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

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Message-ID: <2025062607-hardener-splotchy-1e70@gregkh>
Date: Thu, 26 Jun 2025 15:57:21 +0100
From: Greg KH <gregkh@...uxfoundation.org>
To: Pranav Tyagi <pranav.tyagi03@...il.com>
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 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