linux-kernel - Re: [RESEND PATCH v2 2/2] drm: add lockdep assert to drm_is_current_master

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Message-ID: <YQu4nfxtxyTCVGhn@phenom.ffwll.local>
Date:   Thu, 5 Aug 2021 12:08:29 +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,
        peterz@...radead.org, mingo@...hat.com, will@...nel.org,
        longman@...hat.com, boqun.feng@...il.com, sumit.semwal@...aro.org,
        christian.koenig@....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: [RESEND PATCH v2 2/2] drm: add lockdep assert to
 drm_is_current_master_locked

On Mon, Aug 02, 2021 at 06:59:57PM +0800, Desmond Cheong Zhi Xi wrote:
> In drm_is_current_master_locked, accessing drm_file.master should be
> protected by either drm_file.master_lookup_lock or
> drm_device.master_mutex. This was previously awkward to assert with
> lockdep.
> 
> Following patch ("locking/lockdep: Provide lockdep_assert{,_once}()
> helpers"), this assertion is now convenient. So we add in the
> assertion and explain this lock design in the kerneldoc.
> 
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
> Acked-by: Boqun Feng <boqun.feng@...il.com>
> Acked-by: Waiman Long <longman@...hat.com>
> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Both patches pushed to drm-misc-next, thanks.
-Daniel

> ---
>  drivers/gpu/drm/drm_auth.c | 6 +++---
>  include/drm/drm_file.h     | 4 ++++
>  2 files changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
> index 9c24b8cc8e36..6f4d7ff23c80 100644
> --- a/drivers/gpu/drm/drm_auth.c
> +++ b/drivers/gpu/drm/drm_auth.c
> @@ -63,9 +63,9 @@
>  
>  static bool drm_is_current_master_locked(struct drm_file *fpriv)
>  {
> -	/* Either drm_device.master_mutex or drm_file.master_lookup_lock
> -	 * should be held here.
> -	 */
> +	lockdep_assert_once(lockdep_is_held(&fpriv->master_lookup_lock) ||
> +			    lockdep_is_held(&fpriv->minor->dev->master_mutex));
> +
>  	return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
>  }
>  
> diff --git a/include/drm/drm_file.h b/include/drm/drm_file.h
> index 726cfe0ff5f5..a3acb7ac3550 100644
> --- a/include/drm/drm_file.h
> +++ b/include/drm/drm_file.h
> @@ -233,6 +233,10 @@ struct drm_file {
>  	 * this only matches &drm_device.master if the master is the currently
>  	 * active one.
>  	 *
> +	 * To update @master, both &drm_device.master_mutex and
> +	 * @master_lookup_lock need to be held, therefore holding either of
> +	 * them is safe and enough for the read side.
> +	 *
>  	 * When dereferencing this pointer, either hold struct
>  	 * &drm_device.master_mutex for the duration of the pointer's use, or
>  	 * use drm_file_get_master() if struct &drm_device.master_mutex is not
> -- 
> 2.25.1
> 

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