drm/vkms: deadlock between dev->event_lock and timer

[Tetsuo Handa]

On 2023/09/14 6:08, Thomas Gleixner wrote:
> Maybe the VKMS people need to understand locking in the first place. The
> first thing I saw in this code is:
> 
> static enum hrtimer_restart vkms_vblank_simulate(struct hrtimer *timer)
> {
>    ...
>    mutex_unlock(&output->enabled_lock);
> 
> What?
> 
> Unlocking a mutex in the context of a hrtimer callback is simply
> violating all mutex locking rules.
> 
> How has this code ever survived lock debugging without triggering a big
> fat warning?

Commit a0e6a017ab56936c ("drm/vkms: Fix race-condition between the hrtimer
and the atomic commit") in 6.6-rc1 replaced spinlock with mutex. So we haven't
tested with the lock debugging yet...

Maíra and Arthur, mutex_unlock() from interrupt context is not permitted.
Please revert that patch immediately.
I guess that a semaphore (down()/up()) could be used instead of a mutex.