[PATCH v2 6/8] drm/panfrost: Use mutex_trylock in panfrost_gem_purge

[Steven Price]

On 23/08/2019 03:12, Rob Herring wrote:
> Lockdep reports a circular locking dependency with pages_lock taken in
> the shrinker callback. The deadlock can't actually happen with current
> users at least as a BO will never be purgeable when pages_lock is held.
> To be safe, let's use mutex_trylock() instead and bail if a BO is locked
> already.
> 
> Cc: Tomeu Vizoso <tomeu.vizoso at collabora.com>
> Cc: Steven Price <steven.price at arm.com>
> Cc: Alyssa Rosenzweig <alyssa.rosenzweig at collabora.com>
> Cc: David Airlie <airlied at linux.ie>
> Cc: Daniel Vetter <daniel at ffwll.ch>
> Signed-off-by: Rob Herring <robh at kernel.org>

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

> ---
> v2: new patch
> 
>   drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c | 11 +++++++----
>   1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c b/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c
> index d191632b6197..458f0fa68111 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c
> @@ -36,15 +36,18 @@ panfrost_gem_shrinker_count(struct shrinker *shrinker, struct shrink_control *sc
>   	return count;
>   }
>   
> -static void panfrost_gem_purge(struct drm_gem_object *obj)
> +static bool panfrost_gem_purge(struct drm_gem_object *obj)
>   {
>   	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
> -	mutex_lock(&shmem->pages_lock);
> +
> +	if (!mutex_trylock(&shmem->pages_lock))
> +		return false;
>   
>   	panfrost_mmu_unmap(to_panfrost_bo(obj));
>   	drm_gem_shmem_purge_locked(obj);
>   
>   	mutex_unlock(&shmem->pages_lock);
> +	return true;
>   }
>   
>   static unsigned long
> @@ -61,8 +64,8 @@ panfrost_gem_shrinker_scan(struct shrinker *shrinker, struct shrink_control *sc)
>   	list_for_each_entry_safe(shmem, tmp, &pfdev->shrinker_list, madv_list) {
>   		if (freed >= sc->nr_to_scan)
>   			break;
> -		if (drm_gem_shmem_is_purgeable(shmem)) {
> -			panfrost_gem_purge(&shmem->base);
> +		if (drm_gem_shmem_is_purgeable(shmem) &&
> +		    panfrost_gem_purge(&shmem->base)) {
>   			freed += shmem->base.size >> PAGE_SHIFT;
>   			list_del_init(&shmem->madv_list);
>   		}
>