Re: [PATCH v19 09/30] drm/shmem-helper: Add and use lockless drm_gem_shmem_get_pages()

[Daniel Vetter]

On Fri, Jan 05, 2024 at 09:46:03PM +0300, Dmitry Osipenko wrote:
> Add lockless drm_gem_shmem_get_pages() helper that skips taking reservation
> lock if pages_use_count is non-zero, leveraging from atomicity of the
> refcount_t. Make drm_gem_shmem_mmap() to utilize the new helper.
> 
> Acked-by: Maxime Ripard <mripard@xxxxxxxxxx>
> Reviewed-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>
> Suggested-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>
> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx>
> ---
>  drivers/gpu/drm/drm_gem_shmem_helper.c | 19 +++++++++++++++----
>  1 file changed, 15 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index cacf0f8c42e2..1c032513abf1 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -226,6 +226,20 @@ void drm_gem_shmem_put_pages_locked(struct drm_gem_shmem_object *shmem)
>  }
>  EXPORT_SYMBOL_GPL(drm_gem_shmem_put_pages_locked);
>  
> +static int drm_gem_shmem_get_pages(struct drm_gem_shmem_object *shmem)
> +{
> +	int ret;

Just random drive-by comment: a might_lock annotation here might be good,
or people could hit some really interesting bugs that are rather hard to
reproduce ...
-Sima

> +
> +	if (refcount_inc_not_zero(&shmem->pages_use_count))
> +		return 0;
> +
> +	dma_resv_lock(shmem->base.resv, NULL);
> +	ret = drm_gem_shmem_get_pages_locked(shmem);
> +	dma_resv_unlock(shmem->base.resv);
> +
> +	return ret;
> +}
> +
>  static int drm_gem_shmem_pin_locked(struct drm_gem_shmem_object *shmem)
>  {
>  	int ret;
> @@ -609,10 +623,7 @@ int drm_gem_shmem_mmap(struct drm_gem_shmem_object *shmem, struct vm_area_struct
>  		return ret;
>  	}
>  
> -	dma_resv_lock(shmem->base.resv, NULL);
> -	ret = drm_gem_shmem_get_pages_locked(shmem);
> -	dma_resv_unlock(shmem->base.resv);
> -
> +	ret = drm_gem_shmem_get_pages(shmem);
>  	if (ret)
>  		return ret;
>  
> -- 
> 2.43.0
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch