sessiond: setup event notifier group for registering app
[lttng-tools.git] / src / bin / lttng-sessiond / ust-app.c
index c03772842b88e39976716c7c9f926217d2668e04..b30089dbc2fd6029f590adf8c05752ddda2d3225 100644 (file)
@@ -1,23 +1,12 @@
 /*
- * Copyright (C) 2011 David Goulet <david.goulet@polymtl.ca>
- * Copyright (C) 2016 Jérémie Galarneau <jeremie.galarneau@efficios.com>
+ * 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,
- * as published by the Free Software Foundation.
+ * SPDX-License-Identifier: GPL-2.0-only
  *
- * This program is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU General Public License for more details.
- *
- * You should have received a copy of the GNU General Public License along
- * with this program; if not, write to the Free Software Foundation, Inc.,
- * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
 #define _LGPL_SOURCE
-#include <errno.h>
 #include <inttypes.h>
 #include <pthread.h>
 #include <stdio.h>
@@ -27,9 +16,9 @@
 #include <sys/types.h>
 #include <unistd.h>
 #include <urcu/compiler.h>
-#include <lttng/ust-error.h>
 #include <signal.h>
 
+#include <common/compat/errno.h>
 #include <common/common.h>
 #include <common/sessiond-comm/sessiond-comm.h>
 
 #include "health-sessiond.h"
 #include "ust-app.h"
 #include "ust-consumer.h"
-#include "ust-ctl.h"
+#include "lttng-ust-ctl.h"
+#include "lttng-ust-error.h"
 #include "utils.h"
 #include "session.h"
 #include "lttng-sessiond.h"
 #include "notification-thread-commands.h"
 #include "rotate.h"
 
+struct lttng_ht *ust_app_ht;
+struct lttng_ht *ust_app_ht_by_sock;
+struct lttng_ht *ust_app_ht_by_notify_sock;
+
 static
 int ust_app_flush_app_session(struct ust_app *app, struct ust_app_session *ua_sess);
 
@@ -251,7 +245,7 @@ static struct ust_registry_session *get_session_registry(
        {
                struct buffer_reg_uid *reg_uid = buffer_reg_uid_find(
                                ua_sess->tracing_id, ua_sess->bits_per_long,
-                               ua_sess->real_credentials.uid);
+                               lttng_credentials_get_uid(&ua_sess->real_credentials));
                if (!reg_uid) {
                        goto error;
                }
@@ -741,6 +735,10 @@ error:
  * nullified. The session lock MUST be held unless the application is
  * in the destroy path.
  *
+ * Do not hold the registry lock while communicating with the consumerd, because
+ * doing so causes inter-process deadlocks between consumerd and sessiond with
+ * the metadata request notification.
+ *
  * Return 0 on success else a negative value.
  */
 static int close_metadata(struct ust_registry_session *registry,
@@ -748,6 +746,8 @@ static int close_metadata(struct ust_registry_session *registry,
 {
        int ret;
        struct consumer_socket *socket;
+       uint64_t metadata_key;
+       bool registry_was_already_closed;
 
        assert(registry);
        assert(consumer);
@@ -755,8 +755,19 @@ static int close_metadata(struct ust_registry_session *registry,
        rcu_read_lock();
 
        pthread_mutex_lock(&registry->lock);
+       metadata_key = registry->metadata_key;
+       registry_was_already_closed = registry->metadata_closed;
+       if (metadata_key != 0) {
+               /*
+                * Metadata closed. Even on error this means that the consumer
+                * is not responding or not found so either way a second close
+                * should NOT be emit for this registry.
+                */
+               registry->metadata_closed = 1;
+       }
+       pthread_mutex_unlock(&registry->lock);
 
-       if (!registry->metadata_key || registry->metadata_closed) {
+       if (metadata_key == 0 || registry_was_already_closed) {
                ret = 0;
                goto end;
        }
@@ -766,23 +777,15 @@ static int close_metadata(struct ust_registry_session *registry,
                        consumer);
        if (!socket) {
                ret = -1;
-               goto error;
+               goto end;
        }
 
-       ret = consumer_close_metadata(socket, registry->metadata_key);
+       ret = consumer_close_metadata(socket, metadata_key);
        if (ret < 0) {
-               goto error;
+               goto end;
        }
 
-error:
-       /*
-        * Metadata closed. Even on error this means that the consumer is not
-        * responding or not found so either way a second close should NOT be emit
-        * for this registry.
-        */
-       registry->metadata_closed = 1;
 end:
-       pthread_mutex_unlock(&registry->lock);
        rcu_read_unlock();
        return ret;
 }
@@ -918,6 +921,28 @@ void delete_ust_app(struct ust_app *app)
        ht_cleanup_push(app->ust_sessions_objd);
        ht_cleanup_push(app->ust_objd);
 
