Re: [PATCH v18 21/26] drm/shmem-helper: Optimize unlocked get_pages_sgt()

[Boris Brezillon <boris.brezillon@collabora.com>]

On Mon, 30 Oct 2023 02:02:00 +0300
Dmitry Osipenko <dmitry.osipenko@collabora.com> wrote:

> SGT isn't refcounted. Once SGT pointer has been obtained, it remains the
> same for both locked and unlocked get_pages_sgt(). Return cached SGT
> directly without taking a potentially expensive lock.
> 
> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
> ---
>  drivers/gpu/drm/drm_gem_shmem_helper.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index 560ce565f376..6dd087f19ea3 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -955,6 +955,9 @@ struct sg_table *drm_gem_shmem_get_pages_sgt(struct drm_gem_shmem_object *shmem)
>  	drm_WARN_ON(obj->dev, drm_gem_shmem_is_evictable(shmem));
>  	drm_WARN_ON(obj->dev, drm_gem_shmem_is_purgeable(shmem));
>  
> +	if (shmem->sgt)
> +		return shmem->sgt;
> +

By doing that, you introduce a race between drm_gem_shmem_put_pages()
and drm_gem_shmem_get_pages_sgt(). I know it's only happening if the
user doesn't follow the rule listed in drm_gem_shmem_get_pages_sgt() doc
(caller must owns a pages ref before calling
drm_gem_shmem_get_pages_sgt()), but now you return a possibly invalid
pointer instead of returning an error when this happens.
If that's what we want, we should at least update the documentation to
make it clear.

>  	ret = dma_resv_lock_interruptible(shmem->base.resv, NULL);
>  	if (ret)
>  		return ERR_PTR(ret);