
-       /* Does not exist so create one */
        ua_event = alloc_ust_app_event(uevent->attr.name, &uevent->attr);
        if (ua_event == NULL) {
                /* Only malloc can failed so something is really wrong */
@@ -2943,7 +3157,7 @@ end:
 
 error:
        /* Valid. Calling here is already in a read side lock */
-       delete_ust_app_event(-1, ua_event);
+       delete_ust_app_event(-1, ua_event, app);
        return ret;
 }
 
@@ -2959,12 +3173,14 @@ static int create_ust_app_metadata(struct ust_app_session *ua_sess,
        struct ust_app_channel *metadata;
        struct consumer_socket *socket;
        struct ust_registry_session *registry;
+       struct ltt_session *session = NULL;
 
        assert(ua_sess);
        assert(app);
        assert(consumer);
 
        registry = get_session_registry(ua_sess);
+       /* The UST app session is held registry shall not be null. */
        assert(registry);
 
        pthread_mutex_lock(&registry->lock);
@@ -3007,6 +3223,12 @@ static int create_ust_app_metadata(struct ust_app_session *ua_sess,
         */
        registry->metadata_key = metadata->key;
 
+       session = session_find_by_id(ua_sess->tracing_id);
+       assert(session);
+
+       assert(pthread_mutex_trylock(&session->lock));
+       assert(session_trylock_list());
+
        /*
         * Ask the metadata channel creation to the consumer. The metadata object
         * will be created by the consumer and kept their. However, the stream is
@@ -3014,7 +3236,7 @@ static int create_ust_app_metadata(struct ust_app_session *ua_sess,
         * consumer.
         */
        ret = ust_consumer_ask_channel(ua_sess, metadata, consumer, socket,
-                       registry);
+                       registry, session->current_trace_chunk);
        if (ret < 0) {
                /* Nullify the metadata key so we don't try to close it later on. */
                registry->metadata_key = 0;
@@ -3042,6 +3264,9 @@ error_consumer:
        delete_ust_app_channel(-1, metadata, app);
 error:
        pthread_mutex_unlock(&registry->lock);
+       if (session) {
+               session_put(session);
+       }
        return ret;
 }
 
@@ -3118,6 +3343,7 @@ struct ust_app *ust_app_create(struct ust_register_msg *msg, int sock)
        lta->v_minor = msg->minor;
        lta->sessions = lttng_ht_new(0, LTTNG_HT_TYPE_U64);
        lta->ust_objd = lttng_ht_new(0, LTTNG_HT_TYPE_ULONG);
+       lta->ust_sessions_objd = lttng_ht_new(0, LTTNG_HT_TYPE_ULONG);
        lta->notify_sock = -1;
 
        /* Copy name and make sure it's NULL terminated. */
@@ -3134,10 +3360,10 @@ struct ust_app *ust_app_create(struct ust_register_msg *msg, int sock)
        lta->pid = msg->pid;
        lttng_ht_node_init_ulong(&lta->pid_n, (unsigned long) lta->pid);
        lta->sock = sock;
+       pthread_mutex_init(&lta->sock_lock, NULL);
        lttng_ht_node_init_ulong(&lta->sock_n, (unsigned long) lta->sock);
 
        CDS_INIT_LIST_HEAD(&lta->teardown_head);
-
 error:
        return lta;
 }
@@ -3150,6 +3376,8 @@ void ust_app_add(struct ust_app *app)
        assert(app);
        assert(app->notify_sock >= 0);
 
+       app->registration_time = time(NULL);
+
        rcu_read_lock();
 
        /*
@@ -3189,7 +3417,9 @@ int ust_app_version(struct ust_app *app)
 
        assert(app);
 
+       pthread_mutex_lock(&app->sock_lock);
        ret = ustctl_tracer_version(app->sock, &app->version);
+       pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret != -LTTNG_UST_ERR_EXITING && ret != -EPIPE) {
                        ERR("UST app %d version failed with ret %d", app->sock, ret);
@@ -3296,8 +3526,8 @@ void ust_app_unregister(int sock)
        /*
         * 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.
+        * either way it's valid. The close of that socket is handled by the
+        * apps_notify_thread.
         */
        iter.iter.node = &lta->notify_sock_n.node;
        (void) lttng_ht_del(ust_app_ht_by_notify_sock, &iter);
@@ -3354,12 +3584,14 @@ int ust_app_list_events(struct lttng_event **events)
                         */
                        continue;
                }
+               pthread_mutex_lock(&app->sock_lock);
                handle = ustctl_tracepoint_list(app->sock);
                if (handle < 0) {
                        if (handle != -EPIPE && handle != -LTTNG_UST_ERR_EXITING) {
                                ERR("UST app list events getting handle failed for app pid %d",
                                                app->pid);
                        }
+                       pthread_mutex_unlock(&app->sock_lock);
                        continue;
                }
 
@@ -3367,6 +3599,8 @@ int ust_app_list_events(struct lttng_event **events)
                                        &uiter)) != -LTTNG_UST_ERR_NOENT) {
                        /* Handle ustctl error. */
                        if (ret < 0) {
+                               int release_ret;
+
                                if (ret != -LTTNG_UST_ERR_EXITING && ret != -EPIPE) {
                                        ERR("UST app tp list get failed for app %d with ret %d",
                                                        app->sock, ret);
@@ -3380,6 +3614,13 @@ int ust_app_list_events(struct lttng_event **events)
                                        break;
                                }
                                free(tmp_event);
+                               release_ret = ustctl_release_handle(app->sock, handle);
+                               if (release_ret < 0 &&
+                                               release_ret != -LTTNG_UST_ERR_EXITING &&
+                                               release_ret != -EPIPE) {
+                                       ERR("Error releasing app handle for app %d with ret %d", app->sock, release_ret);
+                               }
+                               pthread_mutex_unlock(&app->sock_lock);
                                goto rcu_error;
                        }
 
@@ -3395,9 +3636,18 @@ int ust_app_list_events(struct lttng_event **events)
                                new_tmp_event = realloc(tmp_event,
                                        new_nbmem * sizeof(struct lttng_event));
                                if (new_tmp_event == NULL) {
+                                       int release_ret;
+
                                        PERROR("realloc ust app events");
                                        free(tmp_event);
                                        ret = -ENOMEM;
+                                       release_ret = ustctl_release_handle(app->sock, handle);
+                                       if (release_ret < 0 &&
+                                                       release_ret != -LTTNG_UST_ERR_EXITING &&
+                                                       release_ret != -EPIPE) {
+                                               ERR("Error releasing app handle for app %d with ret %d", app->sock, release_ret);
+                                       }
+                                       pthread_mutex_unlock(&app->sock_lock);
                                        goto rcu_error;
                                }
                                /* Zero the new memory */
@@ -3413,6 +3663,11 @@ int ust_app_list_events(struct lttng_event **events)
                        tmp_event[count].enabled = -1;
                        count++;
                }
+               ret = ustctl_release_handle(app->sock, handle);
+               pthread_mutex_unlock(&app->sock_lock);
+               if (ret < 0 && ret != -LTTNG_UST_ERR_EXITING && ret != -EPIPE) {
+                       ERR("Error releasing app handle for app %d with ret %d", app->sock, ret);
+               }
        }
 
        ret = count;
@@ -3460,12 +3715,14 @@ int ust_app_list_event_fields(struct lttng_event_field **fields)
                         */
                        continue;
                }
+               pthread_mutex_lock(&app->sock_lock);
                handle = ustctl_tracepoint_field_list(app->sock);
                if (handle < 0) {
                        if (handle != -EPIPE && handle != -LTTNG_UST_ERR_EXITING) {
                                ERR("UST app list field getting handle failed for app pid %d",
                                                app->pid);
                        }
+                       pthread_mutex_unlock(&app->sock_lock);
                        continue;
                }
 
@@ -3473,6 +3730,8 @@ int ust_app_list_event_fields(struct lttng_event_field **fields)
                                        &uiter)) != -LTTNG_UST_ERR_NOENT) {
                        /* Handle ustctl error. */
                        if (ret < 0) {
+                               int release_ret;
+
                                if (ret != -LTTNG_UST_ERR_EXITING && ret != -EPIPE) {
                                        ERR("UST app tp list field failed for app %d with ret %d",
                                                        app->sock, ret);
@@ -3486,6 +3745,13 @@ int ust_app_list_event_fields(struct lttng_event_field **fields)
                                        break;
                                }
                                free(tmp_event);
+                               release_ret = ustctl_release_handle(app->sock, handle);
+                               pthread_mutex_unlock(&app->sock_lock);
+                               if (release_ret < 0 &&
+                                               release_ret != -LTTNG_UST_ERR_EXITING &&
+                                               release_ret != -EPIPE) {
+                                       ERR("Error releasing app handle for app %d with ret %d", app->sock, release_ret);
+                               }
                                goto rcu_error;
                        }
 
