[FFmpeg-trac] #604(avcodec:new): Multithreaded ffmpeg decode with DXVA2 hangs

FFmpeg trac at avcodec.org
Wed Nov 2 00:53:21 CET 2011

#604: Multithreaded ffmpeg decode with DXVA2 hangs
             Reporter:  IMarshal     |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:  avcodec      |                  Version:  git-
             Keywords:  hang, dxva,  |  master
  pthreads                           |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
 I posted this at http://ffmpeg.zeranoe.com/forum/viewtopic.php?f=23&t=267,
 but it's probably more appropriate here.

 First, I'd like to say how awesome ffmpeg is.  We love it, love it, love

 Anyway, we're seeing a hang attempting to decode H264 video using DXVA2
 (aka pixel format PIX_FMT_DXVA2_VLD). This is in the recently released
 Plex Media Center (http://www.plexapp.com). The only workaround we've
 found is to use a single thread when decoding.

 It would be great if you could help us diagnose the issue. We suspect an
 ffmpeg bug, but it's possible that we're doing something wrong as well.

 In our repro case, we call avcodec_thread_init with 8 threads, then
 avcodec_open, then obviously avcodec_decode_video. If we don't call
 avcodec_thread_init and we leave the context's default of 1 thread, then
 no hang occurs and decoding proceeds smoothly.

 The hang itself occurs on the second or third call to
 avcodec_decode_video. Unfortunately, because of the difficulty of
 debugging ffmpeg code on Windows, I don't have good stack traces to
 provide. All I can see is avcodec waiting on what appear to be pthreads
 condition variables. None of our related threads are hung, and
 avcodec_decode_video is not returning, so my guess would be a deadlock in

 Here's a sample thread in cdb - not very useful, obviously:

 24 Id: 3bc8.15e4 Suspend: 1 Teb: fff50000 Unfrozen
 ChildEBP RetAddr
 1681f728 76eb0bdd ntdll_779a0000!NtWaitForMultipleObjects+0x15
 1681f7c4 75f71a2c KERNELBASE!WaitForMultipleObjectsEx+0x100
 1681f80c 75f74208 KERNEL32!WaitForMultipleObjectsExImplementation+0xe0
 1681f828 6e0c5173 KERNEL32!WaitForMultipleObjects+0x18
 WARNING: Stack unwind information not available. Following frames may be
 1681f858 6e0c5582 libpthread_2!pthreadCancelableTimedWait+0x47
 1681f8d8 6e0c66c3 libpthread_2!sem_timedwait+0x15e
 1681f928 661a1740 libpthread_2!pthread_cond_wait+0xeb
 1681f974 6e0c5121 avcodec_53!av_parser_close+0x71ac
 1681f9a4 76861287 libpthread_2!pthread_exit+0xf5
 1681f9dc 76861328 msvcrt!_endthreadex+0x44
 1681f9e4 75f7339a msvcrt!_endthreadex+0xce
 1681f9f0 779d9ed2 KERNEL32!BaseThreadInitThunk+0xe
 1681fa30 779d9ea5 ntdll_779a0000!__RtlUserThreadStart+0x70
 1681fa48 00000000 ntdll_779a0000!_RtlUserThreadStart+0x1b

 More context: we're using ffmpeg library bits built from current ffmpeg
 sources, using MSYS/MinGW. We have some customizations, but they are
 relatively minor and unrelated to this area.

 The following is our configure command line:

 $ cat configure-windows.sh
 ./configure --disable-static --disable-debug --enable-runtime-cpudetect
 --disable-mmx --disable-mmx2 --arch=i386 --disable-muxers --enable-
 muxer=spdif --enable-muxer=adts --disable-encoders --enable-encoder=ac3
 --enable-encoder=aac --disable-libvorbis --disable-devices --disable-
 ffplay --disable-ffserver --disable-ffmpeg --enable-shared --disable-
 decoder=mpeg_xvmc --enable-postproc --enable-gpl --enable-protocol=http
 --enable-pthreads --enable-runtime-cpudetect --enable-memalign-hack
 --enable-dxva2 --enable-decoder=h264_dxva2 --enable-hwaccel=h264_dxva2

 Is there any other data I can provide that might help to debug this?


Ticket URL: <https://ffmpeg.org/trac/ffmpeg/ticket/604>
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