+       /*
+        * This could be NULL if the event notifier setup failed (e.g the app
+        * was killed or the tracer does not support this feature).
+        */
+       if (app->event_notifier_group.object) {
+               enum lttng_error_code ret_code;
+               const int event_notifier_read_fd = lttng_pipe_get_readfd(
+                               app->event_notifier_group.event_pipe);
+
+               ret_code = notification_thread_command_remove_tracer_event_source(
+                               notification_thread_handle,
+                               event_notifier_read_fd);
+               if (ret_code != LTTNG_OK) {
+                       ERR("Failed to remove application tracer event source from notification thread");
+               }
+
+               ustctl_release_object(sock, app->event_notifier_group.object);
+               free(app->event_notifier_group.object);
+       }
+
+       lttng_pipe_destroy(app->event_notifier_group.event_pipe);
+
        /*
         * Wait until we have deleted the application from the sock hash table
         * before closing this socket, otherwise an application could re-use the
@@ -1016,7 +1041,7 @@ error_free:
  * Alloc new UST app channel.
  */
 static
-struct ust_app_channel *alloc_ust_app_channel(char *name,
+struct ust_app_channel *alloc_ust_app_channel(const char *name,
                struct ust_app_session *ua_sess,
                struct lttng_ust_channel_attr *attr)
 {
@@ -1140,7 +1165,7 @@ struct ust_app_ctx *alloc_ust_app_ctx(struct lttng_ust_context_attr *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;
+                       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);
@@ -1191,7 +1216,7 @@ error:
  * Return allocated filter or NULL on error.
  */
 static struct lttng_ust_filter_bytecode *create_ust_bytecode_from_bytecode(
-               struct lttng_filter_bytecode *orig_f)
+               const struct lttng_filter_bytecode *orig_f)
 {
        struct lttng_ust_filter_bytecode *filter = NULL;
 
@@ -1340,33 +1365,28 @@ error:
 /*
  * Set the filter on the tracer.
  */
-static
-int set_ust_event_filter(struct ust_app_event *ua_event,
-               struct ust_app *app)
+static int set_ust_object_filter(struct ust_app *app,
+               const struct lttng_filter_bytecode *bytecode,
+               struct lttng_ust_object_data *ust_object)
 {
        int ret;
        struct lttng_ust_filter_bytecode *ust_bytecode = NULL;
 
        health_code_update();
 
-       if (!ua_event->filter) {
-               ret = 0;
-               goto error;
-       }
-
-       ust_bytecode = create_ust_bytecode_from_bytecode(ua_event->filter);
+       ust_bytecode = create_ust_bytecode_from_bytecode(bytecode);
        if (!ust_bytecode) {
                ret = -LTTNG_ERR_NOMEM;
                goto error;
        }
        pthread_mutex_lock(&app->sock_lock);
        ret = ustctl_set_filter(app->sock, ust_bytecode,
-                       ua_event->obj);
+                       ust_object);
        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) "
-                                       "with ret %d", ua_event->attr.name, app->pid, ret);
+                       ERR("UST app set object filter failed for object %p of app (pid: %d) "
+                                       "with ret %d", ust_object, app->pid, ret);
                } else {
                        /*
                         * This is normal behavior, an application can die during the
@@ -1374,12 +1394,12 @@ int set_ust_event_filter(struct ust_app_event *ua_event,
                         * continue normally.
                         */
                        ret = 0;
-                       DBG3("UST app filter event failed. Application is dead.");
+                       DBG3("Failed to set UST app object filter. Application is dead.");
                }
                goto error;
        }
 
-       DBG2("UST filter set successfully for event %s", ua_event->name);
+       DBG2("UST filter successfully set for object %p", ust_object);
 
 error:
        health_code_update();
@@ -1389,7 +1409,7 @@ error:
 
 static
 struct lttng_ust_event_exclusion *create_ust_exclusion_from_exclusion(
-               struct lttng_event_exclusion *exclusion)
+               const struct lttng_event_exclusion *exclusion)
 {
        struct lttng_ust_event_exclusion *ust_exclusion = NULL;
        size_t exclusion_alloc_size = sizeof(struct lttng_ust_event_exclusion) +
@@ -1411,33 +1431,30 @@ end:
 /*
  * Set event exclusions on the tracer.
  */
-static
-int set_ust_event_exclusion(struct ust_app_event *ua_event,
-               struct ust_app *app)
+static int set_ust_object_exclusions(struct ust_app *app,
+               const struct lttng_event_exclusion *exclusions,
+               struct lttng_ust_object_data *ust_object)
 {
        int ret;
-       struct lttng_ust_event_exclusion *ust_exclusion = NULL;
+       struct lttng_ust_event_exclusion *ust_exclusions = NULL;
 
-       health_code_update();
+       assert(exclusions && exclusions->count > 0);
 
-       if (!ua_event->exclusion || !ua_event->exclusion->count) {
-               ret = 0;
-               goto error;
-       }
+       health_code_update();
 
-       ust_exclusion = create_ust_exclusion_from_exclusion(
-                       ua_event->exclusion);
-       if (!ust_exclusion) {
+       ust_exclusions = create_ust_exclusion_from_exclusion(
+                       exclusions);
+       if (!ust_exclusions) {
                ret = -LTTNG_ERR_NOMEM;
                goto error;
        }
        pthread_mutex_lock(&app->sock_lock);
-       ret = ustctl_set_exclusion(app->sock, ust_exclusion, ua_event->obj);
+       ret = ustctl_set_exclusion(app->sock, ust_exclusions, ust_object);
        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) "
-                                       "with ret %d", ua_event->attr.name, app->pid, ret);
+                       ERR("Failed to set UST app exclusions for object %p of app (pid: %d) "
+                                       "with ret %d", ust_object, app->pid, ret);
                } else {
                        /*
                         * This is normal behavior, an application can die during the
@@ -1445,37 +1462,36 @@ int set_ust_event_exclusion(struct ust_app_event *ua_event,
                         * continue normally.
                         */
                        ret = 0;
-                       DBG3("UST app event exclusion failed. Application is dead.");
+                       DBG3("Failed to set UST app object exclusions. Application is dead.");
                }
                goto error;
        }
 
-       DBG2("UST exclusion set successfully for event %s", ua_event->name);
+       DBG2("UST exclusions set successfully for object %p", ust_object);
 
 error:
        health_code_update();
-       free(ust_exclusion);
+       free(ust_exclusions);
        return ret;
 }
 
 /*
  * Disable the specified event on to UST tracer for the UST session.
  */
-static int disable_ust_event(struct ust_app *app,
-               struct ust_app_session *ua_sess, struct ust_app_event *ua_event)
+static int disable_ust_object(struct ust_app *app,
+               struct lttng_ust_object_data *object)
 {
        int ret;
 
        health_code_update();
 
        pthread_mutex_lock(&app->sock_lock);
-       ret = ustctl_disable(app->sock, ua_event->obj);
+       ret = ustctl_disable(app->sock, object);
        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) "
-                                       "and session handle %d with ret %d",
-                                       ua_event->attr.name, app->pid, ua_sess->handle, ret);
+                       ERR("Failed to disable UST app object %p app (pid: %d) with ret %d",
+                                       object, app->pid, ret);
                } else {
                        /*
                         * This is normal behavior, an application can die during the
@@ -1483,13 +1499,13 @@ static int disable_ust_event(struct ust_app *app,
                         * continue normally.
                         */
                        ret = 0;
-                       DBG3("UST app disable event failed. Application is dead.");
+                       DBG3("Failed to disable UST app object. Application is dead.");
                }
                goto error;
        }
 
-       DBG2("UST app event %s disabled successfully for app (pid: %d)",
-                       ua_event->attr.name, app->pid);
+       DBG2("UST app object %p disabled successfully for app (pid: %d)",
+                       object, app->pid);
 
 error:
        health_code_update();
@@ -1577,21 +1593,20 @@ error:
 /*
  * Enable the specified event on to UST tracer for the UST session.
  */
-static int enable_ust_event(struct ust_app *app,
-               struct ust_app_session *ua_sess, struct ust_app_event *ua_event)
+static int enable_ust_object(
+               struct ust_app *app, struct lttng_ust_object_data *ust_object)
 {
        int ret;
 
        health_code_update();
 
        pthread_mutex_lock(&app->sock_lock);
-       ret = ustctl_enable(app->sock, ua_event->obj);
+       ret = ustctl_enable(app->sock, ust_object);
        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) "
-                                       "and session handle %d with ret %d",
-                                       ua_event->attr.name, app->pid, ua_sess->handle, ret);
+                       ERR("UST app enable failed for object %p app (pid: %d) with ret %d",
+                                       ust_object, app->pid, ret);
                } else {
                        /*
                         * This is normal behavior, an application can die during the
@@ -1599,13 +1614,13 @@ static int enable_ust_event(struct ust_app *app,
                         * continue normally.
                         */
                        ret = 0;
-                       DBG3("UST app enable event failed. Application is dead.");
+                       DBG3("Failed to enable UST app object. Application is dead.");
                }
                goto error;
        }
 
-       DBG2("UST app event %s enabled successfully for app (pid: %d)",
-                       ua_event->attr.name, app->pid);
+       DBG2("UST app object %p enabled successfully for app (pid: %d)",
+                       ust_object, app->pid);
 
 error:
        health_code_update();
@@ -1701,14 +1716,14 @@ int create_ust_event(struct ust_app *app, struct ust_app_session *ua_sess,
 
        ua_event->handle = ua_event->obj->handle;
 
-       DBG2("UST app event %s created successfully for pid:%d",
-                       ua_event->attr.name, app->pid);
+       DBG2("UST app event %s created successfully for pid:%d object: %p",
+                       ua_event->attr.name, app->pid, ua_event->obj);
 
        health_code_update();
 
        /* Set filter if one is present. */
        if (ua_event->filter) {
-               ret = set_ust_event_filter(ua_event, app);
+               ret = set_ust_object_filter(app, ua_event->filter, ua_event->obj);
                if (ret < 0) {
                        goto error;
                }
@@ -1716,7 +1731,7 @@ int create_ust_event(struct ust_app *app, struct ust_app_session *ua_sess,
 
        /* Set exclusions for the event */
        if (ua_event->exclusion) {
-               ret = set_ust_event_exclusion(ua_event, app);
+               ret = set_ust_object_exclusions(app, ua_event->exclusion, ua_event->obj);
                if (ret < 0) {
                        goto error;
                }
@@ -1728,7 +1743,7 @@ int create_ust_event(struct ust_app *app, struct ust_app_session *ua_sess,
                 * We now need to explicitly enable the event, since it
                 * is now disabled at creation.
                 */
-               ret = enable_ust_event(app, ua_sess, ua_event);
+               ret = enable_ust_object(app, ua_event->obj);
                if (ret < 0) {
                        /*
                         * If we hit an EPERM, something is wrong with our enable call. If
@@ -1844,10 +1859,10 @@ static void shadow_copy_session(struct ust_app_session *ua_sess,
 
        ua_sess->tracing_id = usess->id;
        ua_sess->id = get_next_session_id();
-       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;
+       LTTNG_OPTIONAL_SET(&ua_sess->real_credentials.uid, app->uid);
+       LTTNG_OPTIONAL_SET(&ua_sess->real_credentials.gid, app->gid);
+       LTTNG_OPTIONAL_SET(&ua_sess->effective_credentials.uid, usess->uid);
+       LTTNG_OPTIONAL_SET(&ua_sess->effective_credentials.gid, usess->gid);
        ua_sess->buffer_type = usess->buffer_type;
        ua_sess->bits_per_long = app->bits_per_long;
 