@@ -3501,9 +3767,18 @@ int ust_app_list_event_fields(struct lttng_event_field **fields)
                                new_tmp_event = realloc(tmp_event,
                                        new_nbmem * sizeof(struct lttng_event_field));
                                if (new_tmp_event == NULL) {
+                                       int release_ret;
+
                                        PERROR("realloc ust app event fields");
                                        free(tmp_event);
                                        ret = -ENOMEM;
+                                       release_ret = ustctl_release_handle(app->sock, handle);
+                                       pthread_mutex_unlock(&app->sock_lock);
+                                       if (release_ret &&
+                                                       release_ret != -LTTNG_UST_ERR_EXITING &&
+                                                       release_ret != -EPIPE) {
+                                               ERR("Error releasing app handle for app %d with ret %d", app->sock, release_ret);
+                                       }
                                        goto rcu_error;
                                }
                                /* Zero the new memory */
@@ -3525,6 +3800,13 @@ int ust_app_list_event_fields(struct lttng_event_field **fields)
                        tmp_event[count].event.enabled = -1;
                        count++;
                }
+               ret = ustctl_release_handle(app->sock, handle);
+               pthread_mutex_unlock(&app->sock_lock);
+               if (ret < 0 &&
+                               ret != -LTTNG_UST_ERR_EXITING &&
+                               ret != -EPIPE) {
+                       ERR("Error releasing app handle for app %d with ret %d", app->sock, ret);
+               }
        }
 
        ret = count;
@@ -3554,6 +3836,24 @@ void ust_app_clean_list(void)
 
        rcu_read_lock();
 
+       /* Cleanup notify socket hash table */
+       if (ust_app_ht_by_notify_sock) {
+               cds_lfht_for_each_entry(ust_app_ht_by_notify_sock->ht, &iter.iter, app,
+                               notify_sock_n.node) {
+                       struct cds_lfht_node *node;
+                       struct ust_app *app;
+
+                       node = cds_lfht_iter_get_node(&iter.iter);
+                       if (!node) {
+                               continue;
+                       }
+
+                       app = container_of(node, struct ust_app,
+                                       notify_sock_n.node);
+                       ust_app_notify_sock_unregister(app->notify_sock);
+               }
+       }
+
        if (ust_app_ht) {
                cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                        ret = lttng_ht_del(ust_app_ht, &iter);
@@ -3571,14 +3871,6 @@ void ust_app_clean_list(void)
                }
        }
 
-       /* Cleanup notify socket hash table */
-       if (ust_app_ht_by_notify_sock) {
-               cds_lfht_for_each_entry(ust_app_ht_by_notify_sock->ht, &iter.iter, app,
-                               notify_sock_n.node) {
-                       ret = lttng_ht_del(ust_app_ht_by_notify_sock, &iter);
-                       assert(!ret);
-               }
-       }
        rcu_read_unlock();
 
        /* Destroy is done only when the ht is empty */
@@ -3626,12 +3918,7 @@ int ust_app_disable_channel_glb(struct ltt_ust_session *usess,
        struct ust_app_session *ua_sess;
        struct ust_app_channel *ua_chan;
 
-       if (usess == NULL || uchan == NULL) {
-               ERR("Disabling UST global channel with NULL values");
-               ret = -1;
-               goto error;
-       }
-
+       assert(usess->active);
        DBG2("UST app disabling channel %s from global domain for session id %" PRIu64,
                        uchan->name, usess->id);
 
@@ -3671,8 +3958,6 @@ int ust_app_disable_channel_glb(struct ltt_ust_session *usess,
        }
 
        rcu_read_unlock();
-
-error:
        return ret;
 }
 
@@ -3687,12 +3972,7 @@ int ust_app_enable_channel_glb(struct ltt_ust_session *usess,
        struct ust_app *app;
        struct ust_app_session *ua_sess;
 
-       if (usess == NULL || uchan == NULL) {
-               ERR("Adding UST global channel to NULL values");
-               ret = -1;
-               goto error;
-       }
-
+       assert(usess->active);
        DBG2("UST app enabling channel %s to global domain for session id %" PRIu64,
                        uchan->name, usess->id);
 
@@ -3721,8 +4001,6 @@ int ust_app_enable_channel_glb(struct ltt_ust_session *usess,
        }
 
        rcu_read_unlock();
-
-error:
        return ret;
 }
 
@@ -3734,12 +4012,13 @@ int ust_app_disable_event_glb(struct ltt_ust_session *usess,
 {
        int ret = 0;
        struct lttng_ht_iter iter, uiter;
-       struct lttng_ht_node_str *ua_chan_node, *ua_event_node;
+       struct lttng_ht_node_str *ua_chan_node;
        struct ust_app *app;
        struct ust_app_session *ua_sess;
        struct ust_app_channel *ua_chan;
        struct ust_app_event *ua_event;
 
+       assert(usess->active);
        DBG("UST app disabling event %s for all apps in channel "
                        "%s for session id %" PRIu64,
                        uevent->attr.name, uchan->name, usess->id);
@@ -3771,14 +4050,14 @@ int ust_app_disable_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) {
+               ua_event = find_ust_app_event(ua_chan->events, uevent->attr.name,
+                               uevent->filter, uevent->attr.loglevel,
+                               uevent->exclusion);
+               if (ua_event == NULL) {
                        DBG2("Event %s not found in channel %s for app pid %d."
                                        "Skipping", uevent->attr.name, uchan->name, app->pid);
                        continue;
                }
-               ua_event = caa_container_of(ua_event_node, struct ust_app_event, node);
 
                ret = disable_ust_app_event(ua_sess, ua_event, app);
                if (ret < 0) {
@@ -3788,7 +4067,77 @@ int ust_app_disable_event_glb(struct ltt_ust_session *usess,
        }
 
        rcu_read_unlock();
+       return ret;
+}
+
+/* The ua_sess lock must be held by the caller.  */
+static
+int ust_app_channel_create(struct ltt_ust_session *usess,
+               struct ust_app_session *ua_sess,
+               struct ltt_ust_channel *uchan, struct ust_app *app,
+               struct ust_app_channel **_ua_chan)
+{
+       int ret = 0;
+       struct ust_app_channel *ua_chan = NULL;
+
+       assert(ua_sess);
+       ASSERT_LOCKED(ua_sess->lock);
+
+       if (!strncmp(uchan->name, DEFAULT_METADATA_NAME,
+                    sizeof(uchan->name))) {
+               copy_channel_attr_to_ustctl(&ua_sess->metadata_attr,
+                       &uchan->attr);
+               ret = 0;
+       } else {
+               struct ltt_ust_context *uctx = NULL;
+
+               /*
+                * Create channel onto application and synchronize its
+                * configuration.
+                */
+               ret = ust_app_channel_allocate(ua_sess, uchan,
+                       LTTNG_UST_CHAN_PER_CPU, usess,
+                       &ua_chan);
+               if (ret == 0) {
+                       ret = ust_app_channel_send(app, usess,
+                               ua_sess, ua_chan);
+               } else {
+                       goto end;
+               }
 
+               /* Add contexts. */
+               cds_list_for_each_entry(uctx, &uchan->ctx_list, list) {
+                       ret = create_ust_app_channel_context(ua_chan,
+                               &uctx->ctx, app);
+                       if (ret) {
+                               goto end;
+                       }
+               }
+       }
+       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.
+                        */
+                       ret = 0;        /* Not an error. */
+                       break;
+               case -ENOMEM:
+               default:
+                       break;
+               }
+       }
+end:
+       if (ret == 0 && _ua_chan) {
+               /*
+                * Only return the application's channel on success. Note
+                * that the channel can still be part of the application's
+                * channel hashtable on error.
+                */
+               *_ua_chan = ua_chan;
+       }
        return ret;
 }
 
@@ -3798,32 +4147,26 @@ int ust_app_disable_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;
+       int ret = 0;
+       struct cds_lfht_iter iter;
        struct ust_app *app;
-       struct ust_app_session *ua_sess = NULL;
 
-       /* Very wrong code flow */
        assert(usess);
+       assert(usess->active);
        assert(uchan);
 
        DBG2("UST app adding channel %s to UST domain for session id %" PRIu64,
                        uchan->name, usess->id);
 
        rcu_read_lock();
-
        /* For every registered applications */
-       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;
-               }
-               if (!trace_ust_pid_tracker_lookup(usess, app->pid)) {
-                       /* Skip. */
-                       continue;
+       cds_lfht_for_each_entry(ust_app_ht->ht, &iter, app, pid_n.node) {
+               struct ust_app_session *ua_sess;
+               int session_was_created = 0;
+
+               if (!app->compatible ||
+                               !trace_ust_pid_tracker_lookup(usess, app->pid)) {
+                       goto error_rcu_unlock;
                }
 
