From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751895AbeBTOtc (ORCPT ); Tue, 20 Feb 2018 09:49:32 -0500 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:55810 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751000AbeBTOtb (ORCPT ); Tue, 20 Feb 2018 09:49:31 -0500 Date: Tue, 20 Feb 2018 06:49:39 -0800 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Alan Stern , Andrea Parri , Akira Yokosawa , Kernel development list , mingo@kernel.org, Will Deacon , boqun.feng@gmail.com, npiggin@gmail.com, dhowells@redhat.com, Jade Alglave , Luc Maranget , Patrick Bellasi Subject: Re: [PATCH] tools/memory-model: remove rb-dep, smp_read_barrier_depends, and lockless_dereference Reply-To: paulmck@linux.vnet.ibm.com References: <20180217151413.GA3785@andrea> <20180219194123.GZ3617@linux.vnet.ibm.com> <20180219202844.GW25201@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180219202844.GW25201@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18022014-0008-0000-0000-000002D83572 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00008563; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000254; SDB=6.00992513; UDB=6.00504240; IPR=6.00771859; MB=3.00019656; MTD=3.00000008; XFM=3.00000015; UTC=2018-02-20 14:49:20 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18022014-0009-0000-0000-00003853C93F Message-Id: <20180220144939.GG3617@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2018-02-20_06:,, 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 impostorscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1709140000 definitions=main-1802200189 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Feb 19, 2018 at 09:28:44PM +0100, Peter Zijlstra wrote: > On Mon, Feb 19, 2018 at 11:41:23AM -0800, Paul E. McKenney wrote: > > On Mon, Feb 19, 2018 at 12:14:45PM -0500, Alan Stern wrote: > > > This leaves us with a question: Do we want to change the kernel by > > > adding memory barriers after unsuccessful RMW operations on Alpha, or > > > do we want to change the model by excluding such operations from > > > address dependencies? > > > > I vote for adding the barrier on Alpha. However, I don't know of any > > code in the Linux kernel that relies on read-to-read address dependency > > ordering headed by a failing RMW operation, so I don't feel all that > > strongly about this. > > Right, but not knowing doesn't mean doesn't exist, and most certainly > doesn't mean will never exist. Fair enough, safety first! > > > Note that operations like atomic_add_unless() already include memory > > > barriers. > > > > And I don't see an atomic_add_unless_relaxed(), so we are good on this > > one. So far, anyway! ;-) > > Not the point, add_unless() is a conditional operation, and therefore > doesn't need to imply anything when failing. Plus it doesn't return a pointer, so there is no problem with dereferences. Unless someone wants to use its return value as an array index and rely on dependency ordering to the array, but I would NAK that use case. Thanx, Paul