ALSA: hrtimer - Fix lock-up

The timer stop callback can be called from snd_timer_interrupt(), which
is called from the hrtimer callback.  Since hrtimer_cancel() waits for
the callback completion, this eventually results in a lock-up.

This patch fixes the problem by just toggling a flag at stop callback
and call hrtimer_cancel() later.
Reported-and-tested-by: Wojtek Zabolotny <W.Zabolotny@elka.pw.edu.pl>
Cc: <stable@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/hrtimer.c

@@ -37,14 +37,22 @@ static unsigned int resolution;
 struct snd_hrtimer {
 	struct snd_timer *timer;
 	struct hrtimer hrt;
+	atomic_t running;
 };
 
 static enum hrtimer_restart snd_hrtimer_callback(struct hrtimer *hrt)
 {
 	struct snd_hrtimer *stime = container_of(hrt, struct snd_hrtimer, hrt);
 	struct snd_timer *t = stime->timer;
+
+	if (!atomic_read(&stime->running))
+		return HRTIMER_NORESTART;
+
 	hrtimer_forward_now(hrt, ns_to_ktime(t->sticks * resolution));
 	snd_timer_interrupt(stime->timer, t->sticks);
+
+	if (!atomic_read(&stime->running))
+		return HRTIMER_NORESTART;
 	return HRTIMER_RESTART;
 }
 
@@ -58,6 +66,7 @@ static int snd_hrtimer_open(struct snd_timer *t)
 	hrtimer_init(&stime->hrt, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
 	stime->timer = t;
 	stime->hrt.function = snd_hrtimer_callback;
+	atomic_set(&stime->running, 0);
 	t->private_data = stime;
 	return 0;
 }
@@ -78,16 +87,18 @@ 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_start(&stime->hrt, ns_to_ktime(t->sticks * resolution),
 		      HRTIMER_MODE_REL);
+	atomic_set(&stime->running, 1);
 	return 0;
 }
 
 static int snd_hrtimer_stop(struct snd_timer *t)
 {
 	struct snd_hrtimer *stime = t->private_data;
-
-	hrtimer_cancel(&stime->hrt);
+	atomic_set(&stime->running, 0);
 	return 0;
 }