                /*
@@ -3831,59 +4174,34 @@ 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.
                 */
-               ret = create_ust_app_session(usess, app, &ua_sess, &created);
+               ret = find_or_create_ust_app_session(usess, app, &ua_sess,
+                               &session_was_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.
+                                * 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; it is not an error.
                                 */
-                               ret = 0;        /* Not an error. */
-                               continue;
+                               ret = 0;
+                               goto error_rcu_unlock;
                        case -ENOMEM:
                        default:
                                goto error_rcu_unlock;
                        }
                }
-               assert(ua_sess);
-
-               pthread_mutex_lock(&ua_sess->lock);
 
                if (ua_sess->deleted) {
-                       pthread_mutex_unlock(&ua_sess->lock);
                        continue;
                }
-
-               if (!strncmp(uchan->name, DEFAULT_METADATA_NAME,
-                                       sizeof(uchan->name))) {
-                       copy_channel_attr_to_ustctl(&ua_sess->metadata_attr, &uchan->attr);
-                       ret = 0;
-               } else {
-                       /* Create channel onto application. We don't need the chan ref. */
-                       ret = create_ust_app_channel(ua_sess, uchan, app,
-                                       LTTNG_UST_CHAN_PER_CPU, usess, NULL);
-               }
-               pthread_mutex_unlock(&ua_sess->lock);
-               if (ret < 0) {
-                       /* Cleanup the created session if it's the case. */
-                       if (created) {
+               ret = ust_app_channel_create(usess, ua_sess, uchan, app, NULL);
+               if (ret) {
+                       if (session_was_created) {
                                destroy_app_session(app, ua_sess);
                        }
-                       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.
-                                */
-                               ret = 0;        /* Not an error. */
-                               continue;
-                       case -ENOMEM:
-                       default:
-                               goto error_rcu_unlock;
-                       }
+                       goto error_rcu_unlock;
                }
        }
 
@@ -3906,6 +4224,7 @@ int ust_app_enable_event_glb(struct ltt_ust_session *usess,
        struct ust_app_channel *ua_chan;
        struct ust_app_event *ua_event;
 
+       assert(usess->active);
        DBG("UST app enabling event %s for all apps for session id %" PRIu64,
                        uevent->attr.name, usess->id);
 
@@ -3991,6 +4310,7 @@ int ust_app_create_event_glb(struct ltt_ust_session *usess,
        struct ust_app_session *ua_sess;
        struct ust_app_channel *ua_chan;
 
+       assert(usess->active);
        DBG("UST app creating event %s for all apps for session id %" PRIu64,
                        uevent->attr.name, usess->id);
 
@@ -4040,12 +4360,14 @@ int ust_app_create_event_glb(struct ltt_ust_session *usess,
        }
 
        rcu_read_unlock();
-
        return ret;
 }
 
 /*
  * Start tracing for a specific UST session and app.
+ *
+ * Called with UST app session lock held.
+ *
  */
 static
 int ust_app_start_trace(struct ltt_ust_session *usess, struct ust_app *app)
@@ -4079,19 +4401,6 @@ int ust_app_start_trace(struct ltt_ust_session *usess, struct ust_app *app)
                goto skip_setup;
        }
 
-       /* Create directories if consumer is LOCAL and has a path defined. */
-       if (usess->consumer->type == CONSUMER_DST_LOCAL &&
-                       strlen(usess->consumer->dst.trace_path) > 0) {
-               ret = run_as_mkdir_recursive(usess->consumer->dst.trace_path,
-                               S_IRWXU | S_IRWXG, ua_sess->euid, ua_sess->egid);
-               if (ret < 0) {
-                       if (errno != EEXIST) {
-                               ERR("Trace directory creation error");
-                               goto error_unlock;
-                       }
-               }
-       }
-
        /*
         * Create the metadata for the application. This returns gracefully if a
         * metadata was already set for the session.
@@ -4105,7 +4414,9 @@ int ust_app_start_trace(struct ltt_ust_session *usess, struct ust_app *app)
 
 skip_setup:
        /* This start the UST tracing */
+       pthread_mutex_lock(&app->sock_lock);
        ret = ustctl_start_session(app->sock, ua_sess->handle);
+       pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
                        ERR("Error starting tracing for app pid: %d (ret: %d)",
@@ -4131,7 +4442,9 @@ skip_setup:
        health_code_update();
 
        /* Quiescent wait after starting trace */
+       pthread_mutex_lock(&app->sock_lock);
        ret = ustctl_wait_quiescent(app->sock);
+       pthread_mutex_unlock(&app->sock_lock);
        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);
@@ -4192,7 +4505,9 @@ int ust_app_stop_trace(struct ltt_ust_session *usess, struct ust_app *app)
        health_code_update();
 
        /* This inhibits UST tracing */
+       pthread_mutex_lock(&app->sock_lock);
        ret = ustctl_stop_session(app->sock, ua_sess->handle);
+       pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
                        ERR("Error stopping tracing for app pid: %d (ret: %d)",
@@ -4212,7 +4527,9 @@ int ust_app_stop_trace(struct ltt_ust_session *usess, struct ust_app *app)
        health_code_update();
 
        /* Quiescent wait after stopping trace */
+       pthread_mutex_lock(&app->sock_lock);
        ret = ustctl_wait_quiescent(app->sock);
+       pthread_mutex_unlock(&app->sock_lock);
        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);
@@ -4221,6 +4538,8 @@ int ust_app_stop_trace(struct ltt_ust_session *usess, struct ust_app *app)
        health_code_update();
 
        registry = get_session_registry(ua_sess);
+
+       /* The UST app session is held registry shall not be null. */
        assert(registry);
 
        /* Push metadata for application before freeing the application. */
@@ -4275,7 +4594,6 @@ int ust_app_flush_app_session(struct ust_app *app,
                cds_lfht_for_each_entry(ua_sess->channels->ht, &iter.iter, ua_chan,
                                node.node) {
                        health_code_update();
-                       assert(ua_chan->is_sent);
                        ret = consumer_flush_channel(socket, ua_chan->key);
                        if (ret) {
                                ERR("Error flushing consumer channel");
@@ -4378,6 +4696,155 @@ int ust_app_flush_session(struct ltt_ust_session *usess)
        return ret;
 }
 
+static
+int ust_app_clear_quiescent_app_session(struct ust_app *app,
+               struct ust_app_session *ua_sess)
+{
+       int ret = 0;
+       struct lttng_ht_iter iter;
+       struct ust_app_channel *ua_chan;
+       struct consumer_socket *socket;
+
+       DBG("Clearing stream quiescent state for ust app pid %d", app->pid);
+
+       rcu_read_lock();
+
+       if (!app->compatible) {
+               goto end_not_compatible;
+       }
+
+       pthread_mutex_lock(&ua_sess->lock);
+
+       if (ua_sess->deleted) {
+               goto end_unlock;
+       }
+
+       health_code_update();
+
+       socket = consumer_find_socket_by_bitness(app->bits_per_long,
+                       ua_sess->consumer);
+       if (!socket) {
+               ERR("Failed to find consumer (%" PRIu32 ") socket",
+                               app->bits_per_long);
+               ret = -1;
+               goto end_unlock;
+       }
+
+       /* Clear quiescent state. */
+       switch (ua_sess->buffer_type) {
+       case LTTNG_BUFFER_PER_PID:
+               cds_lfht_for_each_entry(ua_sess->channels->ht, &iter.iter,
+                               ua_chan, node.node) {
+                       health_code_update();
+                       ret = consumer_clear_quiescent_channel(socket,
+                                       ua_chan->key);
+                       if (ret) {
+                               ERR("Error clearing quiescent state for consumer channel");
+                               ret = -1;
+                               continue;
+                       }
+               }
+               break;
+       case LTTNG_BUFFER_PER_UID:
+       default:
+               assert(0);
+               ret = -1;
+               break;
+       }
+
+       health_code_update();
+
+end_unlock:
+       pthread_mutex_unlock(&ua_sess->lock);
+
+end_not_compatible:
+       rcu_read_unlock();
+       health_code_update();
+       return ret;
+}
+
+/*
+ * Clear quiescent state in each stream for all applications for a
+ * specific UST session.
+ * Called with UST session lock held.
+ */
+static
+int ust_app_clear_quiescent_session(struct ltt_ust_session *usess)
+
+{
+       int ret = 0;
+
+       DBG("Clearing stream quiescent state for all ust apps");
+
+       rcu_read_lock();
+
+       switch (usess->buffer_type) {
+       case LTTNG_BUFFER_PER_UID:
+       {
+               struct lttng_ht_iter iter;
+               struct buffer_reg_uid *reg;
+
+               /*
+                * Clear quiescent for all per UID buffers associated to
+                * that session.
+                */
+               cds_list_for_each_entry(reg, &usess->buffer_reg_uid_list, lnode) {
+                       struct consumer_socket *socket;
+                       struct buffer_reg_channel *reg_chan;
+
+                       /* Get associated consumer socket.*/
+                       socket = consumer_find_socket_by_bitness(
+                                       reg->bits_per_long, usess->consumer);
+                       if (!socket) {
+                               /*
+                                * Ignore request if no consumer is found for
+                                * the session.
+                                */
+                               continue;
+                       }
+
+                       cds_lfht_for_each_entry(reg->registry->channels->ht,
+                                       &iter.iter, reg_chan, node.node) {
+                               /*
+                                * The following call will print error values so
+                                * the return code is of little importance
+                                * because whatever happens, we have to try them
+                                * all.
+                                */
+                               (void) consumer_clear_quiescent_channel(socket,
+                                               reg_chan->consumer_key);
+                       }
+               }
+               break;
+       }
+       case LTTNG_BUFFER_PER_PID:
+       {
+               struct ust_app_session *ua_sess;
+               struct lttng_ht_iter iter;
+               struct ust_app *app;
+
+               cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app,
+                               pid_n.node) {
+                       ua_sess = lookup_session_by_app(usess, app);
+                       if (ua_sess == NULL) {
+                               continue;
+                       }
+                       (void) ust_app_clear_quiescent_app_session(app,
+                                       ua_sess);
+               }
+               break;
+       }
+       default:
+               ret = -1;
+               assert(0);
+               break;
+       }
+
+       rcu_read_unlock();
+       health_code_update();
+       return ret;
+}
+
 /*
  * Destroy a specific UST session in apps.
  */
@@ -4410,7 +4877,9 @@ static int destroy_trace(struct ltt_ust_session *usess, struct ust_app *app)
        health_code_update();
 
