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SubjectRe: [PATCH 23/25] ALSA/dummy: Replace tasklet with softirq hrtimer
Date
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;
> }

From 07d61ba2a1c0e06e914443225e194d99f2d8c58d Mon Sep 17 00:00:00 2001
From: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Date: Fri, 1 Sep 2017 19:10:18 +0900
Subject: [PATCH] ALSA: dummy: avoid stall due to a call of hrtimer_cancel() on
a callback of hrtimer

A call of 'htrimer_cancel()' on a callback of hrtimer brings endless loop
because 'struct hrtimer_clock_base.running' is not NULL on the callback.
In hrtimer subsystem, this member is used to indicate the instance of
hrtimer gets callbacks and there's a helper function,
'hrtimer_callback_running()' to check it.

ALSA dummy driver uses hrtimer to emulate hardware interrupt per period
of PCM buffer. When XRUN occurs on PCM substream, in a call of
'snd_pcm_period_elapsed()', 'struct snd_pcm_ops.stop()' is called to
stop the substream. In current implementation, 'hrtimer_cancel()' is
used to wait for cancellation of hrtimer. However, as described, this
brings endless loop.

For this problem, this commit uses 'hrtimer_callback_running()' to
detect whether to be on a callback of hrtimer or not, then skip
cancellation of hrtimer in hrtimer callbacks. Furthermore, at a case of
XRUN, hrtimer callback returns HRTIMER_NORESTART after a call of
'snd_pcm_period_elapsed()' to discontinue hrtimr because cancellation is
skipped.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
---
sound/drivers/dummy.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/sound/drivers/dummy.c b/sound/drivers/dummy.c
index 273d60c42125..9caf754c6135 100644
--- a/sound/drivers/dummy.c
+++ b/sound/drivers/dummy.c
@@ -387,7 +387,11 @@ static enum hrtimer_restart dummy_hrtimer_callback(struct hrtimer *timer)
if (!atomic_read(&dpcm->running))
return HRTIMER_NORESTART;

+ /* In a case of XRUN, this calls .trigger to stop PCM substream. */
snd_pcm_period_elapsed(dpcm->substream);
+ if (!atomic_read(&dpcm->running))
+ return HRTIMER_NORESTART;
+
hrtimer_forward_now(timer, dpcm->period_time);
return HRTIMER_RESTART;
}
@@ -407,7 +411,8 @@ 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);
+ if (!hrtimer_callback_running(&dpcm->timer))
+ hrtimer_cancel(&dpcm->timer);
return 0;
}

--
2.11.0

Regards

Takashi Sakamoto

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