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

[Helen Koike]

On 14/09/2023 05:12, Daniel Vetter wrote:
> On Thu, Sep 14, 2023 at 03:33:41PM +0900, Tetsuo Handa wrote:
>> 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...
> 
> Yeah that needs an immediate revert, there's not much that looks legit in
> that patch. I'll chat with Maira.
> 
> Also yes how that landed without anyone running lockdep is ... not good. I
> guess we need a lockdep enabled drm ci target that runs vkms tests asap
> :-)

btw, I just executed a draft version of vkms targed on the ci, we do get 
the warning:

https://gitlab.freedesktop.org/helen.fornazier/linux/-/jobs/49156305#L623

I'm just not sure if tests would fail (since it is a warning) and it has 
a chance to be ignored if people don't look at the logs (unless if igt 
already handles that).

I still need to do some adjustments (it seems the tests is hanging 
somewhere and we got a timeout) but we should have vkms in drm ci soon.

Regards,
Helen


> 
>> 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.
> 
>  From a quick look this smells like a classic "try to use locking when you
> want synchronization primitives", so semaphore here doesn't look any
> better. The vkms_set_composer() function was originally for crc
> generation, where it's userspace's job to make sure they wait for all the
> crc they need to be generated before they shut it down again. But for
> writeback the kernel must guarantee that the compositiona actually
> happens, and the current function just doesn't make any such guarantees.
> 
> Cheers, Daniel