        /* Quiescent wait after stopping trace */
+       pthread_mutex_lock(&app->sock_lock);
        ret = ustctl_wait_quiescent(app->sock);
+       pthread_mutex_unlock(&app->sock_lock);
        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);
@@ -4426,20 +4895,29 @@ end:
  */
 int ust_app_start_trace_all(struct ltt_ust_session *usess)
 {
-       int ret = 0;
        struct lttng_ht_iter iter;
        struct ust_app *app;
 
        DBG("Starting all UST traces");
 
+       /*
+        * Even though the start trace might fail, flag this session active so
+        * other application coming in are started by default.
+        */
+       usess->active = 1;
+
        rcu_read_lock();
 
+       /*
+        * In a start-stop-start use-case, we need to clear the quiescent state
+        * of each channel set by the prior stop command, thus ensuring that a
+        * following stop or destroy is sure to grab a timestamp_end near those
+        * operations, even if the packet is empty.
+        */
+       (void) ust_app_clear_quiescent_session(usess);
+
        cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app, pid_n.node) {
-               ret = ust_app_start_trace(usess, app);
-               if (ret < 0) {
-                       /* Continue to next apps even on error */
-                       continue;
-               }
+               ust_app_global_update(usess, app);
        }
 
        rcu_read_unlock();
@@ -4459,6 +4937,12 @@ int ust_app_stop_trace_all(struct ltt_ust_session *usess)
 
        DBG("Stopping all UST traces");
 
+       /*
+        * Even though the stop trace might fail, flag this session inactive so
+        * other application coming in are not started by default.
+        */
+       usess->active = 0;
+
        rcu_read_lock();
 
        cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app, pid_n.node) {
@@ -4502,92 +4986,147 @@ int ust_app_destroy_trace_all(struct ltt_ust_session *usess)
        return 0;
 }
 
+/* The ua_sess lock must be held by the caller. */
 static
-void ust_app_global_create(struct ltt_ust_session *usess, struct ust_app *app)
+int find_or_create_ust_app_channel(
+               struct ltt_ust_session *usess,
+               struct ust_app_session *ua_sess,
+               struct ust_app *app,
+               struct ltt_ust_channel *uchan,
+               struct ust_app_channel **ua_chan)
 {
        int ret = 0;
-       struct lttng_ht_iter iter, uiter;
-       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;
-       int is_created = 0;
+       struct lttng_ht_iter iter;
+       struct lttng_ht_node_str *ua_chan_node;
 
-       ret = create_ust_app_session(usess, app, &ua_sess, &is_created);
-       if (ret < 0) {
-               /* Tracer is probably gone or ENOMEM. */
-               goto error;
-       }
-       if (!is_created) {
-               /* App session already created. */
+       lttng_ht_lookup(ua_sess->channels, (void *) uchan->name, &iter);
+       ua_chan_node = lttng_ht_iter_get_node_str(&iter);
+       if (ua_chan_node) {
+               *ua_chan = caa_container_of(ua_chan_node,
+                       struct ust_app_channel, node);
                goto end;
        }
-       assert(ua_sess);
 
-       pthread_mutex_lock(&ua_sess->lock);
-
-       if (ua_sess->deleted) {
-               pthread_mutex_unlock(&ua_sess->lock);
+       ret = ust_app_channel_create(usess, ua_sess, uchan, app, ua_chan);
+       if (ret) {
                goto end;
        }
+end:
+       return ret;
+}
 
-       /*
-        * 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 = do_create_channel(app, usess, ua_sess, ua_chan);
-               if (ret < 0 && ret != -ENOTCONN) {
-                       /*
-                        * 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. The only exception is
-                        * -ENOTCONN, which indicates that the application
-                        * has exit.
-                        */
-                       goto error_unlock;
-               }
+static
+int ust_app_channel_synchronize_event(struct ust_app_channel *ua_chan,
+               struct ltt_ust_event *uevent, struct ust_app_session *ua_sess,
+               struct ust_app *app)
+{
+       int ret = 0;
+       struct ust_app_event *ua_event = NULL;
+
+       ua_event = find_ust_app_event(ua_chan->events, uevent->attr.name,
+               uevent->filter, uevent->attr.loglevel, uevent->exclusion);
+       if (!ua_event) {
+               ret = create_ust_app_event(ua_sess, ua_chan, uevent, app);
+               if (ret < 0) {
+                       goto end;
+               }
+       } else {
+               if (ua_event->enabled != uevent->enabled) {
+                       ret = uevent->enabled ?
+                               enable_ust_app_event(ua_sess, ua_event, app) :
+                               disable_ust_app_event(ua_sess, ua_event, app);
+               }
+       }
+
+end:
+       return ret;
+}
+
+/*
+ * The caller must ensure that the application is compatible and is tracked
+ * by the PID tracker.
+ */
+static
+void ust_app_synchronize(struct ltt_ust_session *usess,
+               struct ust_app *app)
+{
+       int ret = 0;
+       struct cds_lfht_iter uchan_iter;
+       struct ltt_ust_channel *uchan;
+       struct ust_app_session *ua_sess = NULL;
+
+       /*
+        * The application's configuration should only be synchronized for
+        * active sessions.
+        */
+       assert(usess->active);
+
+       ret = find_or_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);
+       if (ua_sess->deleted) {
+               pthread_mutex_unlock(&ua_sess->lock);
+               goto end;
+       }
+
+       rcu_read_lock();
+       cds_lfht_for_each_entry(usess->domain_global.channels->ht, &uchan_iter,
+                       uchan, node.node) {
+               struct ust_app_channel *ua_chan;
+               struct cds_lfht_iter uevent_iter;
+               struct ltt_ust_event *uevent;
 
                /*
-                * Add context using the list so they are enabled in the same order the
-                * user added them.
+                * Search for a matching ust_app_channel. If none is found,
+                * create it. Creating the channel will cause the ua_chan
+                * structure to be allocated, the channel buffers to be
+                * allocated (if necessary) and sent to the application, and
+                * all enabled contexts will be added to the channel.
                 */
-               cds_list_for_each_entry(ua_ctx, &ua_chan->ctx_list, list) {
-                       ret = create_ust_channel_context(ua_chan, ua_ctx, app);
-                       if (ret < 0) {
-                               goto error_unlock;
-                       }
+               ret = find_or_create_ust_app_channel(usess, ua_sess,
+                       app, uchan, &ua_chan);
+               if (ret) {
+                       /* Tracer is probably gone or ENOMEM. */
+                       goto error_unlock;
                }
 
+               if (!ua_chan) {
+                       /* ua_chan will be NULL for the metadata channel */
+                       continue;
+               }
 
-               /* For each events */
-               cds_lfht_for_each_entry(ua_chan->events->ht, &uiter.iter, ua_event,
+               cds_lfht_for_each_entry(uchan->events->ht, &uevent_iter, uevent,
                                node.node) {
-                       ret = create_ust_event(app, ua_sess, ua_chan, ua_event);
-                       if (ret < 0) {
+                       ret = ust_app_channel_synchronize_event(ua_chan,
+                               uevent, ua_sess, app);
+                       if (ret) {
                                goto error_unlock;
                        }
                }
-       }
-
-       pthread_mutex_unlock(&ua_sess->lock);
 
-       if (usess->active) {
-               ret = ust_app_start_trace(usess, app);
-               if (ret < 0) {
-                       goto error;
+               if (ua_chan->enabled != uchan->enabled) {
+                       ret = uchan->enabled ?
+                               enable_ust_app_channel(ua_sess, uchan, app) :
+                               disable_ust_app_channel(ua_sess, ua_chan, app);
+                       if (ret) {
+                               goto error_unlock;
+                       }
                }
-
-               DBG2("UST trace started for app pid %d", app->pid);
        }
+       rcu_read_unlock();
+
 end:
+       pthread_mutex_unlock(&ua_sess->lock);
        /* Everything went well at this point. */
        return;
 
 error_unlock:
+       rcu_read_unlock();
        pthread_mutex_unlock(&ua_sess->lock);
 error:
        if (ua_sess) {
@@ -4617,6 +5156,7 @@ void ust_app_global_destroy(struct ltt_ust_session *usess, struct ust_app *app)
 void ust_app_global_update(struct ltt_ust_session *usess, struct ust_app *app)
 {
        assert(usess);
+       assert(usess->active);
 
