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

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Date:   Fri, 30 Jul 2021 14:08:13 +0800
From:   Boqun Feng <boqun.feng@...il.com>
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, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, skhan@...uxfoundation.org,
        gregkh@...uxfoundation.org,
        linux-kernel-mentees@...ts.linuxfoundation.org
Subject: Re: [PATCH 2/2] drm: add lockdep assert to
 drm_is_current_master_locked

On Fri, Jul 30, 2021 at 12:15:15PM +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 it in.
> 
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
> ---
>  drivers/gpu/drm/drm_auth.c | 6 +++---
>  1 file changed, 3 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));
> +

I think it's better to also add the lockdep_assert() of & (i.e. both
held) in the updater side, and have comments pointing to each other.

Is it convenient to do in this patchset? If the updater side doesn't
need to put the lockdep_assert() (maybe the lock acquire code and the
update code are in the same function), it's still better to add some
comments like:

	/*
	 * To update drm_file.master, both drm_file.master_lookup_lock
	 * and drm_device.master_mutex are needed, therefore holding
	 * either of them is safe and enough for the read side.
	 */

Just feel it's better to explain the lock design either in the
lockdep_assert() or comments.

Regards,
Boqun

>  	return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
>  }
>  
> -- 
> 2.25.1
>