[FFmpeg-devel] [PATCH] submit_thread should also lock the mutex that guards the source avctx (prev_thread->avctx) when calling update_context_from_thread.

[Wan-Teh Chang]

This is because the codec->decode() call in frame_worker_thread may be
modifying that avctx at the same time. This data race is reported by
ThreadSanitizer.

Although submit_thread holds two locks simultaneously, it always
acquires them in the same order because |prev_thread| is always the
array element before |p| (with a wraparound at the end of array), and
submit_thread is the only function that acquires those two locks, so
this won't result in a deadlock.
---
 libavcodec/pthread_frame.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/libavcodec/pthread_frame.c b/libavcodec/pthread_frame.c
index b77dd1e..e7879e3 100644
--- a/libavcodec/pthread_frame.c
+++ b/libavcodec/pthread_frame.c
@@ -330,7 +330,9 @@ static int submit_packet(PerThreadContext *p, AVPacket *avpkt)
             pthread_mutex_unlock(&prev_thread->progress_mutex);
         }
 
+        pthread_mutex_lock(&prev_thread->mutex);
         err = update_context_from_thread(p->avctx, prev_thread->avctx, 0);
+        pthread_mutex_unlock(&prev_thread->mutex);
         if (err) {
             pthread_mutex_unlock(&p->mutex);
             return err;
-- 
2.7.0.rc3.207.g0ac5344