linux-kernel - Re: [PATCH 1/3] drm: use the lookup lock in drm_is_current

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Date:   Tue, 27 Jul 2021 16:37:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     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 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 ?

---
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)