From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_NEOMUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 945E4C04AB5 for ; Thu, 6 Jun 2019 09:29:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 69C3B20684 for ; Thu, 6 Jun 2019 09:29:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727862AbfFFJ3N (ORCPT ); Thu, 6 Jun 2019 05:29:13 -0400 Received: from helcar.hmeau.com ([216.24.177.18]:46636 "EHLO deadmen.hmeau.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727833AbfFFJ3N (ORCPT ); Thu, 6 Jun 2019 05:29:13 -0400 Received: from gondobar.mordor.me.apana.org.au ([192.168.128.4] helo=gondobar) by deadmen.hmeau.com with esmtps (Exim 4.89 #2 (Debian)) id 1hYohm-0001wm-23; Thu, 06 Jun 2019 17:29:06 +0800 Received: from herbert by gondobar with local (Exim 4.89) (envelope-from ) id 1hYohb-000735-8z; Thu, 06 Jun 2019 17:28:55 +0800 Date: Thu, 6 Jun 2019 17:28:55 +0800 From: Herbert Xu To: "Paul E. McKenney" Cc: Alan Stern , Boqun Feng , Linus Torvalds , Frederic Weisbecker , Fengguang Wu , LKP , LKML , Netdev , "David S. Miller" , Andrea Parri , Luc Maranget , Jade Alglave Subject: Re: rcu_read_lock lost its compiler barrier Message-ID: <20190606092855.dfeuvyk5lbvm4zbf@gondor.apana.org.au> References: <20190603200301.GM28207@linux.ibm.com> <20190606045109.zjfxxbkzq4wb64bj@gondor.apana.org.au> <20190606060511.GA28207@linux.ibm.com> <20190606061438.nyzaeppdbqjt3jbp@gondor.apana.org.au> <20190606090619.GC28207@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190606090619.GC28207@linux.ibm.com> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 06, 2019 at 02:06:19AM -0700, Paul E. McKenney wrote: > > Or is your point instead that given the initial value of "a" being > zero and the value stored to "a" being one, there is no way that > any possible load and store tearing (your slicing and dicing) could > possibly mess up the test of the value loaded from "a"? Exactly. If you can dream up of a scenario where the compiler can get this wrong I'm all ears. > > But I do concede that in the general RCU case you must have the > > READ_ONCE/WRITE_ONCE calls for rcu_dereference/rcu_assign_pointer. > > OK, good that we are in agreement on this part, at least! ;-) Well only because we're allowing crazy compilers that can turn a simple word-aligned word assignment (a = b) into two stores. Cheers, -- Email: Herbert Xu Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt From mboxrd@z Thu Jan 1 00:00:00 1970 Content-Type: multipart/mixed; boundary="===============2713219447130643993==" MIME-Version: 1.0 From: Herbert Xu To: lkp@lists.01.org Subject: Re: rcu_read_lock lost its compiler barrier Date: Thu, 06 Jun 2019 17:28:55 +0800 Message-ID: <20190606092855.dfeuvyk5lbvm4zbf@gondor.apana.org.au> In-Reply-To: <20190606090619.GC28207@linux.ibm.com> List-Id: --===============2713219447130643993== Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable On Thu, Jun 06, 2019 at 02:06:19AM -0700, Paul E. McKenney wrote: > > Or is your point instead that given the initial value of "a" being > zero and the value stored to "a" being one, there is no way that > any possible load and store tearing (your slicing and dicing) could > possibly mess up the test of the value loaded from "a"? Exactly. If you can dream up of a scenario where the compiler can get this wrong I'm all ears. > > But I do concede that in the general RCU case you must have the > > READ_ONCE/WRITE_ONCE calls for rcu_dereference/rcu_assign_pointer. > = > OK, good that we are in agreement on this part, at least! ;-) Well only because we're allowing crazy compilers that can turn a simple word-aligned word assignment (a =3D b) into two stores. Cheers, -- = Email: Herbert Xu Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt --===============2713219447130643993==--