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>

1 files changed, 6 insertions, 1 deletions

diff --git a/kernel/locking/mutex-debug.c b/kernel/locking/mutex-debug.c
index 7e3443fe1f48..faf6f5b53e77 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);
 }