[alsa-devel] Races in alsa-lib with threads

Rémi Denis-Courmont remi at remlab.net
Wed Nov 14 21:03:52 CET 2012


Le mercredi 14 novembre 2012 21:49:36, Trent Piepho a écrit :
> > You claim that the Gstreamer ALSA sink plugin accesses alsa-lib from two
> > threads simultaneously. Could you elaborate on how this can happen, maybe
> > it is easy to fix?
> gstreamer has no lock around the call to snd_pcm_delay().  So it can
> race with snd_pcm_wait() or snd_pcm_writei(), which are called in
> another thread.  There is a lock around the block of code calling
> wait()/writei(), but this lock isn't used for calling delay().

Use non-blocking I/O and poll(). Then the snd_pcm_write() calls will not sleep 
and can be interlocked with snd_pcm_delay() calls.

> It seems that they didn't know there would be a problem.

Eh? It is rather the exception for a library to be thread-safe when using the 
same object in multiple threads.

It looks more like "they" did not pay attention to what they were doing.

Rémi Denis-Courmont

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