linux-kernel - Re: [PATCH] drm/vkms: Fix race-condition between the hrtimer and the atomic commit

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Message-ID: <CAH4c4jJqX6C4+xvHKqC2fmN7HRhKPgD2XcjnwO2MR0xtMyEhRQ@mail.gmail.com>
Date: Thu, 26 Jun 2025 20:38:09 +0530
From: Pranav Tyagi <pranav.tyagi03@...il.com>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
	rodrigosiqueiramelo@...il.com, melissa.srw@...il.com, hamohammed.sa@...il.com, 
	daniel@...ll.ch, airlied@...ux.ie, mcanal@...lia.com, arthurgrillo@...eup.net, 
	mairacanal@...eup.net, skhan@...uxfoundation.org, 
	linux-kernel-mentees@...ts.linux.dev, stable@...r.kernel.org, 
	sashal@...nel.org
Subject: Re: [PATCH] drm/vkms: Fix race-condition between the hrtimer and the
 atomic commit

On Thu, Jun 26, 2025 at 8:27 PM Greg KH <gregkh@...uxfoundation.org> wrote:
>
> On Thu, Jun 26, 2025 at 07:52:43PM +0530, Pranav Tyagi wrote:
> > From: Maíra Canal <mcanal@...lia.com>
> >
> > [ Upstream commit a0e6a017ab56936c0405fe914a793b241ed25ee0 ]
> >
> > Currently, it is possible for the composer to be set as enabled and then
> > as disabled without a proper call for the vkms_vblank_simulate(). This
> > is problematic, because the driver would skip one CRC output, causing CRC
> > tests to fail. Therefore, we need to make sure that, for each time the
> > composer is set as enabled, a composer job is added to the queue.
> >
> > In order to provide this guarantee, add a mutex that will lock before
> > the composer is set as enabled and will unlock only after the composer
> > job is added to the queue. This way, we can have a guarantee that the
> > driver won't skip a CRC entry.
> >
> > This race-condition is affecting the IGT test "writeback-check-output",
> > making the test fail and also, leaking writeback framebuffers, as the
> > writeback job is queued, but it is not signaled. This patch avoids both
> > problems.
> >
> > [v2]:
> >     * Create a new mutex and keep the spinlock across the atomic commit in
> >       order to avoid interrupts that could result in deadlocks.
> >
> > [ Backport to 5.15: context cleanly applied with no semantic changes.
> > Build-tested. ]
>
> Did you forget about 6.1.y?
>
> confused,
>
> greg k-h

Apologies for the oversight.

Regards
Pranav Tyagi