From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752132AbeDFLeY (ORCPT ); Fri, 6 Apr 2018 07:34:24 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:35432 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751540AbeDFLeX (ORCPT ); Fri, 6 Apr 2018 07:34:23 -0400 Date: Fri, 6 Apr 2018 12:34:36 +0100 From: Will Deacon To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, mingo@kernel.org, boqun.feng@gmail.com, paulmck@linux.vnet.ibm.com, catalin.marinas@arm.com, Waiman Long Subject: Re: [PATCH 10/10] locking/qspinlock: Elide back-to-back RELEASE operations with smp_wmb() Message-ID: <20180406113436.GC27619@arm.com> References: <1522947547-24081-1-git-send-email-will.deacon@arm.com> <1522947547-24081-11-git-send-email-will.deacon@arm.com> <20180405172808.GG4082@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180405172808.GG4082@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 05, 2018 at 07:28:08PM +0200, Peter Zijlstra wrote: > On Thu, Apr 05, 2018 at 05:59:07PM +0100, Will Deacon wrote: > > @@ -340,12 +341,17 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val) > > goto release; > > > > /* > > + * Ensure that the initialisation of @node is complete before we > > + * publish the updated tail and potentially link @node into the > > + * waitqueue. > > + */ > > + smp_wmb(); > > Maybe an explicit note to where the matching barrier lives.. Oh man, that's not a simple thing to write: there isn't a matching barrier! Instead, we rely on dependency ordering for two cases: * We access a node by decoding the tail we get back from the xchg - or - * We access a node by following our own ->next pointer I could say something like: "Pairs with dependency ordering from both xchg_tail and explicit dereferences of node->next" but it's a bit cryptic :( Will