Re: [PATCH 1/3] drm: use the lookup lock in drm_is_current_master

[Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>]

On 29/7/21 3:00 pm, Daniel Vetter wrote:
> 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@xxxxxxxx>
> > > > Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
> > > > ---
> > > >   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

Sounds good, will do. Thanks for the patch, Peter.

Just going to make a small edit:
s/LOCK_STAT_NOT_HELD/LOCK_STATE_NOT_HELD/

Best wishes,
Desmond

> > ---
> > 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@xxxxxxxxxxxxx>
> > ---
> > 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)

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