Re: [PATCH] ALSA: aloop: Fix spinlock deadlock in loopback_hrtimer_stop()

[Takashi Iwai]

On Fri, 31 Jul 2026 09:39:35 +0200,
Yu-Hsuan Hsu wrote:
> 
> 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: bf08a5f698dc ("ALSA: aloop: Add 'hrtimer' option to timer_source")
> Signed-off-by: Yu-Hsuan Hsu <yuhsuan@xxxxxxxxxxxx>

While I find it's fine to change like this, I wonder whether you
really hit a CPU deadlock.  Or it's just hypothetical?


thanks,

Takashi