Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754102Ab3I0QMN (ORCPT ); Fri, 27 Sep 2013 12:12:13 -0400 Received: from g5t0006.atlanta.hp.com ([15.192.0.43]:16581 "EHLO g5t0006.atlanta.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753599Ab3I0QMJ (ORCPT ); Fri, 27 Sep 2013 12:12:09 -0400 Message-ID: <1380298323.2031.13.camel@j-VirtualBox> Subject: Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and locking code into its own file From: Jason Low To: Ingo Molnar Cc: Tim Chen , Davidlohr Bueso , Ingo Molnar , Andrew Morton , Andrea Arcangeli , Alex Shi , Andi Kleen , Michel Lespinasse , Davidlohr Bueso , Matthew R Wilcox , Dave Hansen , Peter Zijlstra , Rik van Riel , Peter Hurley , linux-kernel@vger.kernel.org, linux-mm Date: Fri, 27 Sep 2013 09:12:03 -0700 In-Reply-To: <20130927060213.GA6673@gmail.com> References: <1380147049.3467.67.camel@schen9-DESK> <1380226007.2170.2.camel@buesod1.americas.hpqcorp.net> <1380226997.2602.11.camel@j-VirtualBox> <1380228059.2170.10.camel@buesod1.americas.hpqcorp.net> <1380229794.2602.36.camel@j-VirtualBox> <1380231702.3467.85.camel@schen9-DESK> <1380235333.3229.39.camel@j-VirtualBox> <1380236265.3467.103.camel@schen9-DESK> <20130927060213.GA6673@gmail.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.2.3-0ubuntu6 Content-Transfer-Encoding: 7bit Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1710 Lines: 52 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 --- 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/