From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Fri, 1 Nov 2019 20:23:05 +0000 (-0400)
Subject: Fix: sessiond: ust: deadlock with per-pid buffers
X-Git-Tag: v2.10.10~7
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=c27a42efc458d8f868c4183424ea85c0bf3663e7;p=lttng-tools.git

Fix: sessiond: ust: deadlock with per-pid buffers

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.

The deadlock involves both sessiond and consumerd:

* lttng-sessiond:

thread 11 - thread_application_management

close_metadata()
  pthread_mutex_lock(&registry->lock);
  consumer_close_metadata()
    pthread_mutex_lock(socket->lock);

thread 15 - thread_consumer_management

ust_consumer_metadata_request()
  pthread_mutex_lock(&ust_reg->lock);

thread 8 - thread_manage_clients

consumer_is_data_pending
  pthread_mutex_lock(socket->lock);
  consumer_socket_recv()

* lttng-consumerd:

thread 4 - consumer_timer_thread

sample_channel_positions()
  pthread_mutex_lock(&stream->lock);

thread 8 - consumer_thread_sessiond_poll
  consumer_data_pending
  pthread_mutex_lock(&consumer_data.lock);
  pthread_mutex_lock(&stream->lock);

thread 7 - consumer_thread_data_poll

lttng_consumer_read_subbuffer
  pthread_mutex_lock(&stream->chan->lock);
  pthread_mutex_lock(&stream->lock);
  do_sync_metadata
    pthread_mutex_lock(&metadata->lock);
    lttng_ustconsumer_sync_metadata
      pthread_mutex_unlock(&metadata_stream->lock);
      lttng_ustconsumer_request_metadata()
        pthread_mutex_lock(&ctx->metadata_socket_lock);
        lttcomm_recv_unix_sock()

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
---

diff --git a/src/bin/lttng-sessiond/ust-app.c b/src/bin/lttng-sessiond/ust-app.c
index d3bcfd3d7..efc82b052 100644
--- a/src/bin/lttng-sessiond/ust-app.c
+++ b/src/bin/lttng-sessiond/ust-app.c
@@ -731,6 +731,10 @@ 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,
@@ -738,6 +742,8 @@ 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);
@@ -745,8 +751,19 @@ 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;
 	}
@@ -756,23 +773,15 @@ 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;
 }