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Date:   Thu, 22 Jul 2021 21:02:41 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        VMware Graphics <linux-graphics-maintainer@...are.com>,
        Zack Rusin <zackr@...are.com>, Dave Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        intel-gfx <intel-gfx@...ts.freedesktop.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Greg KH <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 6:00 PM Boqun Feng <boqun.feng@...il.com> wrote:
>
> 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 ;-)

The use case is essentially 2 nesting locks, and only the innermost is
used to update a field. So when you only read this field, it's safe if
either of these two locks are held. Essentially this is a read/write lock
type of thing, except for various reasons the two locks might not be of
the same type (like here where the write lock is a mutex, but the read
lock is a spinlock).

It's a bit like the rcu_derefence macro where it's ok to either be in a
rcu_read_lock() section, or holding the relevant lock that's used to
update the value. We do _not_ have two different locks that allow writing
to the same X.

Does that make it clearer what's the use-case here?

In an example:

void * interesting_pointer.

do_update_interesting_pointer()
{
	mutex_lock(A);
	/* do more stuff to prepare things */
	spin_lock(B);
	interesting_pointer = new_value;
	spin_unlock(B);
	mutex_unlock(A);
}

read_interesting_thing_locked()
{
	lockdep_assert_held_either(A, B);

	return interesting_pointer->thing;
}

read_interesting_thing()
{
	int thing;
	spin_lock(B);
	thing = interesting_pointer->thing;
	spin_unlock(B);

	return B;
}

spinlock might also be irqsafe here if this can be called from irq
context.

Cheers, Daniel

> 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



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

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