Commit 2ba1fe7a authored Jan 18, 2016 by Takashi Iwai

ALSA: hrtimer: Fix stall by hrtimer_cancel()



hrtimer_cancel() waits for the completion from the callback, thus it
must not be called inside the callback itself.  This was already a
problem in the past with ALSA hrtimer driver, and the early commit
[fcfdebe7: ALSA: hrtimer - Fix lock-up] tried to address it.

However, the previous fix is still insufficient: it may still cause a
lockup when the ALSA timer instance reprograms itself in its callback.
Then it invokes the start function even in snd_timer_interrupt() that
is called in hrtimer callback itself, results in a CPU stall.  This is
no hypothetical problem but actually triggered by syzkaller fuzzer.

This patch tries to fix the issue again.  Now we call
hrtimer_try_to_cancel() at both start and stop functions so that it
won't fall into a deadlock, yet giving some chance to cancel the queue
if the functions have been called outside the callback.  The proper
hrtimer_cancel() is called in anyway at closing, so this should be
enough.

Reported-and-tested-by: Dmitry Vyukov <dvyukov@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

parent db8948e6

sound/core/hrtimer.c

+2 −1

Original line number	Diff line number	Diff line
		@@ -90,7 +90,7 @@ static int snd_hrtimer_start(struct snd_timer *t)
		struct snd_hrtimer *stime = t->private_data;

		atomic_set(&stime->running, 0);
		hrtimer_cancel(&stime->hrt);
		hrtimer_try_to_cancel(&stime->hrt);
		hrtimer_start(&stime->hrt, ns_to_ktime(t->sticks * resolution),
		HRTIMER_MODE_REL);
		atomic_set(&stime->running, 1);
		@@ -101,6 +101,7 @@ static int snd_hrtimer_stop(struct snd_timer *t)
		{
		struct snd_hrtimer *stime = t->private_data;
		atomic_set(&stime->running, 0);
		hrtimer_try_to_cancel(&stime->hrt);
		return 0;
		}