        DBG2("UST app global update for app sock %d for session id %" PRIu64,
                        app->sock, usess->id);
@@ -4624,9 +5164,13 @@ void ust_app_global_update(struct ltt_ust_session *usess, struct ust_app *app)
        if (!app->compatible) {
                return;
        }
-
        if (trace_ust_pid_tracker_lookup(usess, app->pid)) {
-               ust_app_global_create(usess, app);
+               /*
+                * Synchronize the application's internal tracing configuration
+                * and start tracing.
+                */
+               ust_app_synchronize(usess, app);
+               ust_app_start_trace(usess, app);
        } else {
                ust_app_global_destroy(usess, app);
        }
@@ -4660,8 +5204,9 @@ int ust_app_add_ctx_channel_glb(struct ltt_ust_session *usess,
        struct ust_app_session *ua_sess;
        struct ust_app *app;
 
-       rcu_read_lock();
+       assert(usess->active);
 
+       rcu_read_lock();
        cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                if (!app->compatible) {
                        /*
@@ -4690,7 +5235,7 @@ int ust_app_add_ctx_channel_glb(struct ltt_ust_session *usess,
                }
                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);
+               ret = create_ust_app_channel_context(ua_chan, &uctx->ctx, app);
                if (ret < 0) {
                        goto next_app;
                }
@@ -4702,125 +5247,6 @@ int ust_app_add_ctx_channel_glb(struct ltt_ust_session *usess,
        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 ret = 0;
-       struct lttng_ht_iter iter;
-       struct lttng_ht_node_str *ua_chan_node;
-       struct ust_app *app;
-       struct ust_app_session *ua_sess;
-       struct ust_app_channel *ua_chan;
-       struct ust_app_event *ua_event;
-
-       DBG("UST app enabling event %s for PID %d", uevent->attr.name, pid);
-
-       rcu_read_lock();
-
-       app = ust_app_find_by_pid(pid);
-       if (app == NULL) {
-               ERR("UST app enable event per PID %d not found", pid);
-               ret = -1;
-               goto end;
-       }
-
-       if (!app->compatible) {
-               ret = 0;
-               goto end;
-       }
-
-       ua_sess = lookup_session_by_app(usess, app);
-       if (!ua_sess) {
-               /* The application has problem or is probably dead. */
-               ret = 0;
-               goto end;
-       }
-
-       pthread_mutex_lock(&ua_sess->lock);
-
-       if (ua_sess->deleted) {
-               ret = 0;
-               goto end_unlock;
-       }
-
-       /* Lookup channel in the ust app session */
-       lttng_ht_lookup(ua_sess->channels, (void *)uchan->name, &iter);
-       ua_chan_node = lttng_ht_iter_get_node_str(&iter);
-       /* If the channel is not found, there is a code flow error */
-       assert(ua_chan_node);
-
-       ua_chan = caa_container_of(ua_chan_node, struct ust_app_channel, node);
-
-       ua_event = find_ust_app_event(ua_chan->events, uevent->attr.name,
-                       uevent->filter, uevent->attr.loglevel, uevent->exclusion);
-       if (ua_event == NULL) {
-               ret = create_ust_app_event(ua_sess, ua_chan, uevent, app);
-               if (ret < 0) {
-                       goto end_unlock;
-               }
-       } else {
-               ret = enable_ust_app_event(ua_sess, ua_event, app);
-               if (ret < 0) {
-                       goto end_unlock;
-               }
-       }
-
-end_unlock:
-       pthread_mutex_unlock(&ua_sess->lock);
-end:
-       rcu_read_unlock();
-       return ret;
-}
-
-/*
- * Calibrate registered applications.
- */
-int ust_app_calibrate_glb(struct lttng_ust_calibrate *calibrate)
-{
-       int ret = 0;
-       struct lttng_ht_iter iter;
-       struct ust_app *app;
-
-       rcu_read_lock();
-
-       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;
-               }
-
-               health_code_update();
-
-               ret = ustctl_calibrate(app->sock, calibrate);
-               if (ret < 0) {
-                       switch (ret) {
-                       case -ENOSYS:
-                               /* Means that it's not implemented on the tracer side. */
-                               ret = 0;
-                               break;
-                       default:
-                               DBG2("Calibrate app PID %d returned with error %d",
-                                               app->pid, ret);
-                               break;
-                       }
-               }
-       }
-
-       DBG("UST app global domain calibration finished");
-
-       rcu_read_unlock();
-
-       health_code_update();
-
-       return ret;
-}
-
 /*
  * Receive registration and populate the given msg structure.
  *
@@ -4870,6 +5296,33 @@ error:
        return ret;
 }
 
+/*
+ * Return a ust app session object using the application object and the
+ * session object descriptor has a key. If not found, NULL is returned.
+ * A RCU read side lock MUST be acquired when calling this function.
+*/
+static struct ust_app_session *find_session_by_objd(struct ust_app *app,
+               int objd)
+{
+       struct lttng_ht_node_ulong *node;
+       struct lttng_ht_iter iter;
+       struct ust_app_session *ua_sess = NULL;
+
+       assert(app);
+
+       lttng_ht_lookup(app->ust_sessions_objd, (void *)((unsigned long) objd), &iter);
+       node = lttng_ht_iter_get_node_ulong(&iter);
+       if (node == NULL) {
+               DBG2("UST app session find by objd %d not found", objd);
+               goto error;
+       }
+
+       ua_sess = caa_container_of(node, struct ust_app_session, ust_objd_node);
+
+error:
+       return ua_sess;
+}
+
 /*
  * 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
@@ -4905,11 +5358,11 @@ error:
  *
  * On success 0 is returned else a negative value.
  */
-static int reply_ust_register_channel(int sock, int sobjd, int cobjd,
+static int reply_ust_register_channel(int sock, int cobjd,
                size_t nr_fields, struct ustctl_field *fields)
 {
        int ret, ret_code = 0;
-       uint32_t chan_id, reg_count;
+       uint32_t chan_id;
        uint64_t chan_reg_key;
        enum ustctl_channel_header type;
        struct ust_app *app;
@@ -4923,19 +5376,17 @@ static int reply_ust_register_channel(int sock, int sobjd, int cobjd,
        /* 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",
+               DBG("Application socket %d is being torn down. Abort event notify",
                                sock);
                ret = 0;
-               free(fields);
                goto error_rcu_unlock;
        }
 
        /* Lookup channel by UST object descriptor. */
        ua_chan = find_channel_by_objd(app, cobjd);
        if (!ua_chan) {
-               DBG("Application channel is being teardown. Abort event notify");
+               DBG("Application channel is being torn down. Abort event notify");
                ret = 0;
-               free(fields);
                goto error_rcu_unlock;
        }
 
@@ -4944,7 +5395,11 @@ static int reply_ust_register_channel(int sock, int sobjd, int cobjd,
 
        /* Get right session registry depending on the session buffer type. */
        registry = get_session_registry(ua_sess);
-       assert(registry);
+       if (!registry) {
+               DBG("Application session is being torn down. Abort event notify");
+               ret = 0;
+               goto error_rcu_unlock;
+       };
 
        /* Depending on the buffer type, a different channel key is used. */
        if (ua_sess->buffer_type == LTTNG_BUFFER_PER_UID) {
@@ -4959,22 +5414,19 @@ static int reply_ust_register_channel(int sock, int sobjd, int cobjd,
        assert(chan_reg);
 
        if (!chan_reg->register_done) {
-               reg_count = ust_registry_get_event_count(chan_reg);
-               if (reg_count < 31) {
-                       type = USTCTL_CHANNEL_HEADER_COMPACT;
-               } else {
-                       type = USTCTL_CHANNEL_HEADER_LARGE;
-               }
-
+               /*
+                * TODO: eventually use the registry event count for
+                * this channel to better guess header type for per-pid
+                * buffers.
+                */
+               type = USTCTL_CHANNEL_HEADER_LARGE;
                chan_reg->nr_ctx_fields = nr_fields;
                chan_reg->ctx_fields = fields;
+               fields = NULL;
                chan_reg->header_type = type;
        } else {
                /* Get current already assigned values. */
                type = chan_reg->header_type;
-               free(fields);
-               /* Set to NULL so the error path does not do a double free. */
-               fields = NULL;
        }
        /* Channel id is set during the object creation. */
        chan_id = chan_reg->chan_id;
@@ -5010,9 +5462,7 @@ error:
        pthread_mutex_unlock(&registry->lock);
 error_rcu_unlock:
        rcu_read_unlock();
-       if (ret) {
-               free(fields);
-       }
+       free(fields);
        return ret;
 }
 
@@ -5026,8 +5476,8 @@ error_rcu_unlock:
  * 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)
+               char *sig, size_t nr_fields, struct ustctl_field *fields,
+               int loglevel_value, char *model_emf_uri)
 {
        int ret, ret_code;
        uint32_t event_id = 0;
@@ -5042,23 +5492,17 @@ static int add_event_ust_registry(int sock, int sobjd, int cobjd, char *name,
        /* 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",
+               DBG("Application socket %d is being torn down. Abort event notify",
                                sock);
                ret = 0;
-               free(sig);
-               free(fields);
-               free(model_emf_uri);
                goto error_rcu_unlock;
        }
 
        /* Lookup channel by UST object descriptor. */
        ua_chan = find_channel_by_objd(app, cobjd);
        if (!ua_chan) {
-               DBG("Application channel is being teardown. Abort event notify");
+               DBG("Application channel is being torn down. Abort event notify");
                ret = 0;
-               free(sig);
-               free(fields);
-               free(model_emf_uri);
                goto error_rcu_unlock;
        }
 
