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

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Message-ID: <YQPK3jMU69WMsCjT@boqun-archlinux>
Date:   Fri, 30 Jul 2021 17:48:14 +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 04:06:44PM +0800, Desmond Cheong Zhi Xi wrote:
> On 30/7/21 2:08 pm, Boqun Feng wrote:
> > 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
> 
> Thanks for the feedback, Boqun.
> 
> Yeah, I think the updater side maybe doesn't need new lockdep_assert()
> because what currently happens is either
> 
> 	lockdep_assert_held_once(&dev->master_mutex);
> 	/* 6 lines of prep */
> 	spin_lock(&fpriv->master_lookup_lock);
> 	fpriv->master = new_value;
> or
> 	mutex_lock(&dev->master_mutex);
> 	/* 3 lines of checks */
> 		spin_lock(&file_priv->master_lookup_lock);
> 		file_priv->master = new_value;
> 
> > 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.
> > 
> 
> But clarifying the lock design in the documentation sounds like a really
> good idea.
> 
> Probably a good place for this would be in the kerneldoc where we also
> explain the lifetime rules and usage of the pointer outside drm_auth.c:
> 
> 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

Works for me ;-)

For the whole series, feel free to add:

Acked-by: Boqun Feng <boqun.feng@...il.com>

Regards,
Boqun

> 
> Best wishes,
> Desmond
> 
> > Regards,
> > Boqun
> > 
> > >   	return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
> > >   }
> > > -- 
> > > 2.25.1
> > > 
>