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Date: Thu, 25 Jan 2024 18:24:04 +0100
From: Daniel Vetter <daniel@ffwll.ch>
To: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Cc: David Airlie <airlied@gmail.com>, Gerd Hoffmann <kraxel@redhat.com>,
	Gurchetan Singh <gurchetansingh@chromium.org>,
	Chia-I Wu <olvaffe@gmail.com>, Daniel Vetter <daniel@ffwll.ch>,
	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
	Maxime Ripard <mripard@kernel.org>,
	Thomas Zimmermann <tzimmermann@suse.de>,
	Christian =?iso-8859-1?Q?K=F6nig?= <christian.koenig@amd.com>,
	Qiang Yu <yuq825@gmail.com>, Steven Price <steven.price@arm.com>,
	Boris Brezillon <boris.brezillon@collabora.com>,
	Emma Anholt <emma@anholt.net>, Melissa Wen <mwen@igalia.com>,
	dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
	kernel@collabora.com, virtualization@lists.linux-foundation.org
Subject: Re: [PATCH v19 09/30] drm/shmem-helper: Add and use lockless
 drm_gem_shmem_get_pages()
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References: <20240105184624.508603-1-dmitry.osipenko@collabora.com>
 <20240105184624.508603-10-dmitry.osipenko@collabora.com>
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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@kernel.org>
> Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
> Suggested-by: Boris Brezillon <boris.brezillon@collabora.com>
> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@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 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