linux-kernel - [tip:locking/core] locking/qspinlock: Use __this_cpu_dec() instead of full-blown this_cpu

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Date:	Mon, 27 Jun 2016 05:56:29 -0700
From:	tip-bot for Pan Xinhui <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	mingo@...nel.org, torvalds@...ux-foundation.org, efault@....de,
	hpa@...or.com, peterz@...radead.org, tglx@...utronix.de,
	xinhui.pan@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org
Subject: [tip:locking/core] locking/qspinlock: Use __this_cpu_dec() instead
 of full-blown this_cpu_dec()

Commit-ID:  0dceeaf599e6d9b8bd908ba4bd3dfee84aa26be2
Gitweb:     http://git.kernel.org/tip/0dceeaf599e6d9b8bd908ba4bd3dfee84aa26be2
Author:     Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>
AuthorDate: Tue, 14 Jun 2016 14:37:27 +0800
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Mon, 27 Jun 2016 11:37:41 +0200

locking/qspinlock: Use __this_cpu_dec() instead of full-blown this_cpu_dec()

queued_spin_lock_slowpath() should not worry about another
queued_spin_lock_slowpath() running in interrupt context and
changing node->count by accident, because node->count keeps
the same value every time we enter/leave queued_spin_lock_slowpath().

On some architectures this_cpu_dec() will save/restore irq flags,
which has high overhead. Use the much cheaper __this_cpu_dec() instead.

Signed-off-by: Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Mike Galbraith <efault@....de>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Waiman.Long@....com
Link: http://lkml.kernel.org/r/1465886247-3773-1-git-send-email-xinhui.pan@linux.vnet.ibm.com
[ Rewrote changelog. ]
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/locking/qspinlock.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 7306555..b2caec7 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -619,7 +619,7 @@ release:
 	/*
 	 * release the node
 	 */
-	this_cpu_dec(mcs_nodes[0].count);
+	__this_cpu_dec(mcs_nodes[0].count);
 }
 EXPORT_SYMBOL(queued_spin_lock_slowpath);