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

[Liviu Dudau <liviu.dudau@arm.com>]

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.
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.

Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>

Best regards,
Liviu

> 
> Fixes: fb42964e2a76 ("drm/panthor: Add a GEM shrinker")
> Reported-by: Chia-I Wu <olvaffe@gmail.com>
> Closes: https://gitlab.freedesktop.org/panfrost/linux/-/work_items/86
> Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
> Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
> ---
>  drivers/gpu/drm/panthor/panthor_device.h | 10 ++++++++++
>  drivers/gpu/drm/panthor/panthor_gem.c    |  5 ++++-
>  2 files changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_device.h b/drivers/gpu/drm/panthor/panthor_device.h
> index 4e4607bca7cc..45b71546f83c 100644
> --- a/drivers/gpu/drm/panthor/panthor_device.h
> +++ b/drivers/gpu/drm/panthor/panthor_device.h
> @@ -190,6 +190,16 @@ struct panthor_device {
>  		/** @reclaim.lock: Lock protecting all LRUs */
>  		struct mutex lock;
>  
> +		/**
> +		 * @reclaim.unreclaimable: unreclaimable BOs
> +		 *
> +		 * Either the BO is unreclaimable because it has no pages allocated,
> +		 * or it's unreclaimable because pages are pinned.
> +		 *
> +		 * All BOs start in that list at creation time.
> +		 */
> +		struct drm_gem_lru unreclaimable;
> +
>  		/**
>  		 * @reclaim.unused: BOs with unused pages
>  		 *
> diff --git a/drivers/gpu/drm/panthor/panthor_gem.c b/drivers/gpu/drm/panthor/panthor_gem.c
> index 13295d7a593d..8e31740126e7 100644
> --- a/drivers/gpu/drm/panthor/panthor_gem.c
> +++ b/drivers/gpu/drm/panthor/panthor_gem.c
> @@ -204,7 +204,7 @@ void panthor_gem_update_reclaim_state_locked(struct panthor_gem_object *bo,
>  		drm_gem_lru_move_tail(&ptdev->reclaim.gpu_mapped_shared, &bo->base);
>  		break;
>  	case PANTHOR_GEM_UNRECLAIMABLE:
> -		drm_gem_lru_remove(&bo->base);
> +		drm_gem_lru_move_tail(&ptdev->reclaim.unreclaimable, &bo->base);
>  		break;
>  	default:
>  		drm_WARN(&ptdev->base, true, "invalid GEM reclaim state (%d)\n", new_state);
> @@ -994,6 +994,7 @@ static struct panthor_gem_object *
>  panthor_gem_create(struct drm_device *dev, size_t size, uint32_t flags,
>  		   struct panthor_vm *exclusive_vm, u32 usage_flags)
>  {
> +	struct panthor_device *ptdev = container_of(dev, struct panthor_device, base);
>  	struct panthor_gem_object *bo;
>  	int ret;
>  
> @@ -1026,6 +1027,7 @@ panthor_gem_create(struct drm_device *dev, size_t size, uint32_t flags,
>  	}
>  
>  	panthor_gem_debugfs_set_usage_flags(bo, usage_flags);
> +	drm_gem_lru_move_tail(&ptdev->reclaim.unreclaimable, &bo->base);
>  	return bo;
>  
>  err_put:
> @@ -1551,6 +1553,7 @@ int panthor_gem_shrinker_init(struct panthor_device *ptdev)
>  		return ret;
>  
>  	INIT_LIST_HEAD(&ptdev->reclaim.vms);
> +	drm_gem_lru_init(&ptdev->reclaim.unreclaimable, &ptdev->reclaim.lock);
>  	drm_gem_lru_init(&ptdev->reclaim.unused, &ptdev->reclaim.lock);
>  	drm_gem_lru_init(&ptdev->reclaim.mmapped, &ptdev->reclaim.lock);
>  	drm_gem_lru_init(&ptdev->reclaim.gpu_mapped_shared, &ptdev->reclaim.lock);
> 
> -- 
> 2.54.0
> 

-- 
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| but they're not |
| giving me the   |
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