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From: Boqun Feng <boqun.feng@gmail.com>
To: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	linux-graphics-maintainer@vmware.com, zackr@vmware.com,
	airlied@linux.ie, maarten.lankhorst@linux.intel.com,
	mripard@kernel.org, tzimmermann@suse.de,
	dri-devel@lists.freedesktop.org, intel-gfx@lists.freedesktop.org,
	skhan@linuxfoundation.org, gregkh@linuxfoundation.org,
	linux-kernel-mentees@lists.linuxfoundation.org
Subject: Re: [Intel-gfx] [PATCH 1/3] drm: use the lookup lock in drm_is_current_master
Date: Thu, 22 Jul 2021 23:04:49 +0800	[thread overview]
Message-ID: <YPmJEYrnB0j17cZV@boqun-archlinux> (raw)
In-Reply-To: <YPlKkvelm/mcnCj0@phenom.ffwll.local>

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@ffwll.ch>
> > Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.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@ffwll.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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  reply	other threads:[~2021-07-22 16:06 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-22  9:29 [Intel-gfx] [PATCH 0/3] drm, drm/vmwgfx: fixes and updates related to drm_master Desmond Cheong Zhi Xi
2021-07-22  9:29 ` [Intel-gfx] [PATCH 1/3] drm: use the lookup lock in drm_is_current_master Desmond Cheong Zhi Xi
2021-07-22 10:38   ` Daniel Vetter
2021-07-22 15:04     ` Boqun Feng [this message]
2021-07-22 19:02       ` Daniel Vetter
2021-07-23  7:16         ` Boqun Feng
2021-07-27 14:37     ` Peter Zijlstra
2021-07-29  7:00       ` Daniel Vetter
2021-07-29 14:32         ` Desmond Cheong Zhi Xi
2021-07-29 14:45           ` Peter Zijlstra
2021-07-22  9:29 ` [Intel-gfx] [PATCH 2/3] drm: clarify lifetime/locking for drm_master's lease fields Desmond Cheong Zhi Xi
2021-07-22 10:35   ` Daniel Vetter
2021-07-22 13:02     ` Desmond Cheong Zhi Xi
2021-07-22 14:17       ` Daniel Vetter
2021-07-22  9:29 ` [Intel-gfx] [PATCH 3/3] drm/vmwgfx: fix potential UAF in vmwgfx_surface.c Desmond Cheong Zhi Xi
2021-07-22 10:39   ` Daniel Vetter
2021-07-22 19:17   ` Zack Rusin
2021-07-23  6:44     ` Desmond Cheong Zhi Xi
2021-07-22 14:05 ` [Intel-gfx] ✗ Fi.CI.SPARSE: warning for drm, drm/vmwgfx: fixes and updates related to drm_master Patchwork
2021-07-22 14:34 ` [Intel-gfx] ✗ Fi.CI.BAT: failure " Patchwork
2021-07-27 17:42 ` [Intel-gfx] ✗ Fi.CI.BUILD: failure for drm, drm/vmwgfx: fixes and updates related to drm_master (rev2) Patchwork

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