linux-kernel - Re: [PATCH v18 21/26] drm/shmem-helper: Optimize unlocked get_pages

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Date:   Mon, 13 Nov 2023 10:49:56 +0100
From:   Boris Brezillon <boris.brezillon@...labora.com>
To:     Dmitry Osipenko <dmitry.osipenko@...labora.com>
Cc:     David Airlie <airlied@...il.com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        Chia-I Wu <olvaffe@...il.com>, Daniel Vetter <daniel@...ll.ch>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Christian König <christian.koenig@....com>,
        Qiang Yu <yuq825@...il.com>,
        Steven Price <steven.price@....com>,
        Emma Anholt <emma@...olt.net>, Melissa Wen <mwen@...lia.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        kernel@...labora.com, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v18 21/26] drm/shmem-helper: Optimize unlocked
 get_pages_sgt()

On Mon, 30 Oct 2023 02:02:00 +0300
Dmitry Osipenko <dmitry.osipenko@...labora.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@...labora.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);