linux-kernel - Re: drm/vkms: deadlock between dev->event

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Message-ID: <167ee2ad-6a7e-876c-f5c9-f0a227070a78@I-love.SAKURA.ne.jp>
Date:   Thu, 14 Sep 2023 15:33:41 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Maira Canal <mairacanal@...eup.net>,
        Arthur Grillo <arthurgrillo@...eup.net>
Cc:     Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>,
        Melissa Wen <melissa.srw@...il.com>,
        Haneen Mohammed <hamohammed.sa@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...il.com>,
        DRI <dri-devel@...ts.freedesktop.org>,
        syzkaller@...glegroups.com, LKML <linux-kernel@...r.kernel.org>,
        Hillf Danton <hdanton@...a.com>,
        Sanan Hasanov <Sanan.Hasanov@....edu>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: drm/vkms: deadlock between dev->event_lock and timer

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.