[PATCH 2/8] drm/panfrost: Fix a race in panfrost_ioctl_madvise()

[Steven Price]

On 29/11/2019 13:59, Boris Brezillon wrote:
> If 2 threads change the MADVISE property of the same BO in parallel we
> might end up with an shmem->madv value that's inconsistent with the
> presence of the BO in the shrinker list.

I'm a bit worried from the point of view of user space sanity that you
observed this - but clearly the kernel should be robust!

> 
> The easiest solution to fix that is to protect the
> drm_gem_shmem_madvise() call with the shrinker lock.
> 
> Fixes: 013b65101315 ("drm/panfrost: Add madvise and shrinker support")
> Cc: <stable at vger.kernel.org>
> Signed-off-by: Boris Brezillon <boris.brezillon at collabora.com>

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

> ---
>  drivers/gpu/drm/panfrost/panfrost_drv.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panfrost/panfrost_drv.c b/drivers/gpu/drm/panfrost/panfrost_drv.c
> index f21bc8a7ee3a..efc0a24d1f4c 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_drv.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_drv.c
> @@ -347,20 +347,19 @@ static int panfrost_ioctl_madvise(struct drm_device *dev, void *data,
>  		return -ENOENT;
>  	}
>  
> +	mutex_lock(&pfdev->shrinker_lock);
>  	args->retained = drm_gem_shmem_madvise(gem_obj, args->madv);
>  
>  	if (args->retained) {
>  		struct panfrost_gem_object *bo = to_panfrost_bo(gem_obj);
>  
> -		mutex_lock(&pfdev->shrinker_lock);
> -
>  		if (args->madv == PANFROST_MADV_DONTNEED)
> -			list_add_tail(&bo->base.madv_list, &pfdev->shrinker_list);
> +			list_add_tail(&bo->base.madv_list,
> +				      &pfdev->shrinker_list);
>  		else if (args->madv == PANFROST_MADV_WILLNEED)
>  			list_del_init(&bo->base.madv_list);
> -
> -		mutex_unlock(&pfdev->shrinker_lock);
>  	}
> +	mutex_unlock(&pfdev->shrinker_lock);
>  
>  	drm_gem_object_put_unlocked(gem_obj);
>  	return 0;
>