rtmutex: Adjust pi_lock usage in wakeup

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From: Peter W.Morreale <pmorreale@novell.com>

In wakeup_next_waiter(), we take the pi_lock, and then find out whether
we have another waiter to add to the pending owner.  We can reduce
contention on the pi_lock for the pending owner if we first obtain the
pointer to the next waiter outside of the pi_lock.
Signed-off-by: Peter W. Morreale <pmorreale@novell.com>
Signed-off-by: Gregory Haskins <ghaskins@novell.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/rtmutex.c

@@ -507,6 +507,7 @@ static void wakeup_next_waiter(struct rt_mutex *lock, int savestate)
 {
 	struct rt_mutex_waiter *waiter;
 	struct task_struct *pendowner;
+	struct rt_mutex_waiter *next;
 
 	atomic_spin_lock(&current->pi_lock);
 
@@ -569,6 +570,12 @@ static void wakeup_next_waiter(struct rt_mutex *lock, int savestate)
 	 * waiter with higher priority than pending-owner->normal_prio
 	 * is blocked on the unboosted (pending) owner.
 	 */
+
+	if (rt_mutex_has_waiters(lock))
+		next = rt_mutex_top_waiter(lock);
+	else
+		next = NULL;
+
 	atomic_spin_lock(&pendowner->pi_lock);
 
 	WARN_ON(!pendowner->pi_blocked_on);
@@ -577,12 +584,9 @@ static void wakeup_next_waiter(struct rt_mutex *lock, int savestate)
 
 	pendowner->pi_blocked_on = NULL;
 
-	if (rt_mutex_has_waiters(lock)) {
-		struct rt_mutex_waiter *next;
-
-		next = rt_mutex_top_waiter(lock);
+	if (next)
 		plist_add(&next->pi_list_entry, &pendowner->pi_waiters);
-	}
+
 	atomic_spin_unlock(&pendowner->pi_lock);
 }