[PATCH v16 12/20] drm/shmem-helper: Add and use lockless drm_gem_shmem_get_pages()

[Boris Brezillon]

On Sun,  3 Sep 2023 20:07:28 +0300
Dmitry Osipenko <dmitry.osipenko at collabora.com> 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.
> 
> Suggested-by: Boris Brezillon <boris.brezillon at collabora.com>
> Signed-off-by: Dmitry Osipenko <dmitry.osipenko at collabora.com>

Reviewed-by: Boris Brezillon <boris.brezillon at collabora.com>

> ---
>  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 a0faef3e762d..d93ebfef20c7 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -227,6 +227,20 @@ 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;
> +
> +	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;
> @@ -610,10 +624,7 @@ 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;
>