@@ -1869,7 +1884,7 @@ static void shadow_copy_session(struct ust_app_session *ua_sess,
        case LTTNG_BUFFER_PER_UID:
                ret = snprintf(ua_sess->path, sizeof(ua_sess->path),
                                DEFAULT_UST_TRACE_UID_PATH,
-                               ua_sess->real_credentials.uid,
+                               lttng_credentials_get_uid(&ua_sess->real_credentials),
                                app->bits_per_long);
                break;
        default:
@@ -1892,12 +1907,12 @@ static void shadow_copy_session(struct ust_app_session *ua_sess,
                switch (ua_sess->buffer_type) {
                case LTTNG_BUFFER_PER_PID:
                        ret = snprintf(tmp_shm_path, sizeof(tmp_shm_path),
-                                       DEFAULT_UST_TRACE_PID_PATH "/%s-%d-%s",
+                                       "/" DEFAULT_UST_TRACE_PID_PATH "/%s-%d-%s",
                                        app->name, app->pid, datetime);
                        break;
                case LTTNG_BUFFER_PER_UID:
                        ret = snprintf(tmp_shm_path, sizeof(tmp_shm_path),
-                                       DEFAULT_UST_TRACE_UID_PATH,
+                                       "/" DEFAULT_UST_TRACE_UID_PATH,
                                        app->uid, app->bits_per_long);
                        break;
                default:
@@ -1992,8 +2007,9 @@ static int setup_buffer_reg_pid(struct ust_app_session *ua_sess,
                        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->effective_credentials.uid,
-                       ua_sess->effective_credentials.gid, ua_sess->tracing_id,
+                       lttng_credentials_get_uid(&ua_sess->effective_credentials),
+                       lttng_credentials_get_gid(&ua_sess->effective_credentials),
+                       ua_sess->tracing_id,
                        app->uid);
        if (ret < 0) {
                /*
@@ -2293,7 +2309,7 @@ end:
  */
 static
 int create_ust_app_channel_context(struct ust_app_channel *ua_chan,
-               struct lttng_ust_context_attr *uctx,
+               struct lttng_ust_context_attr *uctx,
                struct ust_app *app)
 {
        int ret = 0;
@@ -2338,7 +2354,7 @@ int enable_ust_app_event(struct ust_app_session *ua_sess,
 {
        int ret;
 
-       ret = enable_ust_event(app, ua_sess, ua_event);
+       ret = enable_ust_object(app, ua_event->obj);
        if (ret < 0) {
                goto error;
        }
@@ -2357,7 +2373,7 @@ static int disable_ust_app_event(struct ust_app_session *ua_sess,
 {
        int ret;
 
-       ret = disable_ust_event(app, ua_sess, ua_event);
+       ret = disable_ust_object(app, ua_event->obj);
        if (ret < 0) {
                goto error;
        }
@@ -2884,8 +2900,9 @@ static int create_channel_per_uid(struct ust_app *app,
 
        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,
+                       lttng_credentials_get_uid(&ua_sess->effective_credentials),
+                       lttng_credentials_get_gid(&ua_sess->effective_credentials),
+                       ua_chan->name,
                        ua_chan->key, LTTNG_DOMAIN_UST,
                        ua_chan->attr.subbuf_size * ua_chan->attr.num_subbuf);
        if (notification_ret != LTTNG_OK) {
@@ -2984,8 +3001,9 @@ static int create_channel_per_pid(struct ust_app *app,
 
        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,
+                       lttng_credentials_get_uid(&ua_sess->effective_credentials),
+                       lttng_credentials_get_gid(&ua_sess->effective_credentials),
+                       ua_chan->name,
                        ua_chan->key, LTTNG_DOMAIN_UST,
                        ua_chan->attr.subbuf_size * ua_chan->attr.num_subbuf);
        if (cmd_ret != LTTNG_OK) {
@@ -3316,6 +3334,7 @@ error:
 struct ust_app *ust_app_create(struct ust_register_msg *msg, int sock)
 {
        struct ust_app *lta = NULL;
+       struct lttng_pipe *event_notifier_event_source_pipe = NULL;
 
        assert(msg);
        assert(sock >= 0);
@@ -3332,12 +3351,21 @@ struct ust_app *ust_app_create(struct ust_register_msg *msg, int sock)
                goto error;
        }
 
+       event_notifier_event_source_pipe = lttng_pipe_open(FD_CLOEXEC);
+       if (!event_notifier_event_source_pipe) {
+               PERROR("Failed to open application event source pipe: '%s' (ppid = %d)",
+                               msg->name, msg->ppid);
+               goto error;
+       }
+
        lta = zmalloc(sizeof(struct ust_app));
        if (lta == NULL) {
                PERROR("malloc");
-               goto error;
+               goto error_free_pipe;
        }
 
+       lta->event_notifier_group.event_pipe = event_notifier_event_source_pipe;
+
        lta->ppid = msg->ppid;
        lta->uid = msg->uid;
        lta->gid = msg->gid;
@@ -3375,8 +3403,12 @@ struct ust_app *ust_app_create(struct ust_register_msg *msg, int sock)
        lttng_ht_node_init_ulong(&lta->sock_n, (unsigned long) lta->sock);
 
        CDS_INIT_LIST_HEAD(&lta->teardown_head);
-error:
        return lta;
+
+error_free_pipe:
+       lttng_pipe_destroy(event_notifier_event_source_pipe);
+error:
+       return NULL;
 }
 
