Re: [PATCH 6.18.y 0/5] drm/vkms: Backport generic vblank timer to fix ABBA deadlock

[Maarten Lankhorst]

Hello,

Den 2026-05-26 kl. 14:06, skrev w15303746062:
> 
> Hi Sasha,
> 
>> Looking at the five commits:
>>
>>  - 1/5 (74afeb812850) is the one that actually fixes the ABBA
>>    deadlock you observed under Syzkaller; it adds the generic vblank
>>    timer that replaces the open-coded vkms hrtimer path.
>>
>>  - 2/5 (d54dbb5963bd) adds new CRTC helpers for "simple use cases".
>>    No Fixes:/Cc:stable, no described bug.
>>
>>  - 3/5 (02e2681ffe1a) is a refactor that converts vkms to the new
>>    helpers. No Fixes:/Cc:stable, no described bug.
>>
>>  - 4/5 (79ae8510b5b8) is a v7.1-rc1 timeout bump that depends on 1/5.
>>    It is not yet in any released stable, so applying it to 6.18.y
>>    would put it on an LTS before any LTS contains it.
>>
>>  - 5/5 (3946d3ba9934) is a doc fix for 1/5.
>>
>> Per stable-kernel-rules, what I need to queue is the minimum set that
>> fixes the bug. Could you explain, per patch, why 2/5..5/5 are required
>> to make 1/5 work / are required to actually fix the deadlock? If only
>> 1/5 is needed, please resend just that one with your Signed-off-by
>> added (the carried patches today only have Thomas's S-o-b, which
>> breaks the chain of custody on a stable submission).
> 
> Thanks for the quick review and for pointing out the missing Signed-off-by. I apologize for that omission; it was my mistake during the cherry-pick process.
> 
> Regarding the dependency chain, I would like to clarify why commit 1/5 alone cannot fix the issue:
> 
> Commits 1/5 and 2/5 introduce the new generic vblank timer infrastructure to the DRM core but do *not* touch the vkms driver at all. 
> Commit 3/5 (02e2681ffe1a) is the actual fix that modifies `drivers/gpu/drm/vkms/vkms_crtc.c`. It removes the buggy open-coded hrtimer that causes the ABBA deadlock and switches vkms to use the new infrastructure introduced in 1/5 and 2/5. 
> 
> Therefore, 1/5, 2/5, and 3/5 form an indivisible set. Applying only 1/5 would leave the deadlock in vkms completely unpatched.
> 
> As for 4/5 and 5/5 (the timeout bump and doc fix), Maarten Lankhorst (DRM maintainer) explicitly recommended pulling in this exact 5-commit list as the proper upstream fix for this specific vkms issue (see the mailing list link in this thread). 
> 
> However, if you feel 4/5 and 5/5 introduce unnecessary risk for the 6.18.y stable tree, I can absolutely drop them and only submit 1/5, 2/5, and 3/5. 
> 
> I am preparing a v2 patch series now with my Signed-off-by added to the chain of custody. Could you let me know if you prefer the full 5-patch series as recommended by DRM maintainers, or just the minimal 3-patch series?
> 
> Best regards,
> Mingyu

5/5 might strictly speaking not be needed as it's a documentation fix and I have no idea of the policy about those.

The reporter made a bug report of an ABBA deadlock that was fixed in upstream by the first 4 patches, perhaps it's good to those attach here to this discussion.

Kind regards,
~Maarten Lankhorst