[FFmpeg-devel] [PATCH] avutil/threadmessage: fix condition broadcasting

[Clément Bœsch]

From: Clément Bœsch <clement at stupeflix.com>

Fix a dead lock under certain conditions. Let's assume we have a queue of 1
message max, 2 senders, and 1 receiver.

Scenario (real record obtained with debug added):
    [...]
    SENDER #0: acquired lock
    SENDER #0: queue is full, wait
    SENDER #1: acquired lock
    SENDER #1: queue is full, wait
    RECEIVER: acquired lock
    RECEIVER: reading a msg from the queue
    RECEIVER: signal the cond
    RECEIVER: acquired lock
    RECEIVER: queue is empty, wait
    SENDER #0: writing a msg the queue
    SENDER #0: signal the cond
    SENDER #0: acquired lock
    SENDER #0: queue is full, wait
    SENDER #1: queue is full, wait

Translated:
 - initially the queue contains 1/1 message with 2 senders blocking on
   it, waiting to push another message.
 - Meanwhile the receiver is obtaining the lock, read the message,
   signal & release the lock. For some reason it is able to acquire the
   lock again before the signal wakes up one of the sender. Since it
   just emptied the queue, the reader waits for the queue to fill up
   again.
 - The signal finally reaches one of the sender, which writes a message
   and then signal the condition. Unfortunately, instead of waking up
   the reader, it actually wakes up the other worker (signal = notify
   the condition just for 1 waiter), who can't push another message in
   the queue because it's full.
 - Meanwhile, the receiver is still waiting. Deadlock.

This scenario can be triggered with for example:
    tests/api/api-threadmessage-test 2 1 100 100 999

The fix is simply to make sure to notify everyone when work is done (be
it reading or writing). Another solution would be to use 2 distincts
conditions.
---
 libavutil/threadmessage.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/libavutil/threadmessage.c b/libavutil/threadmessage.c
index b7fcbe2..2089c6d 100644
--- a/libavutil/threadmessage.c
+++ b/libavutil/threadmessage.c
@@ -107,7 +107,7 @@ static int av_thread_message_queue_send_locked(AVThreadMessageQueue *mq,
     if (mq->err_send)
         return mq->err_send;
     av_fifo_generic_write(mq->fifo, msg, mq->elsize, NULL);
-    pthread_cond_signal(&mq->cond);
+    pthread_cond_broadcast(&mq->cond);
     return 0;
 }
 
@@ -123,7 +123,7 @@ static int av_thread_message_queue_recv_locked(AVThreadMessageQueue *mq,
     if (av_fifo_size(mq->fifo) < mq->elsize)
         return mq->err_recv;
     av_fifo_generic_read(mq->fifo, msg, mq->elsize, NULL);
-    pthread_cond_signal(&mq->cond);
+    pthread_cond_broadcast(&mq->cond);
     return 0;
 }
 
-- 
2.6.2