[alsa-devel] Hang in snd_pcm_writei with alsa-pulse plugin

[Raymond Yau]

2010/11/2 Colin Guthrie <gmane at colin.guthr.ie>

> 'Twas brillig, and Matthew Gregan at 01/11/10 01:38 did gyre and gimble:
> > Hi,
> >
> > I think I'm seeing a bug in the alsa-pulse plugin where the buffer
> > management ends up corrupt and results in a deadlock waiting for free
> buffer
> > space.  This occurs when resuming from pause using snd_pcm_pause.  After
> > resuming, my application tries to write a fixed block of data, expecting
> > snd_pcm_writei to block if the data is larger than the available buffer
> size
> > (the result of snd_pcm_avail_update).
> >
> > I originally observed this in the wild in Firefox, which pauses and
> resumes
> > the sound device whenever network buffering occurs.  I'm planning to
> include
> > the workaround mentioned below in the next Firefox release (Mozilla bug
> > 573924).
> >
> > What happens is that, after resuming with snd_pcm_pause, a call to
> > snd_pcm_writei never returns.  This happens on the write call that would
> > have exceeded the available buffer size, which I would expect to block
> only
> > until sufficient buffer space became available.
> >
> > It's possible to get into a similar situation using SND_PCM_NONBLOCK and
> > waiting on the sound device if it returns EAGAIN, except that
> snd_pcm_writei
> > always returns EAGAIN and snd_pcm_wait returns 1 immediately, resulting
> in a
> > tight loop in the calling code.
> >
> > I discovered that I can reliably workaround the problem by ensuring the
> > first writes after resuming from pause are never larger than what
> > snd_pcm_avail_update returns.  After writing enough to fill (but not
> exceed)
> > the available buffer size, the code returns to the fixed buffer size per
> > write strategy and continues as normal.
> >
> > The problem occurs with the following stack:
> >
> > #0  __poll (fds=<value optimized out>,
> >     nfds=<value optimized out>, timeout=<value optimized out>)
> >     at ../sysdeps/unix/sysv/linux/poll.c:87
> > #1  snd1_pcm_wait_nocheck (pcm=0x1b9a780, timeout=-1)
> >     at pcm.c:2367
> > #2  snd1_pcm_write_areas (pcm=0x1b9a780,
> >     areas=0x7fff4ce9b890, offset=<value optimized out>, size=30000,
> >     func=0x339ba91d10 <ioplug_priv_transfer_areas>) at pcm.c:6655
> > #3  snd_pcm_ioplug_writei (pcm=0x1b9a780,
> >     buffer=<value optimized out>, size=30000) at pcm_ioplug.c:561
> > #4  bwrite (pcm=0x1b9a780, towrite=30000) at
> >     atest2.c:29
> > #5  main (argc=1, argv=0x7fff4ce9ba68) at atest2.c:86
> >
> > I'm Fedora 13 x86_64 with all updates from updates-testing.  ALSA is
> > 1.0.22-1, PulseAudio is 0.9.21-6, and the kernel is 2.6.34.7-61.  I've
> also
> > tested against the current git versions of alsa-libs and alsa-plugins and
> > can still reproduce the problem.
> >
> > I've attached a simple test program that reproduces this problem reliably
> on
> > my machine.  It writes a period sized buffer in a loop, waiting half a
> > period until the next attempt.  Every few iterations, it pauses the sound
> > device for half a period and then resumes it.  It usually hangs within
> 2-3
> > pause/resume cycles.  Running the test with "-r" enables the recovery
> code I
> > mentioned above.  It never hangs when tested using the hardware ALSA
> backend
> > with alsa-pulse disabled, but my sound hardware doesn't seem to support
> > snd_pcm_pause.
>
> Reproduced here I presume:
> [colin at jimmy pulseaudio (master)]$ ./atest2
> playback, wrote 3000 frames (needed 0)
> playback, wrote 3000 frames (needed 1480)
> playback, wrote 3000 frames (needed 2643)
> playback, wrote 3000 frames (needed 3526)
> playback, wrote 3000 frames (needed 526)
> playback, wrote 3000 frames (needed 1313)
> pausing playback
> resuming playback
> ^C
> [colin at jimmy pulseaudio (master)]$ ./atest2
> playback, wrote 3000 frames (needed 0)
> playback, wrote 3000 frames (needed 1584)
> playback, wrote 3000 frames (needed 2764)
> playback, wrote 3000 frames (needed 3648)
> playback, wrote 3000 frames (needed 648)
> playback, wrote 3000 frames (needed 1426)
> pausing playback
> resuming playback
> ^C
>
> This is with latest PA from stable-queue.
>
> Not sure whether this is "expected" or not, but it's probably worth you
> posting this on PA devel list too for some further thought if this
> thread doesn't garner much response here.
>
>
No much response because there are bugs in atest2.c

  /* prefill sound buffers and begin playback */
  fill(pcm);

  while (++count) {

The program had filled the buffer but the output does not indicate those
write


I can confirm that the program seem hang after a few pause/unpause when
using alsa-pulse plugin

However it assert when using hw device

  assert(bsize / psize >= 4);