From: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Date: Wed, 9 Mar 2016 20:34:52 +0000 (-0500)
Subject: Fix: session_find_by_id can return NULL legitimately
X-Git-Tag: v2.8.0-rc1~94
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=d68ec9741e83c99eea549243eec46c37b4eea61f;p=lttng-tools.git

Fix: session_find_by_id can return NULL legitimately

session_find_by_id can return NULL when the backing hash table
is NULL. This is not an error in the context of
its caller, save_per_pid_lost_discarded_counters(), since the
ltt_session can be destroyed before an ust app session is torn down.

See the comment in save_per_pid_lost_discarded_counters() for more
information.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
---

diff --git a/src/bin/lttng-sessiond/session.c b/src/bin/lttng-sessiond/session.c
index 456901685..5a10340a7 100644
--- a/src/bin/lttng-sessiond/session.c
+++ b/src/bin/lttng-sessiond/session.c
@@ -298,17 +298,21 @@ struct ltt_session *session_find_by_id(uint64_t id)
 	struct lttng_ht_iter iter;
 	struct ltt_session *ls;
 
+	if (!ltt_sessions_ht_by_id) {
+		goto end;
+	}
+
 	lttng_ht_lookup(ltt_sessions_ht_by_id, &id, &iter);
 	node = lttng_ht_iter_get_node_u64(&iter);
 	if (node == NULL) {
-		goto error;
+		goto end;
 	}
 	ls = caa_container_of(node, struct ltt_session, node);
 
 	DBG3("Session %" PRIu64 " found by id.", id);
 	return ls;
 
-error:
+end:
 	DBG3("Session %" PRIu64 " NOT found by id", id);
 	return NULL;
 }
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;
 	}