Data race in mpeg4video.h

[Zu-Ming]

Note: To prove the data race is real, I set breakpoints before these two access when when they are running in the call stack described above. I find that the breakpoints can be activated simultaneously, and the address of race variables are same. I think these result can prove that the race is real.

Summary of the race

Execute command: ./ffmpeg -y -threads 4 -i small.mp4 output.avi
Version: git-master

Thread 1:
Variable: `dc_val[-wrap]'
Access: c = dc_val[-wrap];
Line number: mpeg4video.h:224
Call stack:

1. ff_mpeg4_pred_dc
2. ff_mpeg4_encode_mb
3. encode_thread
4. worker_func
5. run_jobs
6. thread_worker

Lock:

1. w->mutex (Call stack: thread_worker->pthread_mutex_lock)

Thread 2:
Variable: dc_val[0]
Access: dc_val[0] = level;
Line number: mpeg4video.h; 274
Call stack:

1. ff_mpeg4_pred_dc
2. ff_mpeg4_encode_mb
3. encode_thread
4. worker_func
5. run_jobs
6. thread_worker

Lock:

1. w->mutex (Call stack: thread_worker->pthread_mutex_lock)

Additional information
dc_val[-wrap] in thread 1 and dc_val[0] in thread 2 are the same variable becasue their memory address are same. Althougth having the same name, the lock w->mutex in thread 1 is different from the one in thread

My data race fuzzer, conzzer, finds that these 2 accesses can be executed concurrently, and they are protected by different locks, so my fuzzer report this race.