 /*
@@ -3442,6 +3474,61 @@ int ust_app_version(struct ust_app *app)
        return ret;
 }
 
+/*
+ * Setup the base event notifier group.
+ *
+ * Return 0 on success else a negative value either an errno code or a
+ * LTTng-UST error code.
+ */
+int ust_app_setup_event_notifier_group(struct ust_app *app)
+{
+       int ret;
+       int event_pipe_write_fd;
+       struct lttng_ust_object_data *event_notifier_group = NULL;
+       enum lttng_error_code lttng_ret;
+
+       assert(app);
+
+       /* Get the write side of the pipe. */
+       event_pipe_write_fd = lttng_pipe_get_writefd(
+                       app->event_notifier_group.event_pipe);
+
+       pthread_mutex_lock(&app->sock_lock);
+       ret = ustctl_create_event_notifier_group(app->sock,
+                       event_pipe_write_fd, &event_notifier_group);
+       pthread_mutex_unlock(&app->sock_lock);
+       if (ret < 0) {
+               if (ret != -LTTNG_UST_ERR_EXITING && ret != -EPIPE) {
+                       ERR("Failed to create application event notifier group: ret = %d, app socket fd = %d, event_pipe_write_fd = %d",
+                                       ret, app->sock, event_pipe_write_fd);
+               } else {
+                       DBG("Failed to create application event notifier group (application is dead): app socket fd = %d",
+                                       app->sock);
+               }
+
+               goto error;
+       }
+
+       lttng_ret = notification_thread_command_add_tracer_event_source(
+                       notification_thread_handle,
+                       lttng_pipe_get_readfd(app->event_notifier_group.event_pipe),
+                       LTTNG_DOMAIN_UST);
+       if (lttng_ret != LTTNG_OK) {
+               ERR("Failed to add tracer event source to notification thread");
+               ret = - 1;
+               goto error;
+       }
+
+       /* Assign handle only when the complete setup is valid. */
+       app->event_notifier_group.object = event_notifier_group;
+       return ret;
+
+error:
+       ustctl_release_object(app->sock, app->event_notifier_group.object);
+       free(app->event_notifier_group.object);
+       return ret;
+}
+
 /*
  * Unregister app by removing it from the global traceable app list and freeing
  * the data struct.
@@ -4109,11 +4196,14 @@ int ust_app_channel_create(struct ltt_ust_session *usess,
                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;
+               if (ret < 0) {
+                       goto error;
+               }
+
+               ret = ust_app_channel_send(app, usess,
+                       ua_sess, ua_chan);
+               if (ret) {
+                       goto error;
                }
 
                /* Add contexts. */
@@ -4121,10 +4211,12 @@ int ust_app_channel_create(struct ltt_ust_session *usess,
                        ret = create_ust_app_channel_context(ua_chan,
                                &uctx->ctx, app);
                        if (ret) {
-                               goto end;
+                               goto error;
                        }
                }
        }
+
+error:
        if (ret < 0) {
                switch (ret) {
                case -ENOTCONN:
@@ -4140,7 +4232,7 @@ int ust_app_channel_create(struct ltt_ust_session *usess,
                        break;
                }
        }
-end:
+
        if (ret == 0 && _ua_chan) {
                /*
                 * Only return the application's channel on success. Note
@@ -4152,75 +4244,6 @@ end:
        return ret;
 }
 
-/*
- * For a specific UST session, create the channel for all registered apps.
- */
-int ust_app_create_channel_glb(struct ltt_ust_session *usess,
-               struct ltt_ust_channel *uchan)
-{
-       int ret = 0;
-       struct cds_lfht_iter iter;
-       struct ust_app *app;
-
-       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, 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;
-               }
-
-               /*
-                * Create session on the tracer side and add it to app session HT. Note
-                * that if session exist, it will simply return a pointer to the ust
-                * app session.
-                */
-               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; it is not an error.
-                                */
-                               ret = 0;
-                               goto error_rcu_unlock;
-                       case -ENOMEM:
-                       default:
-                               goto error_rcu_unlock;
-                       }
-               }
-
-               if (ua_sess->deleted) {
-                       continue;
-               }
-               ret = ust_app_channel_create(usess, ua_sess, uchan, app, NULL);
-               if (ret) {
-                       if (session_was_created) {
-                               destroy_app_session(app, ua_sess);
-                       }
-                       /* Continue to the next application. */
-               }
-       }
-
-error_rcu_unlock:
-       rcu_read_unlock();
-       return ret;
-}
-
 /*
  * Enable event for a specific session and channel on the tracer.
  */
@@ -4407,20 +4430,16 @@ int ust_app_start_trace(struct ltt_ust_session *usess, struct ust_app *app)
                goto end;
        }
 
+       if (ua_sess->enabled) {
+               pthread_mutex_unlock(&ua_sess->lock);
+               goto end;
+       }
+
        /* Upon restart, we skip the setup, already done */
        if (ua_sess->started) {
                goto skip_setup;
        }
 
-       /*
-        * Create the metadata for the application. This returns gracefully if a
-        * metadata was already set for the session.
-        */
-       ret = create_ust_app_metadata(ua_sess, app, usess->consumer);
-       if (ret < 0) {
-               goto error_unlock;
-       }
-
        health_code_update();
 
 skip_setup:
@@ -4447,6 +4466,7 @@ skip_setup:
 
        /* Indicate that the session has been started once */
        ua_sess->started = 1;
+       ua_sess->enabled = 1;
 
        pthread_mutex_unlock(&ua_sess->lock);
 
