[alsa-devel] [PATCH 23/25] ALSA/dummy: Replace tasklet with softirq hrtimer

[Takashi Iwai]

On Thu, 31 Aug 2017 14:23:45 +0200,
Anna-Maria Gleixner wrote:
> 
> From: Thomas Gleixner <tglx at linutronix.de>
> 
> The tasklet is used to defer the execution of snd_pcm_period_elapsed() to
> the softirq context. Using the CLOCK_MONOTONIC_SOFT base invokes the timer
> callback in softirq context as well which renders the tasklet useless.
> 
> Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
> Signed-off-by: Anna-Maria Gleixner <anna-maria at linutronix.de>
> Cc: Jaroslav Kysela <perex at perex.cz>
> Cc: Takashi Iwai <tiwai at suse.com>
> Cc: Takashi Sakamoto <o-takashi at sakamocchi.jp>
> Cc: alsa-devel at alsa-project.org

I gave it at try, but it caused a kernel hang, unfortunately.

The reason is that snd_pcm_period_elapased() may stop the stream
(e.g. when reaching at the end).  With this patchset, it'll lead to
the call of hrtimer_cancel() from the hrtimer callback itself, thus it
stalls.

Below is the additional fix over your patch for working around it.
I believe it should cover most corner cases, and seems working fine
through quick tests, so far.


thanks,

Takashi

---
diff --git a/sound/drivers/dummy.c b/sound/drivers/dummy.c
index 273d60c42125..b5dd64e3dab1 100644
--- a/sound/drivers/dummy.c
+++ b/sound/drivers/dummy.c
@@ -375,6 +375,7 @@ struct dummy_hrtimer_pcm {
 	ktime_t base_time;
 	ktime_t period_time;
 	atomic_t running;
+	atomic_t callback_running;
 	struct hrtimer timer;
 	struct snd_pcm_substream *substream;
 };
@@ -387,8 +388,15 @@ static enum hrtimer_restart dummy_hrtimer_callback(struct hrtimer *timer)
 	if (!atomic_read(&dpcm->running))
 		return HRTIMER_NORESTART;
 
+	atomic_inc(&dpcm->callback_running);
 	snd_pcm_period_elapsed(dpcm->substream);
+	atomic_dec(&dpcm->callback_running);
+	/* may be flipped during snd_pcm_period_elapsed() */
+	if (!atomic_read(&dpcm->running))
+		return HRTIMER_NORESTART;
+
 	hrtimer_forward_now(timer, dpcm->period_time);
+	atomic_dec(&dpcm->callback_running);
 	return HRTIMER_RESTART;
 }
 
@@ -407,7 +415,9 @@ static int dummy_hrtimer_stop(struct snd_pcm_substream *substream)
 	struct dummy_hrtimer_pcm *dpcm = substream->runtime->private_data;
 
 	atomic_set(&dpcm->running, 0);
-	hrtimer_cancel(&dpcm->timer);
+	/* issue hrtimer_cancel() only when called outside the callback */
+	if (!atomic_read(&dpcm->callback_running))
+		hrtimer_cancel(&dpcm->timer);
 	return 0;
 }
 
@@ -462,6 +472,7 @@ static int dummy_hrtimer_create(struct snd_pcm_substream *substream)
 	dpcm->timer.function = dummy_hrtimer_callback;
 	dpcm->substream = substream;
 	atomic_set(&dpcm->running, 0);
+	atomic_set(&dpcm->callback_running, 0);
 	return 0;
 }