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Message-ID: <YQJSE3TMRydDNhqT@phenom.ffwll.local>
Date: Thu, 29 Jul 2021 09:00:35 +0200
From: Daniel Vetter <daniel@...ll.ch>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>,
Boqun Feng <boqun.feng@...il.com>,
LKML <linux-kernel@...r.kernel.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 Tue, Jul 27, 2021 at 04:37:22PM +0200, Peter Zijlstra 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.
> >
> > Adding lockdep folks, maybe they have ideas.
>
> #ifdef CONFIG_LOCKDEP
> WARN_ON_ONCE(debug_locks && !(lockdep_is_held(&drm_device.master_mutex) ||
> lockdep_is_held(&drm_file.master_lookup_lock)));
> #endif
>
> doesn't exactly roll off the tongue, but should do as you want I
> suppose.
>
> Would something like:
>
> #define lockdep_assert(cond) WARN_ON_ONCE(debug_locks && !(cond))
>
> Such that we can write:
>
> lockdep_assert(lockdep_is_held(&drm_device.master_mutex) ||
> lockdep_is_held(&drm_file.master_lookup_lock));
>
> make it better ?
Yeah I think that's pretty tidy and flexible.
Desmond, can you pls give this a shot with Peter's patch below?
-Daniel
>
> ---
> Subject: locking/lockdep: Provide lockdep_assert{,_once}() helpers
>
> Extract lockdep_assert{,_once}() helpers to more easily write composite
> assertions like, for example:
>
> lockdep_assert(lockdep_is_held(&drm_device.master_mutex) ||
> lockdep_is_held(&drm_file.master_lookup_lock));
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
> diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
> index 5cf387813754..0da67341c1fb 100644
> --- a/include/linux/lockdep.h
> +++ b/include/linux/lockdep.h
> @@ -306,31 +306,29 @@ extern void lock_unpin_lock(struct lockdep_map *lock, struct pin_cookie);
>
> #define lockdep_depth(tsk) (debug_locks ? (tsk)->lockdep_depth : 0)
>
> -#define lockdep_assert_held(l) do { \
> - WARN_ON(debug_locks && \
> - lockdep_is_held(l) == LOCK_STATE_NOT_HELD); \
> - } while (0)
> +#define lockdep_assert(cond) \
> + do { WARN_ON(debug_locks && !(cond)); } while (0)
>
> -#define lockdep_assert_not_held(l) do { \
> - WARN_ON(debug_locks && \
> - lockdep_is_held(l) == LOCK_STATE_HELD); \
> - } while (0)
> +#define lockdep_assert_once(cond) \
> + do { WARN_ON_ONCE(debug_locks && !(cond)); } while (0)
>
> -#define lockdep_assert_held_write(l) do { \
> - WARN_ON(debug_locks && !lockdep_is_held_type(l, 0)); \
> - } while (0)
> +#define lockdep_assert_held(l) \
> + lockdep_assert(lockdep_is_held(l) != LOCK_STAT_NOT_HELD)
>
> -#define lockdep_assert_held_read(l) do { \
> - WARN_ON(debug_locks && !lockdep_is_held_type(l, 1)); \
> - } while (0)
> +#define lockdep_assert_not_held(l) \
> + lockdep_assert(lockdep_is_held(l) != LOCK_STATE_HELD)
>
> -#define lockdep_assert_held_once(l) do { \
> - WARN_ON_ONCE(debug_locks && !lockdep_is_held(l)); \
> - } while (0)
> +#define lockdep_assert_held_write(l) \
> + lockdep_assert(lockdep_is_held_type(l, 0))
>
> -#define lockdep_assert_none_held_once() do { \
> - WARN_ON_ONCE(debug_locks && current->lockdep_depth); \
> - } while (0)
> +#define lockdep_assert_held_read(l) \
> + lockdep_assert(lockdep_is_held_type(l, 1))
> +
> +#define lockdep_assert_held_once(l) \
> + lockdep_assert_once(lockdep_is_held(l) != LOCK_STAT_NOT_HELD)
> +
> +#define lockdep_assert_none_held_once() \
> + lockdep_assert_once(!current->lockdep_depth)
>
> #define lockdep_recursing(tsk) ((tsk)->lockdep_recursion)
>
> @@ -407,6 +405,9 @@ extern int lock_is_held(const void *);
> extern int lockdep_is_held(const void *);
> #define lockdep_is_held_type(l, r) (1)
>
> +#define lockdep_assert(c) do { } while (0)
> +#define lockdep_assert_once(c) do { } while (0)
> +
> #define lockdep_assert_held(l) do { (void)(l); } while (0)
> #define lockdep_assert_not_held(l) do { (void)(l); } while (0)
> #define lockdep_assert_held_write(l) do { (void)(l); } while (0)
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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