@@ -4536,6 +4556,7 @@ int ust_app_stop_trace(struct ltt_ust_session *usess, struct ust_app *app)
        }
 
        health_code_update();
+       ua_sess->enabled = 0;
 
        /* Quiescent wait after stopping trace */
        pthread_mutex_lock(&app->sock_lock);
@@ -5055,7 +5076,7 @@ end:
 
 /*
  * The caller must ensure that the application is compatible and is tracked
- * by the PID tracker.
+ * by the process attribute trackers.
  */
 static
 void ust_app_synchronize(struct ltt_ust_session *usess,
@@ -5086,6 +5107,7 @@ void ust_app_synchronize(struct ltt_ust_session *usess,
        }
 
        rcu_read_lock();
+
        cds_lfht_for_each_entry(usess->domain_global.channels->ht, &uchan_iter,
                        uchan, node.node) {
                struct ust_app_channel *ua_chan;
@@ -5099,7 +5121,7 @@ void ust_app_synchronize(struct ltt_ust_session *usess,
                 * allocated (if necessary) and sent to the application, and
                 * all enabled contexts will be added to the channel.
                 */
-               ret = find_or_create_ust_app_channel(usess, ua_sess,
+               ret = find_or_create_ust_app_channel(usess, ua_sess,
                        app, uchan, &ua_chan);
                if (ret) {
                        /* Tracer is probably gone or ENOMEM. */
@@ -5129,6 +5151,21 @@ void ust_app_synchronize(struct ltt_ust_session *usess,
                        }
                }
        }
+
+       /*
+        * Create the metadata for the application. This returns gracefully if a
+        * metadata was already set for the session.
+        *
+        * The metadata channel must be created after the data channels as the
+        * consumer daemon assumes this ordering. When interacting with a relay
+        * daemon, the consumer will use this assumption to send the
+        * "STREAMS_SENT" message to the relay daemon.
+        */
+       ret = create_ust_app_metadata(ua_sess, app, usess->consumer);
+       if (ret < 0) {
+               goto error_unlock;
+       }
+
        rcu_read_unlock();
 
 end:
@@ -5175,7 +5212,14 @@ 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)) {
+       if (trace_ust_id_tracker_lookup(LTTNG_PROCESS_ATTR_VIRTUAL_PROCESS_ID,
+                           usess, app->pid) &&
+                       trace_ust_id_tracker_lookup(
+                                       LTTNG_PROCESS_ATTR_VIRTUAL_USER_ID,
+                                       usess, app->uid) &&
+                       trace_ust_id_tracker_lookup(
+                                       LTTNG_PROCESS_ATTR_VIRTUAL_GROUP_ID,
+                                       usess, app->gid)) {
                /*
                 * Synchronize the application's internal tracing configuration
                 * and start tracing.
@@ -5912,6 +5956,7 @@ enum lttng_error_code ust_app_snapshot_record(
                        struct buffer_reg_channel *reg_chan;
                        struct consumer_socket *socket;
                        char pathname[PATH_MAX];
+                       size_t consumer_path_offset = 0;
 
                        if (!reg->registry->reg.ust->metadata_key) {
                                /* Skip since no metadata is present */
@@ -5927,12 +5972,8 @@ enum lttng_error_code ust_app_snapshot_record(
                        }
 
                        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");
@@ -5941,18 +5982,19 @@ enum lttng_error_code ust_app_snapshot_record(
                        }
                        /* Free path allowed on previous iteration. */
                        free(trace_path);
-                       trace_path = setup_channel_trace_path(usess->consumer, pathname);
+                       trace_path = setup_channel_trace_path(usess->consumer, pathname,
+                                               &consumer_path_offset);
                        if (!trace_path) {
                                status = LTTNG_ERR_INVALID;
                                goto error;
                        }
-                        /* Add the UST default trace dir to path. */
+                       /* Add the UST default trace dir to path. */
                        cds_lfht_for_each_entry(reg->registry->channels->ht, &iter.iter,
                                        reg_chan, node.node) {
                                status = consumer_snapshot_channel(socket,
                                                reg_chan->consumer_key,
                                                output, 0, usess->uid,
-                                               usess->gid, trace_path, wait,
+                                               usess->gid, &trace_path[consumer_path_offset], wait,
                                                nb_packets_per_stream);
                                if (status != LTTNG_OK) {
                                        goto error;
@@ -5960,7 +6002,8 @@ enum lttng_error_code ust_app_snapshot_record(
                        }
                        status = consumer_snapshot_channel(socket,
                                        reg->registry->reg.ust->metadata_key, output, 1,
-                                       usess->uid, usess->gid, trace_path, wait, 0);
+                                       usess->uid, usess->gid, &trace_path[consumer_path_offset],
+                                       wait, 0);
                        if (status != LTTNG_OK) {
                                goto error;
                        }
@@ -5976,6 +6019,7 @@ enum lttng_error_code ust_app_snapshot_record(
                        struct ust_app_session *ua_sess;
                        struct ust_registry_session *registry;
                        char pathname[PATH_MAX];
+                       size_t consumer_path_offset = 0;
 
                        ua_sess = lookup_session_by_app(usess, app);
                        if (!ua_sess) {
@@ -5993,7 +6037,7 @@ enum lttng_error_code ust_app_snapshot_record(
 
