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_PASS,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 2F693C43381 for ; Mon, 1 Apr 2019 15:44:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0977F2133D for ; Mon, 1 Apr 2019 15:44:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728648AbfDAPoR (ORCPT ); Mon, 1 Apr 2019 11:44:17 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:52908 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726887AbfDAPoR (ORCPT ); Mon, 1 Apr 2019 11:44:17 -0400 Received: from pps.filterd (m0098410.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.27/8.16.0.27) with SMTP id x31Fi0jQ119450 for ; Mon, 1 Apr 2019 11:44:16 -0400 Received: from e15.ny.us.ibm.com (e15.ny.us.ibm.com [129.33.205.205]) by mx0a-001b2d01.pphosted.com with ESMTP id 2rkmeg4ec8-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Mon, 01 Apr 2019 11:44:16 -0400 Received: from localhost by e15.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 1 Apr 2019 16:44:15 +0100 Received: from b01cxnp22035.gho.pok.ibm.com (9.57.198.25) by e15.ny.us.ibm.com (146.89.104.202) with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Mon, 1 Apr 2019 16:44:11 +0100 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22035.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x31FiAfP15794426 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 1 Apr 2019 15:44:10 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id A3FEAB206C; Mon, 1 Apr 2019 15:44:10 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 86B02B205F; Mon, 1 Apr 2019 15:44:10 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.188]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Mon, 1 Apr 2019 15:44:10 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 9B85516C1B98; Mon, 1 Apr 2019 08:44:14 -0700 (PDT) Date: Mon, 1 Apr 2019 08:44:14 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: "H. Peter Anvin" , Alexander Potapenko , Ingo Molnar , LKML , Dmitriy Vyukov , James Y Knight Subject: Re: Potentially missing "memory" clobbers in bitops.h for x86 Reply-To: paulmck@linux.ibm.com References: <20190328162222.GO4102@linux.ibm.com> <8e32ab34-c14c-1ccb-76f9-0dcd729a0ef6@zytor.com> <20190329210918.GZ4102@linux.ibm.com> <8092b8aa-bb1c-0266-b308-5cebfb25e2ef@zytor.com> <20190329220554.GD4102@linux.ibm.com> <20190401105348.GL11158@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190401105348.GL11158@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19040115-0068-0000-0000-000003AE4E8A X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010855; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000283; SDB=6.01182835; UDB=6.00619206; IPR=6.00963577; MB=3.00026247; MTD=3.00000008; XFM=3.00000015; UTC=2019-04-01 15:44:13 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19040115-0069-0000-0000-000048017604 Message-Id: <20190401154414.GM4102@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-04-01_05:,, 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-1904010105 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Apr 01, 2019 at 12:53:48PM +0200, Peter Zijlstra wrote: > On Fri, Mar 29, 2019 at 03:05:54PM -0700, Paul E. McKenney wrote: > > On Fri, Mar 29, 2019 at 02:51:26PM -0700, H. Peter Anvin wrote: > > > On 3/29/19 2:09 PM, Paul E. McKenney wrote: > > > >> > > > >> Note: the atomic versions of these functions obviously need to have > > > >> "volatile" and the clobber anyway, as they are by definition barriers > > > >> and moving memory operations around them would be a very serious error. > > > > > > > > The atomic functions that return void don't need to order anything except > > > > the input and output arguments. The oddness with clear_bit() is that the > > > > memory changed isn't necessarily the quantity referenced by the argument, > > > > if the number of bits specified is large. > > > > > > > > So (for example) atomic_inc() does not need a "memory" clobber, right? > > Correct, and many implementations do not, including x86: > > static __always_inline void arch_atomic_inc(atomic_t *v) > { > asm volatile(LOCK_PREFIX "incl %0" > : "+m" (v->counter)); > } Very good! > > > I don't believe that is true: the code calling it has a reasonable > > > expectation that previous memory operations have finished and later > > > memory operations have not started from the point of view of another > > > processor. You are more of an expert on memory ordering than I am, but > > > I'm 89% sure that there is plenty of code in the kernel which makes that > > > assumption. > > > > From Documentation/core-api/atomic_ops.rst: > > We should delete that file. Only if all of its content is fully present elsewhere. ;-) Thanx, Paul