@@ -5066,7 +5510,11 @@ static int add_event_ust_registry(int sock, int sobjd, int cobjd, char *name,
        ua_sess = ua_chan->session;
 
        registry = get_session_registry(ua_sess);
-       assert(registry);
+       if (!registry) {
+               DBG("Application session is being torn down. Abort event notify");
+               ret = 0;
+               goto error_rcu_unlock;
+       }
 
        if (ua_sess->buffer_type == LTTNG_BUFFER_PER_UID) {
                chan_reg_key = ua_chan->tracing_channel_id;
@@ -5082,9 +5530,12 @@ static int add_event_ust_registry(int sock, int sobjd, int cobjd, char *name,
         * three variables MUST NOT be read/write after this.
         */
        ret_code = ust_registry_create_event(registry, chan_reg_key,
-                       sobjd, cobjd, name, sig, nr_fields, fields, loglevel,
-                       model_emf_uri, ua_sess->buffer_type, &event_id,
-                       app);
+                       sobjd, cobjd, name, sig, nr_fields, fields,
+                       loglevel_value, model_emf_uri, ua_sess->buffer_type,
+                       &event_id, app);
+       sig = NULL;
+       fields = NULL;
+       model_emf_uri = NULL;
 
        /*
         * The return value is returned to ustctl so in case of an error, the
@@ -5112,43 +5563,131 @@ error:
        pthread_mutex_unlock(&registry->lock);
 error_rcu_unlock:
        rcu_read_unlock();
+       free(sig);
+       free(fields);
+       free(model_emf_uri);
        return ret;
 }
 
 /*
- * Handle application notification through the given notify socket.
+ * Add enum to the UST session registry. Once done, this replies to the
+ * application with the appropriate error code.
  *
- * Return 0 on success or else a negative value.
+ * The session UST registry lock is acquired within this function.
+ *
+ * On success 0 is returned else a negative value.
  */
-int ust_app_recv_notify(int sock)
+static int add_enum_ust_registry(int sock, int sobjd, char *name,
+               struct ustctl_enum_entry *entries, size_t nr_entries)
 {
-       int ret;
-       enum ustctl_notify_cmd cmd;
+       int ret = 0, ret_code;
+       struct ust_app *app;
+       struct ust_app_session *ua_sess;
+       struct ust_registry_session *registry;
+       uint64_t enum_id = -1ULL;
 
-       DBG3("UST app receiving notify from sock %d", sock);
+       rcu_read_lock();
 
-       ret = ustctl_recv_notify(sock, &cmd);
+       /* Lookup application. If not found, there is a code flow error. */
+       app = find_app_by_notify_sock(sock);
+       if (!app) {
+               /* Return an error since this is not an error */
+               DBG("Application socket %d is being torn down. Aborting enum registration",
+                               sock);
+               free(entries);
+               goto error_rcu_unlock;
+       }
+
+       /* Lookup session by UST object descriptor. */
+       ua_sess = find_session_by_objd(app, sobjd);
+       if (!ua_sess) {
+               /* Return an error since this is not an error */
+               DBG("Application session is being torn down (session not found). Aborting enum registration.");
+               free(entries);
+               goto error_rcu_unlock;
+       }
+
+       registry = get_session_registry(ua_sess);
+       if (!registry) {
+               DBG("Application session is being torn down (registry not found). Aborting enum registration.");
+               free(entries);
+               goto error_rcu_unlock;
+       }
+
+       pthread_mutex_lock(&registry->lock);
+
+       /*
+        * From this point on, the callee acquires the ownership of
+        * entries. The variable entries MUST NOT be read/written after
+        * call.
+        */
+       ret_code = ust_registry_create_or_find_enum(registry, sobjd, name,
+                       entries, nr_entries, &enum_id);
+       entries = NULL;
+
+       /*
+        * 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_enum(sock, enum_id, ret_code);
        if (ret < 0) {
                if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
-                       ERR("UST app recv notify failed with ret %d", ret);
+                       ERR("UST app reply enum failed with ret %d", ret);
                } else {
-                       DBG3("UST app recv notify failed. Application died");
+                       DBG3("UST app reply enum failed. Application died");
                }
+               /*
+                * No need to wipe the create enum since the application socket will
+                * get close on error hence cleaning up everything by itself.
+                */
                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");
+       DBG3("UST registry enum %s added successfully or already found", name);
 
-               ret = ustctl_recv_register_event(sock, &sobjd, &cobjd, name, &loglevel,
-                               &sig, &nr_fields, &fields, &model_emf_uri);
+error:
+       pthread_mutex_unlock(&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_value;
+               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_value, &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);
@@ -5165,7 +5704,7 @@ int ust_app_recv_notify(int sock)
                 * to the this function.
                 */
                ret = add_event_ust_registry(sock, sobjd, cobjd, name, sig, nr_fields,
-                               fields, loglevel, model_emf_uri);
+                               fields, loglevel_value, model_emf_uri);
                if (ret < 0) {
                        goto error;
                }
@@ -5196,7 +5735,7 @@ int ust_app_recv_notify(int sock)
                 * that if needed it will be freed. After this, it's invalid to access
                 * fields or clean it up.
                 */
-               ret = reply_ust_register_channel(sock, sobjd, cobjd, nr_fields,
+               ret = reply_ust_register_channel(sock, cobjd, nr_fields,
                                fields);
                if (ret < 0) {
                        goto error;
@@ -5204,6 +5743,35 @@ int ust_app_recv_notify(int sock)
 
                break;
        }
+       case USTCTL_NOTIFY_CMD_ENUM:
+       {
+               int sobjd;
+               char name[LTTNG_UST_SYM_NAME_LEN];
+               size_t nr_entries;
+               struct ustctl_enum_entry *entries;
+
+               DBG2("UST app ustctl register enum received");
+
+               ret = ustctl_recv_register_enum(sock, &sobjd, name,
+                               &entries, &nr_entries);
+               if (ret < 0) {
+                       if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
+                               ERR("UST app recv enum failed with ret %d", ret);
+                       } else {
+                               DBG3("UST app recv enum failed. Application died");
+                       }
+                       goto error;
+               }
+
+               /* Callee assumes ownership of entries */
+               ret = add_enum_ust_registry(sock, sobjd, name,
+                               entries, nr_entries);
+               if (ret < 0) {
+                       goto error;
+               }
+
+               break;
+       }
        default:
                /* Should NEVER happen. */
                assert(0);
@@ -5306,17 +5874,18 @@ void ust_app_destroy(struct ust_app *app)
  * Take a snapshot for a given UST session. The snapshot is sent to the given
  * output.
  *
- * Return 0 on success or else a negative value.
+ * Returns LTTNG_OK on success or a LTTNG_ERR error code.
  */
-int ust_app_snapshot_record(struct ltt_ust_session *usess,
-               struct snapshot_output *output, int wait,
+enum lttng_error_code ust_app_snapshot_record(
+               const struct ltt_ust_session *usess,
+               const struct consumer_output *output, int wait,
                uint64_t nb_packets_per_stream)
 {
        int ret = 0;
-       unsigned int snapshot_done = 0;
+       enum lttng_error_code status = LTTNG_OK;
        struct lttng_ht_iter iter;
        struct ust_app *app;
-       char pathname[PATH_MAX];
+       char *trace_path = NULL;
 
        assert(usess);
        assert(output);
@@ -5331,41 +5900,59 @@ int ust_app_snapshot_record(struct ltt_ust_session *usess,
                cds_list_for_each_entry(reg, &usess->buffer_reg_uid_list, lnode) {
                        struct buffer_reg_channel *reg_chan;
                        struct consumer_socket *socket;
+                       char pathname[PATH_MAX];
+
+                       if (!reg->registry->reg.ust->metadata_key) {
+                               /* Skip since no metadata is present */
+                               continue;
+                       }
 
                        /* Get consumer socket to use to push the metadata.*/
                        socket = consumer_find_socket_by_bitness(reg->bits_per_long,
                                        usess->consumer);
                        if (!socket) {
-                               ret = -EINVAL;
+                               status = LTTNG_ERR_INVALID;
                                goto error;
                        }
 
