Re: [PATCH v2 2/4] drm/gem: Fix a race between drm_gem_lru_scan() and drm_gem_object_release()

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

On Fri, May 08, 2026 at 12:40:48PM +0200, Boris Brezillon wrote:
> The following race can currently happen:
> 
> | Thread 0 in `drm_gem_lru_scan`               | Thread 1 in `drm_gem_object_release` |
> | -                                            | -                                    |
> | move obj1 with refcount==0 to `still_in_lru` |                                      |
> | move obj2 with refcount!=0 to `still_in_lru` |                                      |
> | mutex_unlock                                 |                                      |
> | shrink obj2                                  |                                      |
> |                                              | lru = obj1->lru; // `still_in_lru`   |
> | mutex_lock                                   |                                      |
> | move obj1 back to the original lru           |                                      |
> | mutex_unlock                                 |                                      |
> | return                                       |                                      |
> |                                              | dereference `still_in_lru`           |
> 
> Move the drm_gem_lru_move_tail_locked() after the
> kref_get_unless_zero() check so that we don't end up with a
> vanishing LRU when we hit drm_gem_object_release(). We also need to
> remove the skipped object from its LRU, otherwise we'll keep hitting
> it on subsequent loop iterations until it's actually removed from the
> list in the drm_gem_release().
> 
> Fixes: e7c2af13f811 ("drm/gem: Add LRU/shrinker helper")
> 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: Liviu Dudau <liviu.dudau@arm.com>

> ---
>  drivers/gpu/drm/drm_gem.c | 34 ++++++++++++++++++++++++++++------
>  1 file changed, 28 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> index fca42949eb2b..0e087c770883 100644
> --- a/drivers/gpu/drm/drm_gem.c
> +++ b/drivers/gpu/drm/drm_gem.c
> @@ -1573,11 +1573,31 @@ drm_gem_lru_remove(struct drm_gem_object *obj)
>  {
>  	struct drm_gem_lru *lru = obj->lru;
>  
> +	/*
> +	 * We do the lru != NULL check without the lru->lock held, which
> +	 * means we might end up with a stale lru value by the time the
> +	 * lock is acquired.
> +	 *
> +	 * This is deemed safe because:
> +	 * 1. the LRU is assumed to outlive any GEM object it was attached
> +	 *    (LRUs are usually bound to a drm_device). So even if obj->lru
> +	 *    has become NULL, it still point to a valid object that can
> +	 *    safely be dereferenced to get the lock.
> +	 *
> +	 * 2. all LRUs a GEM object might be attached to must share the same
> +	 *    lock (lock that's usually part of the driver-specific device
> +	 *    object), so taking the lock on the 'old' LRU is equivalent
> +	 *    to taking it on the new one (if any)

I like the description, but I think it's worth merging the later comment around
the second check here as that is basically the whole "belt and braces" mechanism
for ensuring correctness.

Best regards,
Liviu

> +	 */
>  	if (!lru)
>  		return;
>  
>  	mutex_lock(lru->lock);
> -	drm_gem_lru_remove_locked(obj);
> +	/* Check a second time with the lock held to make sure we're not racing
> +	 * with another drm_gem_lru_remove[_locked]() call.
> +	 */
> +	if (obj->lru)
> +		drm_gem_lru_remove_locked(obj);
>  	mutex_unlock(lru->lock);
>  }
>  EXPORT_SYMBOL(drm_gem_lru_remove);
> @@ -1660,15 +1680,17 @@ drm_gem_lru_scan(struct drm_gem_lru *lru,
>  		if (!obj)
>  			break;
>  
> -		drm_gem_lru_move_tail_locked(&still_in_lru, obj);
> -
>  		/*
>  		 * If it's in the process of being freed, gem_object->free()
> -		 * may be blocked on lock waiting to remove it.  So just
> -		 * skip it.
> +		 * may be blocked on lock waiting to remove it.  So just remove
> +		 * it from its current LRU and skip it.
>  		 */
> -		if (!kref_get_unless_zero(&obj->refcount))
> +		if (!kref_get_unless_zero(&obj->refcount)) {
> +			drm_gem_lru_remove_locked(obj);
>  			continue;
> +		}
> +
> +		drm_gem_lru_move_tail_locked(&still_in_lru, obj);
>  
>  		/*
>  		 * Now that we own a reference, we can drop the lock for the
> 
> -- 
> 2.54.0
> 

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