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SubjectRe: [PATCH 23/25] ALSA/dummy: Replace tasklet with softirq hrtimer
On Thu, 31 Aug 2017 14:23:45 +0200,
Anna-Maria Gleixner wrote:
>
> From: Thomas Gleixner <tglx@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@linutronix.de>
> Signed-off-by: Anna-Maria Gleixner <anna-maria@linutronix.de>
> Cc: Jaroslav Kysela <perex@perex.cz>
> Cc: Takashi Iwai <tiwai@suse.com>
> Cc: Takashi Sakamoto <o-takashi@sakamocchi.jp>
> Cc: alsa-devel@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;
}

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