linux-kernel - Re: [PATCH v3 2/9] drm: hold master_lookup_lock when releasing a drm

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Date:   Wed, 18 Aug 2021 12:05:01 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
Cc:     maarten.lankhorst@...ux.intel.com, mripard@...nel.org,
        tzimmermann@...e.de, airlied@...ux.ie, daniel@...ll.ch,
        sumit.semwal@...aro.org, christian.koenig@....com, axboe@...nel.dk,
        oleg@...hat.com, tglx@...utronix.de, dvyukov@...gle.com,
        walter-zh.wu@...iatek.com, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, intel-gfx@...ts.freedesktop.org,
        linux-media@...r.kernel.org, linaro-mm-sig@...ts.linaro.org,
        skhan@...uxfoundation.org, gregkh@...uxfoundation.org,
        linux-kernel-mentees@...ts.linuxfoundation.org
Subject: Re: [PATCH v3 2/9] drm: hold master_lookup_lock when releasing a
 drm_file's master

On Wed, Aug 18, 2021 at 03:38:17PM +0800, Desmond Cheong Zhi Xi wrote:
> When drm_file.master changes value, the corresponding
> drm_device.master_lookup_lock should be held.
> 
> In drm_master_release, a call to drm_master_put sets the
> file_priv->master to NULL, so we protect this section with
> drm_device.master_lookup_lock.
> 
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>

At this points all refcounts to drm_file have disappeared, so yeah this is
a lockless access, but also no one can observe it anymore. See also next
patch.

Hence I think the current code is fine.
-Daniel

> ---
>  drivers/gpu/drm/drm_auth.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
> index 8efb58aa7d95..8c0e0dba1611 100644
> --- a/drivers/gpu/drm/drm_auth.c
> +++ b/drivers/gpu/drm/drm_auth.c
> @@ -373,8 +373,11 @@ void drm_master_release(struct drm_file *file_priv)
>  	}
>  
>  	/* drop the master reference held by the file priv */
> -	if (file_priv->master)
> +	if (file_priv->master) {
> +		spin_lock(&dev->master_lookup_lock);
>  		drm_master_put(&file_priv->master);
> +		spin_unlock(&dev->master_lookup_lock);
> +	}
>  	mutex_unlock(&dev->master_mutex);
>  }
>  
> -- 
> 2.25.1
> 

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