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

[Yu-Hsuan Hsu]

Hi Takashi,

Thanks. I actually encountered a CPU deadlock during the aloop stress
test. (Resent as plain text.)

Best,
Yu-Hsuan

Takashi Iwai <tiwai@xxxxxxx> 於 2026年8月1日週六 下午3:33寫道：
>
> 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