Fix: session_find_by_id can return NULL legitimately

[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 331a9d67160d4fee424e4ddb892e1f7057f8983b..9b634facc1422630827e5e92268839ef37b20146 100644 (file)
--- a/src/bin/lttng-sessiond/ust-app.c
+++ b/src/bin/lttng-sessiond/ust-app.c
@@ -392,12 +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 The session is then unpublished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;
        }