[PATCH 1/4] rtmutex: avoid null derefence in WARN_ON

[Darren Hart <dvhltc@us.ibm.com>]

If the pi_blocked_on variable is NULL, the subsequent WARN_ON's
will cause an OOPS. Only perform the susequent checks if
pi_blocked_on is valid.

Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: John Kacur <jkacur@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mike Galbraith <efault@gmx.de>
---
 kernel/rtmutex.c |    7 ++++---
 1 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
index 23dd443..baac7d9 100644
--- a/kernel/rtmutex.c
+++ b/kernel/rtmutex.c
@@ -579,9 +579,10 @@ static void wakeup_next_waiter(struct rt_mutex *lock, int savestate)
 
 	raw_spin_lock(&pendowner->pi_lock);
 
-	WARN_ON(!pendowner->pi_blocked_on);
-	WARN_ON(pendowner->pi_blocked_on != waiter);
-	WARN_ON(pendowner->pi_blocked_on->lock != lock);
+	if (!WARN_ON(!pendowner->pi_blocked_on)) {
+		WARN_ON(pendowner->pi_blocked_on != waiter);
+		WARN_ON(pendowner->pi_blocked_on->lock != lock);
+	}
 
 	pendowner->pi_blocked_on = NULL;
 
-- 
1.7.0.4