Re: [PATCH 1/3] drm/panthor: Don't use the racy drm_gem_lru_remove() helper

[Boris Brezillon <boris.brezillon@collabora.com>]

On Thu, 7 May 2026 11:01:25 +0100
Liviu Dudau <liviu.dudau@arm.com> wrote:

> On Wed, May 06, 2026 at 02:16:26PM +0200, Boris Brezillon wrote:
> > drm_gem_lru_remove() dereference stores drm_gem_object::lru in a local
> > variable that's then dereferenced to acquire the LRU lock. Because this
> > assignment in done without the LRU lock held, it can race with
> > drm_gem_lru_scan() where drm_gem_object::lru is temporarily assigned
> > a stack-allcated LRU that goes away when leaving the function. By
> > the time we dereference this local lru variable, the object might already
> > be gone.
> > 
> > It feels like drm_gem_lru_move_tail() was never meant to be used this
> > way, because there's no easy way we can avoid this race unless we defer
> > the locking to the caller. Let's add an explicit LRU for unreclaimable
> > BOs instead, and have all BOs added to this LRU at creation time.  
> 
> I would argue that drm_gem_lru_scan() is broken by design. If you're going
> to release the LRU lock in the middle of a loop you can expect that someone
> will get hold of your stack-allocated LRU and end up picking the pieces.

I think it's fine as long as you always use the drm_gem_lru helpers to
manipulate the lru field, which is true of a lot of kernel constructs.

> This patch is fine in itself by trying to avoid stepping into the fight,
> but I think we should also add a warning in drm_gem_lru_scan() for future
> users to be aware of the dangers.

Warning the user about what? There's nothing they can do about it, and
I don't even think it's unsafe per-se, unless someone goes off and
stores the drm_gem_object::lru value somewhere else while their shrink()
callback is called, and accesses it later, outside the shrinker path.
Given drm_gem_lru is not refcounted, there's no way one could safely
hold on the LRU they saw in the shrink() callback anyway, so I don't
think that's fair to blame the drm_gem_lru API for this kind of misuse.