                        /* 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;
@@ -6002,20 +6046,19 @@ enum lttng_error_code ust_app_snapshot_record(
                        }
                        /* Free path allowed on previous iteration. */
                        free(trace_path);
-                       trace_path = setup_channel_trace_path(usess->consumer, pathname);
+                       trace_path = setup_channel_trace_path(usess->consumer, pathname,
+                                       &consumer_path_offset);
                        if (!trace_path) {
                                status = LTTNG_ERR_INVALID;
                                goto error;
                        }
-                        cds_lfht_for_each_entry(ua_sess->channels->ht, &chan_iter.iter,
+                       cds_lfht_for_each_entry(ua_sess->channels->ht, &chan_iter.iter,
                                        ua_chan, node.node) {
                                status = consumer_snapshot_channel(socket,
                                                ua_chan->key, output, 0,
-                                               ua_sess->effective_credentials
-                                                               .uid,
-                                               ua_sess->effective_credentials
-                                                               .gid,
-                                               trace_path, wait,
+                                               lttng_credentials_get_uid(&ua_sess->effective_credentials),
+                                               lttng_credentials_get_gid(&ua_sess->effective_credentials),
+                                               &trace_path[consumer_path_offset], wait,
                                                nb_packets_per_stream);
                                switch (status) {
                                case LTTNG_OK:
@@ -6034,9 +6077,9 @@ enum lttng_error_code ust_app_snapshot_record(
                        }
                        status = consumer_snapshot_channel(socket,
                                        registry->metadata_key, output, 1,
-                                       ua_sess->effective_credentials.uid,
-                                       ua_sess->effective_credentials.gid,
-                                       trace_path, wait, 0);
+                                       lttng_credentials_get_uid(&ua_sess->effective_credentials),
+                                       lttng_credentials_get_gid(&ua_sess->effective_credentials),
+                                       &trace_path[consumer_path_offset], wait, 0);
                        switch (status) {
                        case LTTNG_OK:
                                break;
@@ -6316,6 +6359,11 @@ enum lttng_error_code ust_app_rotate_session(struct ltt_session *session)
                        struct buffer_reg_channel *reg_chan;
                        struct consumer_socket *socket;
 
+                       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);
@@ -6386,10 +6434,8 @@ enum lttng_error_code ust_app_rotate_session(struct ltt_session *session)
                                        ua_chan, node.node) {
                                ret = consumer_rotate_channel(socket,
                                                ua_chan->key,
-                                               ua_sess->effective_credentials
-                                                               .uid,
-                                               ua_sess->effective_credentials
-                                                               .gid,
+                                               lttng_credentials_get_uid(&ua_sess->effective_credentials),
+                                               lttng_credentials_get_gid(&ua_sess->effective_credentials),
                                                ua_sess->consumer,
                                                /* is_metadata_channel */ false);
                                if (ret < 0) {
@@ -6405,8 +6451,8 @@ enum lttng_error_code ust_app_rotate_session(struct ltt_session *session)
                        (void) push_metadata(registry, usess->consumer);
                        ret = consumer_rotate_channel(socket,
                                        registry->metadata_key,
-                                       ua_sess->effective_credentials.uid,
-                                       ua_sess->effective_credentials.gid,
+                                       lttng_credentials_get_uid(&ua_sess->effective_credentials),
+                                       lttng_credentials_get_gid(&ua_sess->effective_credentials),
                                        ua_sess->consumer,
                                        /* is_metadata_channel */ true);
                        if (ret < 0) {
@@ -6450,7 +6496,7 @@ enum lttng_error_code ust_app_create_channel_subdirectories(
 
                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,
+                                      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");
@@ -6477,6 +6523,17 @@ enum lttng_error_code ust_app_create_channel_subdirectories(
        {
                struct ust_app *app;
 
+               /*
+                * Create the toplevel ust/ directory in case no apps are running.
+                */
+               chunk_status = lttng_trace_chunk_create_subdirectory(
+                               usess->current_trace_chunk,
+                               DEFAULT_UST_TRACE_DIR);
+               if (chunk_status != LTTNG_TRACE_CHUNK_STATUS_OK) {
+                       ret = LTTNG_ERR_CREATE_DIR_FAIL;
+                       goto error;
+               }
+
                cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app,
                                pid_n.node) {
                        struct ust_app_session *ua_sess;
@@ -6495,7 +6552,7 @@ enum lttng_error_code ust_app_create_channel_subdirectories(
                        }
 
