ALSA: hrtimer: Handle start/stop more properly

This patch tries to address the still remaining issues in ALSA hrtimer
driver:
- Spurious use-after-free was detected in hrtimer callback
- Incorrect rescheduling due to delayed start
- WARN_ON() is triggered in hrtimer_forward() invoked in hrtimer
  callback

The first issue happens only when the new timer is scheduled even
while hrtimer is being closed.  It's related with the second and third
items; since ALSA timer core invokes hw.start callback during hrtimer
interrupt, this may result in the explicit call of hrtimer_start().

Also, the similar problem is seen for the stop; ALSA timer core
invokes hw.stop callback even in the hrtimer handler, too.  Since we
must not call the synced hrtimer_cancel() in such a context, it's just
a hrtimer_try_to_cancel() call that doesn't properly work.

Another culprit of the second and third items is the call of
hrtimer_forward_now() before snd_timer_interrupt().  The timer->stick
value may change during snd_timer_interrupt() call, but this
possibility is ignored completely.

For covering these subtle and messy issues, the following changes have
been done in this patch:
- A new flag, in_callback, is introduced in the private data to
  indicate that the hrtimer handler is being processed.
- Both start and stop callbacks skip when called from (during)
  in_callback flag.
- The hrtimer handler returns properly HRTIMER_RESTART and NORESTART
  depending on the running state now.
- The hrtimer handler reprograms the expiry properly after
  snd_timer_interrupt() call, instead of before.
- The close callback clears running flag and sets in_callback flag
  to block any further start/stop calls.

Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/hrtimer.c

@@ -38,37 +38,53 @@ static unsigned int resolution;
 struct snd_hrtimer {
 	struct snd_timer *timer;
 	struct hrtimer hrt;
-	atomic_t running;
+	bool in_callback;
 };
 
 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;
-	unsigned long oruns;
+	ktime_t delta;
+	unsigned long ticks;
+	enum hrtimer_restart ret = HRTIMER_NORESTART;
 
-	if (!atomic_read(&stime->running))
-		return HRTIMER_NORESTART;
+	spin_lock(&t->lock);
+	if (!t->running)
+		goto out; /* fast path */
+	stime->in_callback = true;
+	ticks = t->sticks;
+	spin_unlock(&t->lock);
 
-	oruns = hrtimer_forward_now(hrt, ns_to_ktime(t->sticks * resolution));
-	snd_timer_interrupt(stime->timer, t->sticks * oruns);
+	/* calculate the drift */
+	delta = ktime_sub(hrt->base->get_time(), hrtimer_get_expires(hrt));
+	if (delta.tv64 > 0)
+		ticks += ktime_divns(delta, ticks * resolution);
 
-	if (!atomic_read(&stime->running))
-		return HRTIMER_NORESTART;
-	return HRTIMER_RESTART;
+	snd_timer_interrupt(stime->timer, ticks);
+
+	spin_lock(&t->lock);
+	if (t->running) {
+		hrtimer_add_expires_ns(hrt, t->sticks * resolution);
+		ret = HRTIMER_RESTART;
+	}
+
+	stime->in_callback = false;
+ out:
+	spin_unlock(&t->lock);
+	return ret;
 }
 
 static int snd_hrtimer_open(struct snd_timer *t)
 {
 	struct snd_hrtimer *stime;
 
-	stime = kmalloc(sizeof(*stime), GFP_KERNEL);
+	stime = kzalloc(sizeof(*stime), GFP_KERNEL);
 	if (!stime)
 		return -ENOMEM;
 	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;
 }
@@ -78,6 +94,11 @@ static int snd_hrtimer_close(struct snd_timer *t)
 	struct snd_hrtimer *stime = t->private_data;
 
 	if (stime) {
+		spin_lock_irq(&t->lock);
+		t->running = 0; /* just to be sure */
+		stime->in_callback = 1; /* skip start/stop */
+		spin_unlock_irq(&t->lock);
+
 		hrtimer_cancel(&stime->hrt);
 		kfree(stime);
 		t->private_data = NULL;
@@ -89,18 +110,19 @@ static int snd_hrtimer_start(struct snd_timer *t)
 {
 	struct snd_hrtimer *stime = t->private_data;
 
-	atomic_set(&stime->running, 0);
-	hrtimer_try_to_cancel(&stime->hrt);
+	if (stime->in_callback)
+		return 0;
 	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;
-	atomic_set(&stime->running, 0);
+
+	if (stime->in_callback)
+		return 0;
 	hrtimer_try_to_cancel(&stime->hrt);
 	return 0;
 }