Re: [PATCH 4/8] drm/panfrost: Fix a race in panfrost_gem_free_object()

[Steven Price <steven.price@arm.com>]

On 29/11/2019 13:59, Boris Brezillon wrote:
> panfrost_gem_shrinker_scan() might purge a BO (release the sgt and
> kill the GPU mapping) that's being freed by panfrost_gem_free_object()
> if we don't remove the BO from the shrinker list at the beginning of
> panfrost_gem_free_object().
> 
> Fixes: 013b65101315 ("drm/panfrost: Add madvise and shrinker support")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>

Reviewed-by: Steven Price <steven.price@arm.com>

> ---
>  drivers/gpu/drm/panfrost/panfrost_gem.c | 15 ++++++++++-----
>  1 file changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panfrost/panfrost_gem.c b/drivers/gpu/drm/panfrost/panfrost_gem.c
> index acb07fe06580..daf4c55a2863 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_gem.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_gem.c
> @@ -19,6 +19,16 @@ static void panfrost_gem_free_object(struct drm_gem_object *obj)
>  	struct panfrost_gem_object *bo = to_panfrost_bo(obj);
>  	struct panfrost_device *pfdev = obj->dev->dev_private;
>  
> +	/*
> +	 * Make sure the BO is no longer inserted in the shrinker list before
> +	 * taking care of the destruction itself. If we don't do that we have a
> +	 * race condition between this function and what's done in
> +	 * panfrost_gem_shrinker_scan().
> +	 */
> +	mutex_lock(&pfdev->shrinker_lock);
> +	list_del_init(&bo->base.madv_list);
> +	mutex_unlock(&pfdev->shrinker_lock);
> +
>  	if (bo->sgts) {
>  		int i;
>  		int n_sgt = bo->base.base.size / SZ_2M;
> @@ -33,11 +43,6 @@ static void panfrost_gem_free_object(struct drm_gem_object *obj)
>  		kfree(bo->sgts);
>  	}
>  
> -	mutex_lock(&pfdev->shrinker_lock);
> -	if (!list_empty(&bo->base.madv_list))
> -		list_del(&bo->base.madv_list);
> -	mutex_unlock(&pfdev->shrinker_lock);
> -
>  	drm_gem_shmem_free_object(obj);
>  }
>  
>