linux-kernel - [PATCH tip/locking/core v10 3/7] locking/qspinlock: Avoid redundant read of next pointer

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Message-Id: <1447114167-47185-4-git-send-email-Waiman.Long@hpe.com>
Date:	Mon,  9 Nov 2015 19:09:23 -0500
From:	Waiman Long <Waiman.Long@....com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Waiman Long <Waiman.Long@....com>
Subject: [PATCH tip/locking/core v10 3/7] 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@....com>
---
 kernel/locking/qspinlock.c |    9 ++++++---
 1 files 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);
-- 
1.7.1

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