X-Git-Url: https://git.lttng.org/?p=lttng-tools.git;a=blobdiff_plain;f=src%2Fbin%2Flttng-sessiond%2Fust-app.c;h=9b634facc1422630827e5e92268839ef37b20146;hp=331a9d67160d4fee424e4ddb892e1f7057f8983b;hb=d68ec9741e83c99eea549243eec46c37b4eea61f;hpb=accdc9bfb89aa7e2f511caa9c69f59a42c55c2af

diff --git a/src/bin/lttng-sessiond/ust-app.c b/src/bin/lttng-sessiond/ust-app.c
index 331a9d671..9b634facc 100644
--- 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 The session is then unpublished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;
 	}