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Date:   Thu, 22 Jul 2021 23:04:49 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-graphics-maintainer@...are.com, zackr@...are.com,
        airlied@...ux.ie, maarten.lankhorst@...ux.intel.com,
        mripard@...nel.org, tzimmermann@...e.de,
        dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
        skhan@...uxfoundation.org, gregkh@...uxfoundation.org,
        linux-kernel-mentees@...ts.linuxfoundation.org
Subject: Re: [PATCH 1/3] drm: use the lookup lock in drm_is_current_master

On Thu, Jul 22, 2021 at 12:38:10PM +0200, Daniel Vetter wrote:
> 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.
> 

The implementation is not hard but I don't understand the usage, for
example, if we have a global variable x, and two locks L1 and L2, and
the function

	void do_something_to_x(void)
	{
		lockdep_assert_held_either(L1, L2);
		x++;
	}

and two call sites:

	void f(void)
	{
		lock(L1);
		do_something_to_x();
		unlock(L1);
	}

	void g(void)
	{
		lock(L2);
		do_something_to_x();
		unlock(L2);
	}

, wouldn't it be racy if f() and g() called by two threads at the same
time? Usually I would expect there exists a third synchronazition
mechanism (say M), which synchronizes the calls to f() and g(), and we
put M in the lockdep_assert_held() check inside do_something_to_x()
like:

	void do_something_to_x(void)
	{
		lockdep_assert_held_once(M);
		x++;
	}

But of course, M may not be a lock, so we cannot put the assert there.

My cscope failed to find ->master_lookup_lock in -rc2 and seems it's not
introduced in the patchset either, could you point me the branch this
patchset is based on, so that I could understand this better, and maybe
come up with a solution? Thanks ;-)

Regards,
Boqun

> 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

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