Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753314AbcCJQFE (ORCPT ); Thu, 10 Mar 2016 11:05:04 -0500 Received: from mail-wm0-f42.google.com ([74.125.82.42]:38883 "EHLO mail-wm0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752574AbcCJQEy (ORCPT ); Thu, 10 Mar 2016 11:04:54 -0500 Subject: Re: [PATCH] KVM: Remove redundant smp_mb() in the kvm_mmu_commit_zap_page() To: Xiao Guangrong , Lan Tianyu , Thomas Gleixner References: <1457055312-27067-1-git-send-email-tianyu.lan@intel.com> <56D9354E.9040908@intel.com> <56D94BFE.1080406@redhat.com> <56DE8F1A.9000401@intel.com> <56DEEF69.20208@redhat.com> <56E1875E.8060007@linux.intel.com> <56E19224.9090906@redhat.com> <56E1933E.20909@redhat.com> <56E196AC.4030103@linux.intel.com> Cc: gleb@kernel.org, mingo@redhat.com, hpa@zytor.com, x86@kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org From: Paolo Bonzini Message-ID: <56E19B21.8090501@redhat.com> Date: Thu, 10 Mar 2016 17:04:49 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0 MIME-Version: 1.0 In-Reply-To: <56E196AC.4030103@linux.intel.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1019 Lines: 33 On 10/03/2016 16:45, Xiao Guangrong wrote: >> >>> Compared to smp_load_acquire(), smp_mb() adds an ordering between stores >>> and loads. >> >> Here, the ordering is load-store, hence... > > Yes, this is why i put smp_mb() in the code. :) Here is a table of barriers: '. after| | before '. | load | store __________'.|___________________|________________________ | | | smp_rmb | smp_load_acquire load | smp_load_acquire | smp_store_release XX | smp_mb | smp_mb ____________|___________________|________________________ | | | | smp_wmb store | smp_mb | smp_store_release | | smp_mb | | Your case is the one marked with XX, so a smp_load_acquire() is enough---and it's preferrable, because it's cheaper than smp_mb() and more self-documenting. Paolo