From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f49.google.com (mail-pa0-f49.google.com [209.85.220.49]) by kanga.kvack.org (Postfix) with ESMTP id A02F66B003A for ; Fri, 27 Sep 2013 12:12:12 -0400 (EDT) Received: by mail-pa0-f49.google.com with SMTP id ld10so2984537pab.22 for ; Fri, 27 Sep 2013 09:12:12 -0700 (PDT) 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 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" Content-Transfer-Encoding: 7bit Mime-Version: 1.0 Sender: owner-linux-mm@kvack.org List-ID: 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 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, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org