From: Daniel Vetter <daniel@ffwll.ch>
To: Boqun Feng <boqun.feng@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
Greg KH <gregkh@linuxfoundation.org>,
intel-gfx <intel-gfx@lists.freedesktop.org>,
LKML <linux-kernel@vger.kernel.org>,
Maxime Ripard <mripard@kernel.org>,
Dave Airlie <airlied@linux.ie>,
VMware Graphics <linux-graphics-maintainer@vmware.com>,
dri-devel <dri-devel@lists.freedesktop.org>,
Thomas Zimmermann <tzimmermann@suse.de>,
Shuah Khan <skhan@linuxfoundation.org>,
Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>,
linux-kernel-mentees@lists.linuxfoundation.org,
Zack Rusin <zackr@vmware.com>
Subject: Re: [Intel-gfx] [PATCH 1/3] drm: use the lookup lock in drm_is_current_master
Date: Thu, 22 Jul 2021 21:02:41 +0200 [thread overview]
Message-ID: <CAKMK7uGSc_YMf2e=oA23KeAvC8i_pqJBU82v8oRGfnwsT41WLQ@mail.gmail.com> (raw)
In-Reply-To: <YPmJEYrnB0j17cZV@boqun-archlinux>
On Thu, Jul 22, 2021 at 6:00 PM Boqun Feng <boqun.feng@gmail.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@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 ;-)
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@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
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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next prev parent reply other threads:[~2021-07-22 19:02 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
2021-07-22 19:02 ` Daniel Vetter [this message]
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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