linux-kernel - Re: [PATCH 1/3] drm: use the lookup lock in drm_is_current

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Date:   Thu, 22 Jul 2021 12:38:10 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     linux-graphics-maintainer@...are.com, zackr@...are.com,
        airlied@...ux.ie, daniel@...ll.ch,
        maarten.lankhorst@...ux.intel.com, mripard@...nel.org,
        tzimmermann@...e.de, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, intel-gfx@...ts.freedesktop.org,
        skhan@...uxfoundation.org, gregkh@...uxfoundation.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        Daniel Vetter <daniel.vetter@...ll.ch>
Subject: Re: [PATCH 1/3] drm: use the lookup lock in drm_is_current_master

On Thu, Jul 22, 2021 at 05:29:27PM +0800, Desmond Cheong Zhi Xi wrote:
> Inside drm_is_current_master, using the outer drm_device.master_mutex
> to protect reads of drm_file.master makes the function prone to creating
> lock hierarchy inversions. Instead, we can use the
> drm_file.master_lookup_lock that sits at the bottom of the lock
> hierarchy.
> 
> Reported-by: Daniel Vetter <daniel.vetter@...ll.ch>
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
> ---
>  drivers/gpu/drm/drm_auth.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
> index f00354bec3fb..9c24b8cc8e36 100644
> --- a/drivers/gpu/drm/drm_auth.c
> +++ b/drivers/gpu/drm/drm_auth.c
> @@ -63,8 +63,9 @@
>  
>  static bool drm_is_current_master_locked(struct drm_file *fpriv)
>  {
> -	lockdep_assert_held_once(&fpriv->minor->dev->master_mutex);
> -
> +	/* Either drm_device.master_mutex or drm_file.master_lookup_lock
> +	 * should be held here.
> +	 */

Disappointing that lockdep can't check or conditions for us, a
lockdep_assert_held_either would be really neat in some cases.

Adding lockdep folks, maybe they have ideas.

On the patch:

Reviewed-by: Daniel Vetter <daniel.vetter@...ll.ch>

>  	return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
>  }
>  
> @@ -82,9 +83,9 @@ bool drm_is_current_master(struct drm_file *fpriv)
>  {
>  	bool ret;
>  
> -	mutex_lock(&fpriv->minor->dev->master_mutex);
> +	spin_lock(&fpriv->master_lookup_lock);
>  	ret = drm_is_current_master_locked(fpriv);
> -	mutex_unlock(&fpriv->minor->dev->master_mutex);
> +	spin_unlock(&fpriv->master_lookup_lock);
>  
>  	return ret;
>  }
> -- 
> 2.25.1
> 

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