                        memset(pathname, 0, sizeof(pathname));
+                       /*
+                        * DEFAULT_UST_TRACE_UID_PATH already contains a path
+                        * separator.
+                        */
                        ret = snprintf(pathname, sizeof(pathname),
-                                       DEFAULT_UST_TRACE_DIR "/" DEFAULT_UST_TRACE_UID_PATH,
+                                       DEFAULT_UST_TRACE_DIR DEFAULT_UST_TRACE_UID_PATH,
                                        reg->uid, reg->bits_per_long);
                        if (ret < 0) {
                                PERROR("snprintf snapshot path");
+                               status = LTTNG_ERR_INVALID;
                                goto error;
                        }
-
-                       /* Add the UST default trace dir to path. */
+                       /* Free path allowed on previous iteration. */
+                       free(trace_path);
+                       trace_path = setup_channel_trace_path(usess->consumer, pathname);
+                       if (!trace_path) {
+                               status = LTTNG_ERR_INVALID;
+                               goto error;
+                       }
+                        /* Add the UST default trace dir to path. */
                        cds_lfht_for_each_entry(reg->registry->channels->ht, &iter.iter,
                                        reg_chan, node.node) {
-                               ret = consumer_snapshot_channel(socket, reg_chan->consumer_key,
-                                               output, 0, usess->uid, usess->gid, pathname, wait,
+                               status = consumer_snapshot_channel(socket,
+                                               reg_chan->consumer_key,
+                                               output, 0, usess->uid,
+                                               usess->gid, trace_path, wait,
                                                nb_packets_per_stream);
-                               if (ret < 0) {
+                               if (status != LTTNG_OK) {
                                        goto error;
                                }
                        }
-                       ret = consumer_snapshot_channel(socket,
+                       status = consumer_snapshot_channel(socket,
                                        reg->registry->reg.ust->metadata_key, output, 1,
-                                       usess->uid, usess->gid, pathname, wait, 0);
-                       if (ret < 0) {
+                                       usess->uid, usess->gid, trace_path, wait, 0);
+                       if (status != LTTNG_OK) {
                                goto error;
                        }
-                       snapshot_done = 1;
                }
                break;
        }
@@ -5377,6 +5964,7 @@ int ust_app_snapshot_record(struct ltt_ust_session *usess,
                        struct ust_app_channel *ua_chan;
                        struct ust_app_session *ua_sess;
                        struct ust_registry_session *registry;
+                       char pathname[PATH_MAX];
 
                        ua_sess = lookup_session_by_app(usess, app);
                        if (!ua_sess) {
@@ -5386,39 +5974,66 @@ int ust_app_snapshot_record(struct ltt_ust_session *usess,
 
                        /* Get the right consumer socket for the application. */
                        socket = consumer_find_socket_by_bitness(app->bits_per_long,
-                                       output->consumer);
+                                       output);
                        if (!socket) {
-                               ret = -EINVAL;
+                               status = LTTNG_ERR_INVALID;
                                goto error;
                        }
 
                        /* Add the UST default trace dir to path. */
                        memset(pathname, 0, sizeof(pathname));
-                       ret = snprintf(pathname, sizeof(pathname), DEFAULT_UST_TRACE_DIR "/%s",
+                       ret = snprintf(pathname, sizeof(pathname), DEFAULT_UST_TRACE_DIR "%s",
                                        ua_sess->path);
                        if (ret < 0) {
+                               status = LTTNG_ERR_INVALID;
                                PERROR("snprintf snapshot path");
                                goto error;
                        }
-
-                       cds_lfht_for_each_entry(ua_sess->channels->ht, &chan_iter.iter,
+                       /* Free path allowed on previous iteration. */
+                       free(trace_path);
+                       trace_path = setup_channel_trace_path(usess->consumer, pathname);
+                       if (!trace_path) {
+                               status = LTTNG_ERR_INVALID;
+                               goto error;
+                       }
+                        cds_lfht_for_each_entry(ua_sess->channels->ht, &chan_iter.iter,
                                        ua_chan, node.node) {
-                               ret = consumer_snapshot_channel(socket, ua_chan->key, output,
-                                               0, ua_sess->euid, ua_sess->egid, pathname, wait,
+                               status = consumer_snapshot_channel(socket,
+                                               ua_chan->key, output, 0,
+                                               ua_sess->effective_credentials
+                                                               .uid,
+                                               ua_sess->effective_credentials
+                                                               .gid,
+                                               trace_path, wait,
                                                nb_packets_per_stream);
-                               if (ret < 0) {
+                               switch (status) {
+                               case LTTNG_OK:
+                                       break;
+                               case LTTNG_ERR_CHAN_NOT_FOUND:
+                                       continue;
+                               default:
                                        goto error;
                                }
                        }
 
                        registry = get_session_registry(ua_sess);
-                       assert(registry);
-                       ret = consumer_snapshot_channel(socket, registry->metadata_key, output,
-                                       1, ua_sess->euid, ua_sess->egid, pathname, wait, 0);
-                       if (ret < 0) {
+                       if (!registry) {
+                               DBG("Application session is being torn down. Skip application.");
+                               continue;
+                       }
+                       status = consumer_snapshot_channel(socket,
+                                       registry->metadata_key, output, 1,
+                                       ua_sess->effective_credentials.uid,
+                                       ua_sess->effective_credentials.gid,
+                                       trace_path, wait, 0);
+                       switch (status) {
+                       case LTTNG_OK:
+                               break;
+                       case LTTNG_ERR_CHAN_NOT_FOUND:
+                               continue;
+                       default:
                                goto error;
                        }
-                       snapshot_done = 1;
                }
                break;
        }
@@ -5427,25 +6042,17 @@ int ust_app_snapshot_record(struct ltt_ust_session *usess,
                break;
        }
 
-       if (!snapshot_done) {
-               /*
-                * If no snapshot was made and we are not in the error path, this means
-                * that there are no buffers thus no (prior) application to snapshot
-                * data from so we have simply NO data.
-                */
-               ret = -ENODATA;
-       }
-
 error:
+       free(trace_path);
        rcu_read_unlock();
-       return ret;
+       return status;
 }
 
 /*
  * Return the size taken by one more packet per stream.
  */
-uint64_t ust_app_get_size_one_more_packet_per_stream(struct ltt_ust_session *usess,
-               uint64_t cur_nr_packets)
+uint64_t ust_app_get_size_one_more_packet_per_stream(
+               const struct ltt_ust_session *usess, uint64_t cur_nr_packets)
 {
        uint64_t tot_size = 0;
        struct ust_app *app;
@@ -5513,3 +6120,398 @@ uint64_t ust_app_get_size_one_more_packet_per_stream(struct ltt_ust_session *use
 
