ALSA: aloop: Fix spinlock deadlock in loopback_hrtimer_stop()

In loopback_hrtimer_stop(), calling hrtimer_cancel() while holding
cable->lock triggers an AB-BA spinlock deadlock if the hrtimer softirq
is executing concurrently on another CPU:

1) CPU A runs loopback_trigger(STOP), acquires spin_lock(&cable->lock),
   and calls hrtimer_cancel(). Since hrtimer_cancel() is synchronous,
   it spins waiting for the executing callback to complete before
   returning.
2) CPU B executes loopback_hrtimer_function(), which immediately tries
   to acquire spin_lock(&cable->lock).

This mutual dependency leads to a CPU hard lockup and NMI watchdog
panic when multiple streams start and stop concurrently with small
period sizes.

Replace hrtimer_cancel() in loopback_hrtimer_stop() with the non-blocking
hrtimer_try_to_cancel(), matching the behavior of jiffies timers
(timer_delete vs timer_delete_sync). If try_to_cancel returns -1
because the handler is running, CPU A releases cable->lock cleanly.
When the running handler subsequently acquires cable->lock, it observes
that the stream is no longer in running state (cleared by trigger STOP)
and terminates without re-arming the timer. Synchronous hrtimer_cancel()
remains preserved in loopback_hrtimer_stop_sync() where cable->lock is
not held.

Fixes: bf08a5f698 ("ALSA: aloop: Add 'hrtimer' option to timer_source")
Signed-off-by: Yu-Hsuan Hsu <yuhsuan@chromium.org>
Link: https://patch.msgid.link/20260731074255.1513402-1-yuhsuan@chromium.org
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/drivers/aloop.c

@@ -303,7 +303,7 @@ static inline int loopback_jiffies_timer_stop(struct loopback_pcm *dpcm)
 /* call in cable->lock */
 static inline int loopback_hrtimer_stop(struct loopback_pcm *dpcm)
 {
-	hrtimer_cancel(&dpcm->hrtimer);
+	hrtimer_try_to_cancel(&dpcm->hrtimer);
 
 	return 0;
 }