[FFmpeg-devel] Threading issue with avcodec_decode_video2 ?

[Michael Niedermayer]

On Sat, Oct 27, 2012 at 10:15:19PM +0200, Hendrik Leppkes wrote:
> On Sat, Oct 27, 2012 at 9:40 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> > On Sat, Oct 27, 2012 at 12:59:37AM -0400, Don Moir wrote:
> >> >I load up multiple instances of the same file. These are used for
> >> >playback, thumbnails, and other things. Each is in a separate
> >> >thread with their own context, variables, etc.
> >> >
> >> >We noticed a crash when decoding WMV3 files. It's was a rare event so hard to track down.
> >> >
> >> >Recently I put some code in to help find the problem. It seems
> >> >that avcodec_decode_video2 is not thread safe at least for WMV3
> >> >decoding.
> >> >
> >> >I have disable-pthreads set. I use av_lockmgr_register and see it
> >> >allows a lock for avformat and avcodec. The lock for avcodec never
> >> >gets called.
> >> >
> >> >If I put a lock around avcodec_decode_video2, the problem goes
> >> >away. After several crashes that led nowhere, I isolated it to
> >> >avcodec_decode_video2.
> >> >
> >> >Shouldn't related decoders be setting the lock via the lock manager if needed ?
> >> >
> >> >I don't want to have to lock the entire avcodec_decode_video2 call
> >> >and lock should be special cased for each decoder and so I am left
> >> >wondering.
> >> >
> >>
> >> Looks like the call to ff_msmpeg4_decode_init is causing the problem.
> >>
> >> About midway down in vc1_decode_frame you will see:
> >>
> >> if (ff_msmpeg4_decode_init(avctx) < 0 || ff_vc1_decode_init_alloc_tables(v) < 0)
> >>
> >> I added avpriv_lock_avcodec and avpriv_unlock_avcodec and wrapped
> >> ff_msmpeg4_decode_init(avctx) with a lock and problem went away. So
> >> there may be more to it but can't get it to crash anymore.
> >> ff_vc1_decode_init_alloc_tables(v) appears to be ok.
> >>
> >> The scary part is what about the other decoders. I have not noticed
> >> a problem outside of this though. There could also be a way to fix
> >> it without using a lock but didn't look further.
> >
> > thanks for pointing me to these functions, they indeed had some
> > thread saftey bugs.
> > should be fixed, does it work without the lock now ?
> >
> 
> Couldn't ff_msmpeg4_decode_init be called from the decoder init
> function, which is already inside a lock, so the static portions get
> initialized there?
> On a quick look, it doesn't seem to depend on any important values
> into avctx which would only be available later.

probably yes, and patch for that is welcome

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Those who are best at talking, realize last or never when they are wrong.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 198 bytes
Desc: Digital signature
URL: <http://ffmpeg.org/pipermail/ffmpeg-devel/attachments/20121027/3154768d/attachment.asc>