snd_pcm_drop causing lockup with DMix

snd_pcm_drop causing lockup with DMix

[John Lindgren]

Hi,

A couple of weeks ago I reported a rather serious bug #4914 that can
cause a multi-threaded using ALSA to lock up so that it can only be
ended with SIGKILL.

I maintain the ALSA output part of Audacious, and I need to be able to
play audio in one thread of the program and then signal that thread to
stop from another.  The way to do this in ALSA, if I understand
correctly, is to interrupt the thread that is sending audio with a call
to snd_pcm_drop.  But using DMix makes the first thread hang up in a
snd_pcm_wait call with a timeout of -1.

Is anyone working on this bug?  Is there any more help I can provide?

John Lindgren

Re: snd_pcm_drop causing lockup with DMix

[Jaroslav Kysela]

On Sun, 28 Feb 2010, John Lindgren wrote:

> Hi,
>
> A couple of weeks ago I reported a rather serious bug #4914 that can
> cause a multi-threaded using ALSA to lock up so that it can only be
> ended with SIGKILL.
>
> I maintain the ALSA output part of Audacious, and I need to be able to
> play audio in one thread of the program and then signal that thread to
> stop from another.  The way to do this in ALSA, if I understand
> correctly, is to interrupt the thread that is sending audio with a call
> to snd_pcm_drop.  But using DMix makes the first thread hang up in a
> snd_pcm_wait call with a timeout of -1.
>
> Is anyone working on this bug?  Is there any more help I can provide?

Could you try this patch?

diff --git a/src/pcm/pcm_direct.c b/src/pcm/pcm_direct.c
index d9e596e..8c7ca06 100644
--- a/src/pcm/pcm_direct.c
+++ b/src/pcm/pcm_direct.c
@@ -1274,6 +1274,7 @@ int snd_pcm_direct_set_timer_params(snd_pcm_direct_t *dmix)
  	snd_timer_params_set_ticks(params, 1);
  	if (dmix->tread) {
  		filter = (1<<SND_TIMER_EVENT_TICK) |
+			 (1<<SND_TIMER_EVENT_STOP) |
  			 dmix->timer_event_suspend |
  			 dmix->timer_event_resume;
  		snd_timer_params_set_filter(params, filter);

 					Jaroslav

-----
Jaroslav Kysela <perex@perex.cz>
Linux Kernel Sound Maintainer
ALSA Project, Red Hat, Inc.

Re: snd_pcm_drop causing lockup with DMix

[John Lindgren]

Unfortunately, it doesn't make a difference.

On Sun, 2010-02-28 at 17:22 +0100, Jaroslav Kysela wrote:
> Could you try this patch?
> 
> diff --git a/src/pcm/pcm_direct.c b/src/pcm/pcm_direct.c
> index d9e596e..8c7ca06 100644
> --- a/src/pcm/pcm_direct.c
> +++ b/src/pcm/pcm_direct.c
> @@ -1274,6 +1274,7 @@ int snd_pcm_direct_set_timer_params(snd_pcm_direct_t *dmix)
>   	snd_timer_params_set_ticks(params, 1);
>   	if (dmix->tread) {
>   		filter = (1<<SND_TIMER_EVENT_TICK) |
> +			 (1<<SND_TIMER_EVENT_STOP) |
>   			 dmix->timer_event_suspend |
>   			 dmix->timer_event_resume;
>   		snd_timer_params_set_filter(params, filter);
> 
>  					Jaroslav
> 
> -----
> Jaroslav Kysela <perex@perex.cz>

Re: snd_pcm_drop causing lockup with DMix

[Jaroslav Kysela]

On Sun, 28 Feb 2010, John Lindgren wrote:

> Unfortunately, it doesn't make a difference.

And this one?

diff --git a/src/pcm/pcm_direct.c b/src/pcm/pcm_direct.c
index d9e596e..ae32fab 100644
--- a/src/pcm/pcm_direct.c
+++ b/src/pcm/pcm_direct.c
@@ -540,7 +540,6 @@ void snd_pcm_direct_clear_timer_queue(snd_pcm_direct_t *dmix)
  int snd_pcm_direct_timer_stop(snd_pcm_direct_t *dmix)
  {
  	snd_timer_stop(dmix->timer);
-	snd_pcm_direct_clear_timer_queue(dmix);
  	return 0;
  }

@@ -1274,6 +1273,7 @@ int snd_pcm_direct_set_timer_params(snd_pcm_direct_t *dmix)
  	snd_timer_params_set_ticks(params, 1);
  	if (dmix->tread) {
  		filter = (1<<SND_TIMER_EVENT_TICK) |
+			 (1<<SND_TIMER_EVENT_STOP) |
  			 dmix->timer_event_suspend |
  			 dmix->timer_event_resume;
  		snd_timer_params_set_filter(params, filter);

-----
Jaroslav Kysela <perex@perex.cz>
Linux Kernel Sound Maintainer
ALSA Project, Red Hat, Inc.

Re: snd_pcm_drop causing lockup with DMix

[John Lindgren]

Still no difference.

