[PATCH] drm/ttm: make ttm_bo_unpin more defensive

[Thomas Hellström (Intel)]

Hi, Christian

On 3/12/21 10:38 AM, Christian König wrote:
> We seem to have some more driver bugs than thought.
>
> Signed-off-by: Christian König <christian.koenig at amd.com>
> ---
>   include/drm/ttm/ttm_bo_api.h | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/include/drm/ttm/ttm_bo_api.h b/include/drm/ttm/ttm_bo_api.h
> index 4fb523dfab32..df9fe596e7c5 100644
> --- a/include/drm/ttm/ttm_bo_api.h
> +++ b/include/drm/ttm/ttm_bo_api.h
> @@ -603,9 +603,11 @@ static inline void ttm_bo_pin(struct ttm_buffer_object *bo)
>   static inline void ttm_bo_unpin(struct ttm_buffer_object *bo)
>   {
>   	dma_resv_assert_held(bo->base.resv);
> -	WARN_ON_ONCE(!bo->pin_count);
>   	WARN_ON_ONCE(!kref_read(&bo->kref));
> -	--bo->pin_count;
> +	if (bo->pin_count)
> +		--bo->pin_count;
> +	else
> +		WARN_ON_ONCE(true);
>   }
>   
>   int ttm_mem_evict_first(struct ttm_device *bdev,

Since I now have been staring for half a year at the code of the driver 
that made pinning an art, I have a couple of suggestions here, Could we 
use an atomic for pin_count, allowing unlocked unpinning but require the 
lock only for pin_count transition 0->1, (but unlocked for 
pin_if_already_pinned). In particular I think vmwgfx would benefit from 
unlocked unpins. Also if the atomic were a refcount_t, that would 
probably give you the above behaviour?

/Thomas