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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_MUTT 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 9B389C04AB5 for ; Thu, 6 Jun 2019 13:49:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7AA182070B for ; Thu, 6 Jun 2019 13:49:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728456AbfFFNtE (ORCPT ); Thu, 6 Jun 2019 09:49:04 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:51656 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726877AbfFFNtE (ORCPT ); Thu, 6 Jun 2019 09:49:04 -0400 Received: from pps.filterd (m0098396.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.27/8.16.0.27) with SMTP id x56DgIo4068662 for ; Thu, 6 Jun 2019 09:49:03 -0400 Received: from e14.ny.us.ibm.com (e14.ny.us.ibm.com [129.33.205.204]) by mx0a-001b2d01.pphosted.com with ESMTP id 2sy1yypyrq-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Thu, 06 Jun 2019 09:49:02 -0400 Received: from localhost by e14.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! 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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Thu, 6 Jun 2019 14:48:58 +0100 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23033.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x56DmvcR37683528 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Thu, 6 Jun 2019 13:48:57 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 1E4DCB2067; Thu, 6 Jun 2019 13:48:57 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id E0D3AB2064; Thu, 6 Jun 2019 13:48:56 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.80.209.205]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Thu, 6 Jun 2019 13:48:56 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 6315916C3783; Thu, 6 Jun 2019 06:48:56 -0700 (PDT) Date: Thu, 6 Jun 2019 06:48:56 -0700 From: "Paul E. McKenney" To: Herbert Xu 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 Reply-To: paulmck@linux.ibm.com 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> <20190606133824.aibysezb5qdo3x27@gondor.apana.org.au> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190606133824.aibysezb5qdo3x27@gondor.apana.org.au> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19060613-0052-0000-0000-000003CC3C7F X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00011223; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000286; SDB=6.01214073; UDB=6.00638158; IPR=6.00995153; MB=3.00027206; MTD=3.00000008; XFM=3.00000015; UTC=2019-06-06 13:49:01 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19060613-0053-0000-0000-00006135265B Message-Id: <20190606134856.GL28207@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-06-06_10:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1906060099 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 09:38:24PM +0800, Herbert Xu wrote: > 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. I disagree. Given a volatile store, the compiler cannot assume that the stored-to location is normal memory at that point in time, and therefore cannot assume that it is safe to invent a store to that location (as shown above). Thus far, the C++ standards committee seems on-board with this, though time will tell. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1382r1.pdf Thanx, Paul