I also noticed that with or without the patch, Audacious will
occasionally not hang up, and instead the snd_pcm_writei call that is
interrupted will return -EBADFD.  I haven't seen the test case do the
same.

On Mon, 2010-03-01 at 07:53 +0100, Jaroslav Kysela wrote:
> And this one?
> 
> diff --git a/src/pcm/pcm_direct.c b/src/pcm/pcm_direct.c
> index d9e596e..ae32fab 100644
> --- a/src/pcm/pcm_direct.c
> +++ b/src/pcm/pcm_direct.c
> @@ -540,7 +540,6 @@ void snd_pcm_direct_clear_timer_queue(snd_pcm_direct_t *dmix)
>   int snd_pcm_direct_timer_stop(snd_pcm_direct_t *dmix)
>   {
>   	snd_timer_stop(dmix->timer);
> -	snd_pcm_direct_clear_timer_queue(dmix);
>   	return 0;
>   }
> 
> @@ -1274,6 +1273,7 @@ int snd_pcm_direct_set_timer_params(snd_pcm_direct_t *dmix)
>   	snd_timer_params_set_ticks(params, 1);
>   	if (dmix->tread) {
>   		filter = (1<<SND_TIMER_EVENT_TICK) |
> +			 (1<<SND_TIMER_EVENT_STOP) |
>   			 dmix->timer_event_suspend |
>   			 dmix->timer_event_resume;
>   		snd_timer_params_set_filter(params, filter);
> 
> -----
> Jaroslav Kysela <perex@perex.cz>
> Linux Kernel Sound Maintainer
> ALSA Project, Red Hat, Inc.

Re: snd_pcm_drop causing lockup with DMix

[Jaroslav Kysela]

On Mon, 1 Mar 2010, John Lindgren wrote:

> Still no difference.
>
> I also noticed that with or without the patch, Audacious will
> occasionally not hang up, and instead the snd_pcm_writei call that is
> interrupted will return -EBADFD.  I haven't seen the test case do the
> same.

Could you provide a simple C program to test this issue? You may modify 
alsa-lib/src/test/pcm_min.c for example.

 					Thanks,
 						Jaroslav

-----
Jaroslav Kysela <perex@perex.cz>
Linux Kernel Sound Maintainer
ALSA Project, Red Hat, Inc.

Re: snd_pcm_drop causing lockup with DMix

[John Lindgren]

On Tue, 2010-03-02 at 14:25 +0100, Jaroslav Kysela wrote:
> Could you provide a simple C program to test this issue? You may modify 
> alsa-lib/src/test/pcm_min.c for example.

There is already one attached to the bug report page:

https://bugtrack.alsa-project.org/alsa-bug/bug_view_advanced_page.php?bug_id=4914

Thanks for investigating this.

John Lindgren

Re: snd_pcm_drop causing lockup with DMix

[Jaroslav Kysela]

On Tue, 2 Mar 2010, John Lindgren wrote:

> On Tue, 2010-03-02 at 14:25 +0100, Jaroslav Kysela wrote:
>> Could you provide a simple C program to test this issue? You may modify
>> alsa-lib/src/test/pcm_min.c for example.
>
> There is already one attached to the bug report page:
>
> https://bugtrack.alsa-project.org/alsa-bug/bug_view_advanced_page.php?bug_id=4914
>
> Thanks for investigating this.

This commit should solve this issue:

http://git.alsa-project.org/?p=alsa-lib.git;a=commitdiff;h=b9dbee694a94a724670b36f4df1aeceb0dd4a261

 					Jaroslav

-----
Jaroslav Kysela <perex@perex.cz>
Linux Kernel Sound Maintainer
ALSA Project, Red Hat, Inc.

Re: snd_pcm_drop causing lockup with DMix

[John Lindgren]

Confirmed, that fixes the hangup.  A minor issue remains that
snd_pcm_writei still always returns -EIO or -EBADFD in this situation.

John Lindgren

On Wed, 2010-03-03 at 11:01 +0100, Jaroslav Kysela wrote:
> This commit should solve this issue:
> 
> http://git.alsa-project.org/?p=alsa-lib.git;a=commitdiff;h=b9dbee694a94a724670b36f4df1aeceb0dd4a261
> 
>  					Jaroslav

Re: snd_pcm_drop causing lockup with DMix

[Jaroslav Kysela]

On Wed, 3 Mar 2010, John Lindgren wrote:

> Confirmed, that fixes the hangup.  A minor issue remains that
> snd_pcm_writei still always returns -EIO or -EBADFD in this situation.

I don't see a problem here. If transfer is stopped, the pcm handle is 
not initialized for I/O transactions, thus upper layers return these 
errors.

Your application should check snd_pcm_state() in case of error and look if 
stream is inactive (back in SND_PCM_STATE_SETUP), because your application 
invoked the drop() call.

 						Jaroslav

-----
Jaroslav Kysela <perex@perex.cz>
Linux Kernel Sound Maintainer
ALSA Project, Red Hat, Inc.