Re: [RFC PATCH v2 0/5] ASoC: soc-pcm: fix trigger race conditions with shared BE

[Takashi Iwai <tiwai@suse.de>]

On Thu, 07 Oct 2021 15:31:49 +0200,
Pierre-Louis Bossart wrote:
> 
> 
> >> Takashi, Mark, do you think that an all/none assumption on FE nonatomic
> >> properties would make sense?
> > 
> > As long as BE's are updated from FE's PCM callback, BE has to follow
> > the atomicity of its FE, so we can't assume all or none globally.
> 
> A BE may have more than one FEs. That's precisely the point of
> mixers/demux, and if the BE has FEs with different 'atomicity' then I
> don't see how locking can work: the BE operations are run in the context
> of each of its FE, typically using the following loop:
> 
> for_each_dpcm_be(fe, stream, dpcm) {
>    do_something();
> }

Do we really have the cases where FEs have different atomicity?
I don't think it's a valid configuration, and we should catch it via
WARN_ON() or such.

> Applications will view multiple FEs as completely independent. They may
> be opened/prepared/started/stopped/paused as needed. When the BE is
> shared, then there is a need for consistency, such as starting the BE
> when the first FE becomes active and stopping it when the last FE stops.
> 
> > How is the expected lock granularity and the protection context?  Do
> > we need a card global lock/mutex, or is it specific to each BE
> > substream?
> 
> We already have a dpcm_lock at the card level, which protects the
> addition of BEs and BE states. That spin_lock is fine for most cases.
> 
> The only real problem is the trigger, which is currently completely
> unprotected: we have to serialize the BE triggers, otherwise you could
> STOP before you START due to scheduling, or other problems that I saw in
> my SoundWire tests with two START triggers, or the STOP never sent.

So it's about calling triggers to the same BE stream concurrently?
If that's the case, can't we simply protect the trigger handling of
each BE like below?

> But how to do this serialization is unclear...
> 
> A lateral thinking approach would be to decouple the BEs entirely, and
> have the FEs 'signal' their change of state. The BE 'thread' run in the
> BE context would then serialize the requests and perform all the BE
> operations, and the same approach could be chained. I am afraid that
> would be a complete rewrite of DPCM, but maybe we have to do so anyways
> if we can't support a basic case of a mixer with 2 streams :-)

Well, let's see whether we can get some improvements by a simple
change at first.


Takashi


--- a/sound/soc/soc-pcm.c
+++ b/sound/soc/soc-pcm.c
@@ -1998,6 +1998,7 @@ int dpcm_be_dai_trigger(struct snd_soc_pcm_runtime *fe, int stream,
 {
 	struct snd_soc_pcm_runtime *be;
 	struct snd_soc_dpcm *dpcm;
+	unsigned long flags;
 	int ret = 0;
 
 	for_each_dpcm_be(fe, stream, dpcm) {
@@ -2006,9 +2007,11 @@ int dpcm_be_dai_trigger(struct snd_soc_pcm_runtime *fe, int stream,
 		be = dpcm->be;
 		be_substream = snd_soc_dpcm_get_substream(be, stream);
 
+		snd_pcm_stream_lock_irqsave(be_substream, flags);
+
 		/* is this op for this BE ? */
 		if (!snd_soc_dpcm_be_can_update(fe, be, stream))
-			continue;
+			goto unlock;
 
 		dev_dbg(be->dev, "ASoC: trigger BE %s cmd %d\n",
 			be->dai_link->name, cmd);
@@ -2018,77 +2021,81 @@ int dpcm_be_dai_trigger(struct snd_soc_pcm_runtime *fe, int stream,
 			if ((be->dpcm[stream].state != SND_SOC_DPCM_STATE_PREPARE) &&
 			    (be->dpcm[stream].state != SND_SOC_DPCM_STATE_STOP) &&
 			    (be->dpcm[stream].state != SND_SOC_DPCM_STATE_PAUSED))
-				continue;
+				goto unlock;
 
 			ret = soc_pcm_trigger(be_substream, cmd);
 			if (ret)
-				goto end;
+				goto unlock;
 
 			be->dpcm[stream].state = SND_SOC_DPCM_STATE_START;
 			break;
 		case SNDRV_PCM_TRIGGER_RESUME:
 			if ((be->dpcm[stream].state != SND_SOC_DPCM_STATE_SUSPEND))
-				continue;
+				goto unlock;
 
 			ret = soc_pcm_trigger(be_substream, cmd);
 			if (ret)
-				goto end;
+				goto unlock;
 
 			be->dpcm[stream].state = SND_SOC_DPCM_STATE_START;
 			break;
 		case SNDRV_PCM_TRIGGER_PAUSE_RELEASE:
 			if ((be->dpcm[stream].state != SND_SOC_DPCM_STATE_PAUSED))
-				continue;
+				goto unlock;
 
 			ret = soc_pcm_trigger(be_substream, cmd);
 			if (ret)
-				goto end;
+				goto unlock;
 
 			be->dpcm[stream].state = SND_SOC_DPCM_STATE_START;
 			break;
 		case SNDRV_PCM_TRIGGER_STOP:
 			if ((be->dpcm[stream].state != SND_SOC_DPCM_STATE_START) &&
 			    (be->dpcm[stream].state != SND_SOC_DPCM_STATE_PAUSED))
-				continue;
+				goto unlock;
 
 			if (!snd_soc_dpcm_can_be_free_stop(fe, be, stream))
-				continue;
+				goto unlock;
 
 			ret = soc_pcm_trigger(be_substream, cmd);
 			if (ret)
-				goto end;
+				goto unlock;
 
 			be->dpcm[stream].state = SND_SOC_DPCM_STATE_STOP;
 			break;
 		case SNDRV_PCM_TRIGGER_SUSPEND:
 			if (be->dpcm[stream].state != SND_SOC_DPCM_STATE_START)
-				continue;
+				goto unlock;
 
 			if (!snd_soc_dpcm_can_be_free_stop(fe, be, stream))
-				continue;
+				goto unlock;
 
 			ret = soc_pcm_trigger(be_substream, cmd);
 			if (ret)
-				goto end;
+				goto unlock;
 
 			be->dpcm[stream].state = SND_SOC_DPCM_STATE_SUSPEND;
 			break;
 		case SNDRV_PCM_TRIGGER_PAUSE_PUSH:
 			if (be->dpcm[stream].state != SND_SOC_DPCM_STATE_START)
-				continue;
+				goto unlock;
 
 			if (!snd_soc_dpcm_can_be_free_stop(fe, be, stream))
-				continue;
+				goto unlock;
 
 			ret = soc_pcm_trigger(be_substream, cmd);
 			if (ret)
-				goto end;
+				goto unlock;
 
 			be->dpcm[stream].state = SND_SOC_DPCM_STATE_PAUSED;
 			break;
 		}
+	unlock:
+		snd_pcm_stream_unlock_irqrestore(be_substream, flags);
+		if (ret < 0)
+			break;
 	}
-end:
+
 	if (ret < 0)
 		dev_err(fe->dev, "ASoC: %s() failed at %s (%d)\n",
 			__func__, be->dai_link->name, ret);