[(RT RFC) PATCH v2 8/9] adjust pi_lock usage in wakeup

[Gregory Haskins <ghaskins@novell.com>]

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.

This patch adds a measureable increase in throughput.

Signed-off-by: Peter W. Morreale <pmorreale@novell.com>
---

 kernel/rtmutex.c |   14 +++++++++-----
 1 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
index ea593e0..b81bbef 100644
--- a/kernel/rtmutex.c
+++ b/kernel/rtmutex.c
@@ -526,6 +526,7 @@ 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;
 
 	spin_lock(&current->pi_lock);
 
@@ -587,6 +588,12 @@ 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;
+
 	spin_lock(&pendowner->pi_lock);
 
 	WARN_ON(!pendowner->pi_blocked_on);
@@ -595,12 +602,9 @@ 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);
-	}
+
 	spin_unlock(&pendowner->pi_lock);
 }