Re: [PATCH 23/25] ALSA/dummy: Replace tasklet with softirq hrtimer

[Takashi Sakamoto]

Hi,

On Sep 1 2017 00:36, Takashi Iwai wrote:
> 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.
 
I can reproduce this bug.

> 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.

This patch looks good to me, too. But I have an alternative.

We can use 'hrtimer_callback_running()' to detect whether to be on hrtimer
callback or not (please read '__run_hrtimer()' in 'kernel/time/hrtimer.c').
Usage of this helper function on .stop callback to skip cancellation can
avoid the stall. In this case, after stopping PCM substream, the hrtimer
callback should return HRTIMER_NORESTART to avoid restarting, as well as
your patch.  Please test a patch in this message.

> ---
> 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;
>   }