linux-kernel - Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and locking code into its own file

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Date:	Fri, 27 Sep 2013 09:12:03 -0700
From:	Jason Low <jason.low2@...com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Tim Chen <tim.c.chen@...ux.intel.com>,
	Davidlohr Bueso <davidlohr@...com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Alex Shi <alex.shi@...aro.org>,
	Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Matthew R Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	linux-kernel@...r.kernel.org, linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and
 locking code into its own file

On Fri, 2013-09-27 at 08:02 +0200, Ingo Molnar wrote:
> Would be nice to have this as a separate, add-on patch. Every single 
> instruction removal that has no downside is an upside!

Okay, so here is a patch. Tim, would you like to add this to v7?

...
Subject: MCS lock: Remove and reorder unnecessary assignments in mcs_spin_lock()

In mcs_spin_lock(), if (likely(prev == NULL)) is true, then the lock is free
and we won't spin on the local node. In that case, we don't have to assign
node->locked because it won't be used. We can also move the node->locked = 0
assignment so that it occurs after the if (likely(prev == NULL)) check.

This might also help make it clearer as to how the node->locked variable
is used in MCS locks.

Signed-off-by: Jason Low <jason.low2@...com>
---
 include/linux/mcslock.h |    3 +--
 1 files changed, 1 insertions(+), 2 deletions(-)

diff --git a/include/linux/mcslock.h b/include/linux/mcslock.h
index 20fd3f0..1167d57 100644
--- a/include/linux/mcslock.h
+++ b/include/linux/mcslock.h
@@ -21,15 +21,14 @@ void mcs_spin_lock(struct mcs_spin_node **lock, struct mcs_spin_node *node)
 	struct mcs_spin_node *prev;
 
 	/* Init node */
-	node->locked = 0;
 	node->next   = NULL;
 
 	prev = xchg(lock, node);
 	if (likely(prev == NULL)) {
 		/* Lock acquired */
-		node->locked = 1;
 		return;
 	}
+	node->locked = 0;
 	ACCESS_ONCE(prev->next) = node;
 	smp_wmb();
 	/* Wait until the lock holder passes the lock down */
-- 
1.7.1


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