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

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Message-ID: <5baab2ed-c48d-41ae-819a-71ca195c4407@igalia.com>
Date: Thu, 26 Jun 2025 12:02:23 -0300
From: Maíra Canal <mcanal@...lia.com>
To: Pranav Tyagi <pranav.tyagi03@...il.com>, linux-kernel@...r.kernel.org,
 dri-devel@...ts.freedesktop.org
Cc: rodrigosiqueiramelo@...il.com, melissa.srw@...il.com,
 hamohammed.sa@...il.com, daniel@...ll.ch, airlied@...ux.ie,
 arthurgrillo@...eup.net, mairacanal@...eup.net, skhan@...uxfoundation.org,
 linux-kernel-mentees@...ts.linux.dev, stable@...r.kernel.org,
 gregkh@...uxfoundation.org, sashal@...nel.org
Subject: Re: [PATCH] drm/vkms: Fix race-condition between the hrtimer and the
 atomic commit

Hi Pranav,

On 26/06/25 11:22, 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. ]
> 
> Signed-off-by: Maíra Canal <mcanal@...lia.com>
> Reviewed-by: Arthur Grillo <arthurgrillo@...eup.net>
> Signed-off-by: Maíra Canal <mairacanal@...eup.net>
> Link: https://patchwork.freedesktop.org/patch/msgid/20230523123207.173976-1-mcanal@igalia.com
> Signed-off-by: Pranav Tyagi <pranav.tyagi03@...il.com>

This patch violates locking rules and it was reversed a while ago.
Please, check commit 7908632f2927 ("Revert "drm/vkms: Fix race-condition
between the hrtimer and the atomic commit"").

Best Regards,
- Maíra