[tip:locking/core] locking/qspinlock: Avoid redundant read of next pointer

[tip-bot for Waiman Long <tipbot@zytor.com>]

Commit-ID:  aa68744f80bfb6f26fbe7f10e42876066f7dac1b
Gitweb:     http://git.kernel.org/tip/aa68744f80bfb6f26fbe7f10e42876066f7dac1b
Author:     Waiman Long <Waiman.Long@hpe.com>
AuthorDate: Mon, 9 Nov 2015 19:09:23 -0500
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 23 Nov 2015 10:02:00 +0100

locking/qspinlock: Avoid redundant read of next pointer

With optimistic prefetch of the next node cacheline, the next pointer
may have been properly inititalized. As a result, the reading
of node->next in the contended path may be redundant. This patch
eliminates the redundant read if the next pointer value is not NULL.

Signed-off-by: Waiman Long <Waiman.Long@hpe.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Douglas Hatch <doug.hatch@hpe.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Scott J Norton <scott.norton@hpe.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1447114167-47185-4-git-send-email-Waiman.Long@hpe.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/locking/qspinlock.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 365b203..9862078 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -396,6 +396,7 @@ queue:
 	 * p,*,* -> n,*,*
 	 */
 	old = xchg_tail(lock, tail);
+	next = NULL;
 
 	/*
 	 * if there was a previous node; link it and wait until reaching the
@@ -463,10 +464,12 @@ queue:
 	}
 
 	/*
-	 * contended path; wait for next, release.
+	 * contended path; wait for next if not observed yet, release.
 	 */
-	while (!(next = READ_ONCE(node->next)))
-		cpu_relax();
+	if (!next) {
+		while (!(next = READ_ONCE(node->next)))
+			cpu_relax();
+	}
 
 	arch_mcs_spin_unlock_contended(&next->locked);
 	pv_kick_node(lock, next);