[PATCH] ALSA: dummy: avoid stall due to a call of hrtimer_cancel() on a callback of hrtimer

[Takashi Sakamoto]

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@xxxxxxxxxxxxx>
---
 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