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

[Takashi Iwai]

On Sun, 02 Aug 2026 15:58:26 +0200,
Yu-Hsuan Hsu wrote:
> 
> Hi Takashi,
> 
> Thanks. I actually encountered a CPU deadlock during the aloop stress
> test. (Resent as plain text.)

Thanks for confirmation.  I asked it to judge for 7.2-rc7 fix, but I
missed the point that it's the fix for the new change for 7.3 :)
Now applied to for-next branch.


thanks,

Takashi


> 
> 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