        return tot_size;
 }
+
+int ust_app_uid_get_channel_runtime_stats(uint64_t ust_session_id,
+               struct cds_list_head *buffer_reg_uid_list,
+               struct consumer_output *consumer, uint64_t uchan_id,
+               int overwrite, uint64_t *discarded, uint64_t *lost)
+{
+       int ret;
+       uint64_t consumer_chan_key;
+
+       *discarded = 0;
+       *lost = 0;
+
+       ret = buffer_reg_uid_consumer_channel_key(
+                       buffer_reg_uid_list, uchan_id, &consumer_chan_key);
+       if (ret < 0) {
+               /* Not found */
+               ret = 0;
+               goto end;
+       }
+
+       if (overwrite) {
+               ret = consumer_get_lost_packets(ust_session_id,
+                               consumer_chan_key, consumer, lost);
+       } else {
+               ret = consumer_get_discarded_events(ust_session_id,
+                               consumer_chan_key, consumer, discarded);
+       }
+
+end:
+       return ret;
+}
+
+int ust_app_pid_get_channel_runtime_stats(struct ltt_ust_session *usess,
+               struct ltt_ust_channel *uchan,
+               struct consumer_output *consumer, int overwrite,
+               uint64_t *discarded, uint64_t *lost)
+{
+       int ret = 0;
+       struct lttng_ht_iter iter;
+       struct lttng_ht_node_str *ua_chan_node;
+       struct ust_app *app;
+       struct ust_app_session *ua_sess;
+       struct ust_app_channel *ua_chan;
+
+       *discarded = 0;
+       *lost = 0;
+
+       rcu_read_lock();
+       /*
+        * Iterate over every registered applications. Sum counters for
+        * all applications containing requested session and channel.
+        */
+       cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app, pid_n.node) {
+               struct lttng_ht_iter uiter;
+
+               ua_sess = lookup_session_by_app(usess, app);
+               if (ua_sess == NULL) {
+                       continue;
+               }
+
+               /* Get channel */
+               lttng_ht_lookup(ua_sess->channels, (void *) uchan->name, &uiter);
+               ua_chan_node = lttng_ht_iter_get_node_str(&uiter);
+               /* If the session is found for the app, the channel must be there */
+               assert(ua_chan_node);
+
+               ua_chan = caa_container_of(ua_chan_node, struct ust_app_channel, node);
+
+               if (overwrite) {
+                       uint64_t _lost;
+
+                       ret = consumer_get_lost_packets(usess->id, ua_chan->key,
+                                       consumer, &_lost);
+                       if (ret < 0) {
+                               break;
+                       }
+                       (*lost) += _lost;
+               } else {
+                       uint64_t _discarded;
+
+                       ret = consumer_get_discarded_events(usess->id,
+                                       ua_chan->key, consumer, &_discarded);
+                       if (ret < 0) {
+                               break;
+                       }
+                       (*discarded) += _discarded;
+               }
+       }
+
+       rcu_read_unlock();
+       return ret;
+}
+
+static
+int ust_app_regenerate_statedump(struct ltt_ust_session *usess,
+               struct ust_app *app)
+{
+       int ret = 0;
+       struct ust_app_session *ua_sess;
+
+       DBG("Regenerating the metadata for ust app pid %d", app->pid);
+
+       rcu_read_lock();
+
+       ua_sess = lookup_session_by_app(usess, app);
+       if (ua_sess == NULL) {
+               /* The session is in teardown process. Ignore and continue. */
+               goto end;
+       }
+
+       pthread_mutex_lock(&ua_sess->lock);
+
+       if (ua_sess->deleted) {
+               goto end_unlock;
+       }
+
+       pthread_mutex_lock(&app->sock_lock);
+       ret = ustctl_regenerate_statedump(app->sock, ua_sess->handle);
+       pthread_mutex_unlock(&app->sock_lock);
+
+end_unlock:
+       pthread_mutex_unlock(&ua_sess->lock);
+
+end:
+       rcu_read_unlock();
+       health_code_update();
+       return ret;
+}
+
+/*
+ * Regenerate the statedump for each app in the session.
+ */
+int ust_app_regenerate_statedump_all(struct ltt_ust_session *usess)
+{
+       int ret = 0;
+       struct lttng_ht_iter iter;
+       struct ust_app *app;
+
+       DBG("Regenerating the metadata for all UST apps");
+
+       rcu_read_lock();
+
+       cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app, pid_n.node) {
+               if (!app->compatible) {
+                       continue;
+               }
+
+               ret = ust_app_regenerate_statedump(usess, app);
+               if (ret < 0) {
+                       /* Continue to the next app even on error */
+                       continue;
+               }
+       }
+
+       rcu_read_unlock();
+
+       return 0;
+}
+
+/*
+ * Rotate all the channels of a session.
+ *
+ * Return LTTNG_OK on success or else an LTTng error code.
+ */
+enum lttng_error_code ust_app_rotate_session(struct ltt_session *session)
+{
+       int ret;
+       enum lttng_error_code cmd_ret = LTTNG_OK;
+       struct lttng_ht_iter iter;
+       struct ust_app *app;
+       struct ltt_ust_session *usess = session->ust_session;
+
+       assert(usess);
+
+       rcu_read_lock();
+
+       switch (usess->buffer_type) {
+       case LTTNG_BUFFER_PER_UID:
+       {
+               struct buffer_reg_uid *reg;
+
+               cds_list_for_each_entry(reg, &usess->buffer_reg_uid_list, lnode) {
+                       struct buffer_reg_channel *reg_chan;
+                       struct consumer_socket *socket;
+
+                       /* Get consumer socket to use to push the metadata.*/
+                       socket = consumer_find_socket_by_bitness(reg->bits_per_long,
+                                       usess->consumer);
+                       if (!socket) {
+                               cmd_ret = LTTNG_ERR_INVALID;
+                               goto error;
+                       }
+
+                       /* Rotate the data channels. */
+                       cds_lfht_for_each_entry(reg->registry->channels->ht, &iter.iter,
+                                       reg_chan, node.node) {
+                               ret = consumer_rotate_channel(socket,
+                                               reg_chan->consumer_key,
+                                               usess->uid, usess->gid,
+                                               usess->consumer,
+                                               /* is_metadata_channel */ false);
+                               if (ret < 0) {
+                                       cmd_ret = LTTNG_ERR_ROTATION_FAIL_CONSUMER;
+                                       goto error;
+                               }
+                       }
+
+                       (void) push_metadata(reg->registry->reg.ust, usess->consumer);
+
+                       ret = consumer_rotate_channel(socket,
+                                       reg->registry->reg.ust->metadata_key,
+                                       usess->uid, usess->gid,
+                                       usess->consumer,
+                                       /* is_metadata_channel */ true);
+                       if (ret < 0) {
+                               cmd_ret = LTTNG_ERR_ROTATION_FAIL_CONSUMER;
+                               goto error;
+                       }
+               }
+               break;
+       }
+       case LTTNG_BUFFER_PER_PID:
+       {
+               cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app, pid_n.node) {
+                       struct consumer_socket *socket;
+                       struct lttng_ht_iter chan_iter;
+                       struct ust_app_channel *ua_chan;
+                       struct ust_app_session *ua_sess;
+                       struct ust_registry_session *registry;
+
+                       ua_sess = lookup_session_by_app(usess, app);
+                       if (!ua_sess) {
+                               /* Session not associated with this app. */
+                               continue;
+                       }
+
+                       /* Get the right consumer socket for the application. */
+                       socket = consumer_find_socket_by_bitness(app->bits_per_long,
+                                       usess->consumer);
+                       if (!socket) {
+                               cmd_ret = LTTNG_ERR_INVALID;
+                               goto error;
+                       }
+
+                       registry = get_session_registry(ua_sess);
+                       if (!registry) {
+                               DBG("Application session is being torn down. Skip application.");
+                               continue;
+                       }
+
+                       /* Rotate the data channels. */
+                       cds_lfht_for_each_entry(ua_sess->channels->ht, &chan_iter.iter,
+                                       ua_chan, node.node) {
+                               ret = consumer_rotate_channel(socket,
+                                               ua_chan->key,
+                                               ua_sess->effective_credentials
+                                                               .uid,
+                                               ua_sess->effective_credentials
+                                                               .gid,
+                                               ua_sess->consumer,
+                                               /* is_metadata_channel */ false);
+                               if (ret < 0) {
+                                       /* Per-PID buffer and application going away. */
+                                       if (ret == -LTTNG_ERR_CHAN_NOT_FOUND)
+                                               continue;
+                                       cmd_ret = LTTNG_ERR_ROTATION_FAIL_CONSUMER;
+                                       goto error;
+                               }
+                       }
+
+                       /* Rotate the metadata channel. */
+                       (void) push_metadata(registry, usess->consumer);
+                       ret = consumer_rotate_channel(socket,
+                                       registry->metadata_key,
+                                       ua_sess->effective_credentials.uid,
+                                       ua_sess->effective_credentials.gid,
+                                       ua_sess->consumer,
+                                       /* is_metadata_channel */ true);
+                       if (ret < 0) {
+                               /* Per-PID buffer and application going away. */
+                               if (ret == -LTTNG_ERR_CHAN_NOT_FOUND)
+                                       continue;
+                               cmd_ret = LTTNG_ERR_ROTATION_FAIL_CONSUMER;
+                               goto error;
+                       }
+               }
+               break;
+       }
+       default:
+               assert(0);
+               break;
+       }
+
+       cmd_ret = LTTNG_OK;
+
+error:
+       rcu_read_unlock();
+       return cmd_ret;
+}
+
+enum lttng_error_code ust_app_create_channel_subdirectories(
+               const struct ltt_ust_session *usess)
+{
+       enum lttng_error_code ret = LTTNG_OK;
+       struct lttng_ht_iter iter;
+       enum lttng_trace_chunk_status chunk_status;
+       char *pathname_index;
+       int fmt_ret;
+
+       assert(usess->current_trace_chunk);
+       rcu_read_lock();
+
+       switch (usess->buffer_type) {
+       case LTTNG_BUFFER_PER_UID:
+       {
+               struct buffer_reg_uid *reg;
+
+               cds_list_for_each_entry(reg, &usess->buffer_reg_uid_list, lnode) {
+                       fmt_ret = asprintf(&pathname_index,
+                                      DEFAULT_UST_TRACE_DIR DEFAULT_UST_TRACE_UID_PATH "/" DEFAULT_INDEX_DIR,
+                                      reg->uid, reg->bits_per_long);
+                       if (fmt_ret < 0) {
+                               ERR("Failed to format channel index directory");
+                               ret = LTTNG_ERR_CREATE_DIR_FAIL;
+                               goto error;
+                       }
+
+                       /*
+                        * Create the index subdirectory which will take care
+                        * of implicitly creating the channel's path.
+                        */
+                       chunk_status = lttng_trace_chunk_create_subdirectory(
+                                       usess->current_trace_chunk,
+                                       pathname_index);
+                       free(pathname_index);
+                       if (chunk_status != LTTNG_TRACE_CHUNK_STATUS_OK) {
+                               ret = LTTNG_ERR_CREATE_DIR_FAIL;
+                               goto error;
+                       }
+               }
+               break;
+       }
+       case LTTNG_BUFFER_PER_PID:
+       {
+               struct ust_app *app;
+
+               cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app,
+                               pid_n.node) {
+                       struct ust_app_session *ua_sess;
+                       struct ust_registry_session *registry;
+
+                       ua_sess = lookup_session_by_app(usess, app);
+                       if (!ua_sess) {
+                               /* Session not associated with this app. */
+                               continue;
+                       }
+
+                       registry = get_session_registry(ua_sess);
+                       if (!registry) {
+                               DBG("Application session is being torn down. Skip application.");
+                               continue;
+                       }
+
+                       fmt_ret = asprintf(&pathname_index,
+                                       DEFAULT_UST_TRACE_DIR "%s/" DEFAULT_INDEX_DIR,
+                                       ua_sess->path);
+                       if (fmt_ret < 0) {
+                               ERR("Failed to format channel index directory");
+                               ret = LTTNG_ERR_CREATE_DIR_FAIL;
+                               goto error;
+                       }
+                       /*
+                        * Create the index subdirectory which will take care
+                        * of implicitly creating the channel's path.
+                        */
+                       chunk_status = lttng_trace_chunk_create_subdirectory(
+                                       usess->current_trace_chunk,
+                                       pathname_index);
+                       free(pathname_index);
+                       if (chunk_status != LTTNG_TRACE_CHUNK_STATUS_OK) {
+                               ret = LTTNG_ERR_CREATE_DIR_FAIL;
+                               goto error;
+                       }
+               }
+               break;
+       }
+       default:
+               abort();
+       }
+
+       ret = LTTNG_OK;
+error:
+       rcu_read_unlock();
+       return ret;
+}
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