[tip:core/locking] mutexes: Give more informative mutex warning in the !lock->owner case

[tip-bot for Chuansheng Liu <tipbot@zytor.com>]

Commit-ID:  91f30a17024ff0d8345e11228af33ee938b13426
Gitweb:     http://git.kernel.org/tip/91f30a17024ff0d8345e11228af33ee938b13426
Author:     Chuansheng Liu <chuansheng.liu@intel.com>
AuthorDate: Wed, 4 Dec 2013 13:58:13 +0800
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 17 Dec 2013 15:35:10 +0100

mutexes: Give more informative mutex warning in the !lock->owner case

When mutex debugging is enabled and an imbalanced mutex_unlock()
is called, we get the following, slightly confusing warning:

  [  364.208284] DEBUG_LOCKS_WARN_ON(lock->owner != current)

But in that case the warning is due to an imbalanced mutex_unlock() call,
and the lock->owner is NULL - so the message is misleading.

So improve the message by testing for this case specifically:

   DEBUG_LOCKS_WARN_ON(!lock->owner)

Signed-off-by: Liu, Chuansheng <chuansheng.liu@intel.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/1386136693.3650.48.camel@cliu38-desktop-build
[ Improved the changelog, changed the patch to use !lock->owner consistently. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/locking/mutex-debug.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/locking/mutex-debug.c b/kernel/locking/mutex-debug.c
index 7e3443f..faf6f5b 100644
--- a/kernel/locking/mutex-debug.c
+++ b/kernel/locking/mutex-debug.c
@@ -75,7 +75,12 @@ void debug_mutex_unlock(struct mutex *lock)
 		return;
 
 	DEBUG_LOCKS_WARN_ON(lock->magic != lock);
-	DEBUG_LOCKS_WARN_ON(lock->owner != current);
+
+	if (!lock->owner)
+		DEBUG_LOCKS_WARN_ON(!lock->owner);
+	else
+		DEBUG_LOCKS_WARN_ON(lock->owner != current);
+
 	DEBUG_LOCKS_WARN_ON(!lock->wait_list.prev && !lock->wait_list.next);
 	mutex_clear_owner(lock);
 }