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,URIBL_BLOCKED,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 77E00C04AB5 for ; Thu, 6 Jun 2019 13:38:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 52F4820868 for ; Thu, 6 Jun 2019 13:38:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728280AbfFFNim (ORCPT ); Thu, 6 Jun 2019 09:38:42 -0400 Received: from helcar.hmeau.com ([216.24.177.18]:40800 "EHLO deadmen.hmeau.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726014AbfFFNik (ORCPT ); Thu, 6 Jun 2019 09:38:40 -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 1hYsbB-0006rm-BN; Thu, 06 Jun 2019 21:38:33 +0800 Received: from herbert by gondobar with local (Exim 4.89) (envelope-from ) id 1hYsb2-0007Ot-8l; Thu, 06 Jun 2019 21:38:24 +0800 Date: Thu, 6 Jun 2019 21:38:24 +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: <20190606133824.aibysezb5qdo3x27@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> <20190606092855.dfeuvyk5lbvm4zbf@gondor.apana.org.au> <20190606105817.GE28207@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190606105817.GE28207@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 03:58:17AM -0700, Paul E. McKenney wrote: > > I cannot immediately think of a way that the compiler could get this > wrong even in theory, but similar code sequences can be messed up. > The reason for this is that in theory, the compiler could use the > stored-to location as temporary storage, like this: > > a = whatever; // Compiler uses "a" as a temporary > do_something(); > whatever = a; > a = 1; // Intended store Well if the compiler is going to do this then surely it would continue to do this even if you used WRITE_ONCE. Remember a is not volatile, only the access of a through WRITE_ONCE is volatile. Cheers, -- Email: Herbert Xu Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt