Re: [PATCH 3/3] drm/gem: Stop exposing the racy/unsafe drm_gem_lru_remove() helper

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

On Wed, May 06, 2026 at 02:16:28PM +0200, Boris Brezillon wrote:
> The only place where it's safe to call drm_gem_lru_remove() is when
> we know the drm_gem_object::lru field can't be concurrently updated,
> which we know is the case when the drm_gem_object is destroyed.
> 
> Rather than trying to make that safe, let's kill the function and inline
> its content in drm_gem_object_release().
> 
> Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
> Reviewed-by: Chia-I Wu <olvaffe@gmail.com>

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

Best regards,
Liviu

> ---
>  drivers/gpu/drm/drm_gem.c | 49 +++++++++++++++++------------------------------
>  include/drm/drm_gem.h     |  1 -
>  2 files changed, 18 insertions(+), 32 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> index 97cf63de0112..b6df4f62f7b5 100644
> --- a/drivers/gpu/drm/drm_gem.c
> +++ b/drivers/gpu/drm/drm_gem.c
> @@ -1108,6 +1108,15 @@ drm_gem_release(struct drm_device *dev, struct drm_file *file_private)
>  	idr_destroy(&file_private->object_idr);
>  }
>  
> +static void
> +drm_gem_lru_remove_locked(struct drm_gem_object *obj)
> +{
> +	obj->lru->count -= obj->size >> PAGE_SHIFT;
> +	WARN_ON(obj->lru->count < 0);
> +	list_del(&obj->lru_node);
> +	obj->lru = NULL;
> +}
> +
>  /**
>   * drm_gem_object_release - release GEM buffer object resources
>   * @obj: GEM buffer object
> @@ -1124,7 +1133,15 @@ drm_gem_object_release(struct drm_gem_object *obj)
>  	drm_gem_private_object_fini(obj);
>  
>  	drm_gem_free_mmap_offset(obj);
> -	drm_gem_lru_remove(obj);
> +
> +	/* If the object refcount drops to zero, it means no one can change
> +	 * the LRU it's inserted into, so it's safe to dereference
> +	 * drm_gem_object::lru without the drm_gem_lru::lock held.
> +	 */
> +	if (obj->lru) {
> +		guard(mutex)(obj->lru->lock);
> +		drm_gem_lru_remove_locked(obj);
> +	}
>  }
>  EXPORT_SYMBOL(drm_gem_object_release);
>  
> @@ -1552,36 +1569,6 @@ drm_gem_lru_init(struct drm_gem_lru *lru, struct mutex *lock)
>  }
>  EXPORT_SYMBOL(drm_gem_lru_init);
>  
> -static void
> -drm_gem_lru_remove_locked(struct drm_gem_object *obj)
> -{
> -	obj->lru->count -= obj->size >> PAGE_SHIFT;
> -	WARN_ON(obj->lru->count < 0);
> -	list_del(&obj->lru_node);
> -	obj->lru = NULL;
> -}
> -
> -/**
> - * drm_gem_lru_remove - remove object from whatever LRU it is in
> - *
> - * If the object is currently in any LRU, remove it.
> - *
> - * @obj: The GEM object to remove from current LRU
> - */
> -void
> -drm_gem_lru_remove(struct drm_gem_object *obj)
> -{
> -	struct drm_gem_lru *lru = obj->lru;
> -
> -	if (!lru)
> -		return;
> -
> -	mutex_lock(lru->lock);
> -	drm_gem_lru_remove_locked(obj);
> -	mutex_unlock(lru->lock);
> -}
> -EXPORT_SYMBOL(drm_gem_lru_remove);
> -
>  /**
>   * drm_gem_lru_move_tail_locked - move the object to the tail of the LRU
>   *
> diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
> index 86f5846154f7..d527df98d142 100644
> --- a/include/drm/drm_gem.h
> +++ b/include/drm/drm_gem.h
> @@ -611,7 +611,6 @@ int drm_gem_dumb_map_offset(struct drm_file *file, struct drm_device *dev,
>  			    u32 handle, u64 *offset);
>  
>  void drm_gem_lru_init(struct drm_gem_lru *lru, struct mutex *lock);
> -void drm_gem_lru_remove(struct drm_gem_object *obj);
>  void drm_gem_lru_move_tail_locked(struct drm_gem_lru *lru, struct drm_gem_object *obj);
>  void drm_gem_lru_move_tail(struct drm_gem_lru *lru, struct drm_gem_object *obj);
>  unsigned long
> 
> -- 
> 2.54.0
> 

-- 
====================
| I would like to |
| fix the world,  |
| but they're not |
| giving me the   |
 \ source code!  /
  ---------------
    ¯\_(ツ)_/¯