                        fmt_ret = asprintf(&pathname_index,
-                                       DEFAULT_UST_TRACE_DIR "%s/" DEFAULT_INDEX_DIR,
+                                       DEFAULT_UST_TRACE_DIR "/%s/" DEFAULT_INDEX_DIR,
                                        ua_sess->path);
                        if (fmt_ret < 0) {
                                ERR("Failed to format channel index directory");
@@ -6526,3 +6583,274 @@ error:
        rcu_read_unlock();
        return ret;
 }
+
+/*
+ * Clear all the channels of a session.
+ *
+ * Return LTTNG_OK on success or else an LTTng error code.
+ */
+enum lttng_error_code ust_app_clear_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();
+
+       if (usess->active) {
+               ERR("Expecting inactive session %s (%" PRIu64 ")", session->name, session->id);
+               cmd_ret = LTTNG_ERR_FATAL;
+               goto end;
+       }
+
+       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_socket;
+                       }
+
+                       /* Clear the data channels. */
+                       cds_lfht_for_each_entry(reg->registry->channels->ht, &iter.iter,
+                                       reg_chan, node.node) {
+                               ret = consumer_clear_channel(socket,
+                                               reg_chan->consumer_key);
+                               if (ret < 0) {
+                                       goto error;
+                               }
+                       }
+
+                       (void) push_metadata(reg->registry->reg.ust, usess->consumer);
+
+                       /*
+                        * Clear the metadata channel.
+                        * Metadata channel is not cleared per se but we still need to
+                        * perform a rotation operation on it behind the scene.
+                        */
+                       ret = consumer_clear_channel(socket,
+                                       reg->registry->reg.ust->metadata_key);
+                       if (ret < 0) {
+                               goto error;
+                       }
+               }
+               break;
+       }
+       case LTTNG_BUFFER_PER_PID:
+       {
+               cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app, pid_n.node) {
+                       struct consumer_socket *socket;
+                       struct lttng_ht_iter chan_iter;
+                       struct ust_app_channel *ua_chan;
+                       struct ust_app_session *ua_sess;
+                       struct ust_registry_session *registry;
+
+                       ua_sess = lookup_session_by_app(usess, app);
+                       if (!ua_sess) {
+                               /* Session not associated with this app. */
+                               continue;
+                       }
+
+                       /* 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_socket;
+                       }
+
+                       registry = get_session_registry(ua_sess);
+                       if (!registry) {
+                               DBG("Application session is being torn down. Skip application.");
+                               continue;
+                       }
+
+                       /* Clear the data channels. */
+                       cds_lfht_for_each_entry(ua_sess->channels->ht, &chan_iter.iter,
+                                       ua_chan, node.node) {
+                               ret = consumer_clear_channel(socket, ua_chan->key);
+                               if (ret < 0) {
+                                       /* Per-PID buffer and application going away. */
+                                       if (ret == -LTTNG_ERR_CHAN_NOT_FOUND) {
+                                               continue;
+                                       }
+                                       goto error;
+                               }
+                       }
+
+                       (void) push_metadata(registry, usess->consumer);
+
+                       /*
+                        * Clear the metadata channel.
+                        * Metadata channel is not cleared per se but we still need to
+                        * perform rotation operation on it behind the scene.
+                        */
+                       ret = consumer_clear_channel(socket, registry->metadata_key);
+                       if (ret < 0) {
+                               /* Per-PID buffer and application going away. */
+                               if (ret == -LTTNG_ERR_CHAN_NOT_FOUND) {
+                                       continue;
+                               }
+                               goto error;
+                       }
+               }
+               break;
+       }
+       default:
+               assert(0);
+               break;
+       }
+
+       cmd_ret = LTTNG_OK;
+       goto end;
+
+error:
+       switch (-ret) {
+       case LTTCOMM_CONSUMERD_RELAYD_CLEAR_DISALLOWED:
+               cmd_ret = LTTNG_ERR_CLEAR_RELAY_DISALLOWED;
+               break;
+       default:
+               cmd_ret = LTTNG_ERR_CLEAR_FAIL_CONSUMER;
+       }
+
+error_socket:
+end:
+       rcu_read_unlock();
+       return cmd_ret;
+}
+
+/*
+ * This function skips the metadata channel as the begin/end timestamps of a
+ * metadata packet are useless.
+ *
+ * Moreover, opening a packet after a "clear" will cause problems for live
+ * sessions as it will introduce padding that was not part of the first trace
+ * chunk. The relay daemon expects the content of the metadata stream of
+ * successive metadata trace chunks to be strict supersets of one another.
+ *
+ * For example, flushing a packet at the beginning of the metadata stream of
+ * a trace chunk resulting from a "clear" session command will cause the
+ * size of the metadata stream of the new trace chunk to not match the size of
+ * the metadata stream of the original chunk. This will confuse the relay
+ * daemon as the same "offset" in a metadata stream will no longer point
+ * to the same content.
+ */
+enum lttng_error_code ust_app_open_packets(struct ltt_session *session)
+{
+       enum lttng_error_code ret = LTTNG_OK;
+       struct lttng_ht_iter iter;
+       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;
+
+                       socket = consumer_find_socket_by_bitness(
+                                       reg->bits_per_long, usess->consumer);
+                       if (!socket) {
+                               ret = LTTNG_ERR_FATAL;
+                               goto error;
+                       }
+
+                       cds_lfht_for_each_entry(reg->registry->channels->ht,
+                                       &iter.iter, reg_chan, node.node) {
+                               const int open_ret =
+                                               consumer_open_channel_packets(
+                                                       socket,
+                                                       reg_chan->consumer_key);
+
+                               if (open_ret < 0) {
+                                       ret = LTTNG_ERR_UNK;
+                                       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 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) {
+                               ret = LTTNG_ERR_FATAL;
+                               goto error;
+                       }
+
+                       registry = get_session_registry(ua_sess);
+                       if (!registry) {
+                               DBG("Application session is being torn down. Skip application.");
+                               continue;
+                       }
+
+                       cds_lfht_for_each_entry(ua_sess->channels->ht,
+                                       &chan_iter.iter, ua_chan, node.node) {
+                               const int open_ret =
+                                               consumer_open_channel_packets(
+                                                       socket,
+                                                       ua_chan->key);
+
+                               if (open_ret < 0) {
+                                       /*
+                                        * Per-PID buffer and application going
+                                        * away.
+                                        */
+                                       if (open_ret == -LTTNG_ERR_CHAN_NOT_FOUND) {
+                                               continue;
+                                       }
+
+                                       ret = LTTNG_ERR_UNK;
+                                       goto error;
+                               }
+                       }
+               }
+               break;
+       }
+       default:
+               abort();
+               break;
+       }
+
+error:
+       rcu_read_unlock();
+       return ret;
+}
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