Store the version of the tracer in the UID registry

[lttng-tools.git] / src / bin / lttng-sessiond / ust-app.c

diff --git a/src/bin/lttng-sessiond/ust-app.c b/src/bin/lttng-sessiond/ust-app.c
index 285467fbbacac990864a0871d18f200a0eb7f67b..9b634facc1422630827e5e92268839ef37b20146 100644 (file)
--- a/src/bin/lttng-sessiond/ust-app.c
+++ b/src/bin/lttng-sessiond/ust-app.c
@@ -376,6 +376,8 @@ void delete_ust_app_channel_rcu(struct rcu_head *head)
  * 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)
@@ -390,12 +392,22 @@ void save_per_pid_lost_discarded_counters(struct ust_app_channel *ua_chan)
 
        rcu_read_lock();
        session = session_find_by_id(ua_chan->session->tracing_id);
-       if (!session) {
-               ERR("Missing LTT session to get discarded events");
-               goto end;
-       }
-       if (!session->ust_session) {
-               ERR("Missing UST session to get discarded events");
+       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 The session is then unpublished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;
        }
 
@@ -426,6 +438,8 @@ end:
 /*
  * 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,
@@ -787,6 +801,8 @@ 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,
@@ -870,6 +886,11 @@ 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;
@@ -908,6 +929,7 @@ void delete_ust_app(struct ust_app *app)
 
        DBG2("UST app pid %d deleted", app->pid);
        free(app);
+       session_unlock_list();
 }
 
 /*
@@ -928,6 +950,8 @@ 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)
@@ -5971,7 +5995,6 @@ int ust_app_pid_get_channel_runtime_stats(struct ltt_ust_session *usess,
                                        ua_chan->key, consumer, discarded);
                        goto end;
                }
-               goto end;
        }
 
 end: