[alsa-devel] Duplicate wake-ups in pcm_lib.c

[pl bossart]

Thanks to Takashi's advice, I managed to find out the reason why I was
seeing null events returned by poll(). This could explain why
PulseAudio doesn't seem to sleep much. It turns out that we have two
calls to wakeup() in pcm_lib.c, and a nice race condition it seems.
See the log below.

A wake-up is generated during the period interrupt, and a second
wake-up is generated during the write loop, after the application was
awaken but just before the pointers are updated. This second wake-up
shouldn't exist, since the write loop actually fills the ring buffer.
By the time the second wake-up is actually handled, there's really no
space left in the buffer and a null event is generated; it'll wake-up
the application a second time for nothing. Maybe we should move the
call to snd_pcm_update_hw_ptr() after the transfer took place?

Can anyone apply the attached patches (only adds printks) and confirm
this issue on their platforms? That would be very useful.
Happy holidays everyone
- Pierre


alsa-lib/test/pcm --verbose -Dhw -c2 -r48000 -f440  --method write_and_poll

[  371.369865] snd_intel8x0_interrupt
[  371.369876]  snd_intel8x0_update
[  371.369884] snd_pcm_period_elapsed
[  371.369890]  snd_pcm_update_hw_ptr_interrupt
[  371.369900]    snd_pcm_update_hw_ptr_post
[  371.369909]     wakeup in snd_pcm_update_hw_ptr_post
sound/core/pcm_lib.c: 214, avail 8194, avail_min 8192
[  371.369935] ALSA: poll event  POLLOUT|POLLWRNORM, avail 8194 avail_min 8192
[  371.375449] snd_pcm_lib_write1: in
[  371.375457] snd_pcm_lib_write1 loop
[  371.375462]  snd_pcm_update_hw_ptr
[  371.375472]    snd_pcm_update_hw_ptr_post
[  371.375481]     wakeup in snd_pcm_update_hw_ptr_post
sound/core/pcm_lib.c: 214, avail 8462, avail_min 8192

>>Between these two points, the appl_ptr is updated, and there's no space left in the buffer. The wake-up is generated too early.

[  371.375514] snd_pcm_lib_write1: out
[  371.375524] ALSA: null poll event, avail 270 avail_min 8192
[  371.540532] snd_intel8x0_interrupt
[  371.540542]  snd_intel8x0_update
[  371.540551] snd_pcm_period_elapsed
[  371.540556]  snd_pcm_update_hw_ptr_interrupt
[  371.540566]    snd_pcm_update_hw_ptr_post
[  371.540575]     wakeup in snd_pcm_update_hw_ptr_post
sound/core/pcm_lib.c: 214, avail 8194, avail_min 8192
[  371.540601] ALSA: poll event  POLLOUT|POLLWRNORM, avail 8194 avail_min 8192
[  371.543734] snd_pcm_lib_write1: in
[  371.543741] snd_pcm_lib_write1 loop
[  371.543746]  snd_pcm_update_hw_ptr
[  371.543755]    snd_pcm_update_hw_ptr_post
[  371.543764]     wakeup in snd_pcm_update_hw_ptr_post
sound/core/pcm_lib.c: 214, avail 8347, avail_min 8192
[  371.543797] snd_pcm_lib_write1: out
[  371.543807] ALSA: null poll event, avail 155 avail_min 8192
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