From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753169AbdK3QOJ (ORCPT ); Thu, 30 Nov 2017 11:14:09 -0500 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:43646 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751992AbdK3QOI (ORCPT ); Thu, 30 Nov 2017 11:14:08 -0500 Date: Thu, 30 Nov 2017 08:14:01 -0800 From: "Paul E. McKenney" To: Alan Stern Cc: Daniel Lustig , Peter Zijlstra , Andrea Parri , Luc Maranget , Jade Alglave , Boqun Feng , Nicholas Piggin , Will Deacon , David Howells , Palmer Dabbelt , Kernel development list Subject: Re: Unlock-lock questions and the Linux Kernel Memory Model Reply-To: paulmck@linux.vnet.ibm.com References: <6b068a40-75bb-4152-b1ec-9ef3beacbdd5@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17113016-2213-0000-0000-00000243B535 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00008128; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000242; SDB=6.00953447; UDB=6.00481754; IPR=6.00733522; BA=6.00005722; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00018269; XFM=3.00000015; UTC=2017-11-30 16:14:05 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17113016-2214-0000-0000-0000584F6B1B Message-Id: <20171130161401.GP3624@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-11-30_05:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=2 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-1711300212 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Nov 30, 2017 at 10:20:02AM -0500, Alan Stern wrote: > On Wed, 29 Nov 2017, Daniel Lustig wrote: > > > On 11/29/2017 12:42 PM, Paul E. McKenney wrote: > > > On Wed, Nov 29, 2017 at 02:53:06PM -0500, Alan Stern wrote: > > >> On Wed, 29 Nov 2017, Peter Zijlstra wrote: > > >> > > >>> On Wed, Nov 29, 2017 at 11:04:53AM -0800, Daniel Lustig wrote: > > >>> > > >>>> While we're here, let me ask about another test which isn't directly > > >>>> about unlock/lock but which is still somewhat related to this > > >>>> discussion: > > >>>> > > >>>> "MP+wmb+xchg-acq" (or some such) > > >>>> > > >>>> {} > > >>>> > > >>>> P0(int *x, int *y) > > >>>> { > > >>>> WRITE_ONCE(*x, 1); > > >>>> smp_wmb(); > > >>>> WRITE_ONCE(*y, 1); > > >>>> } > > >>>> > > >>>> P1(int *x, int *y) > > >>>> { > > >>>> r1 = atomic_xchg_relaxed(y, 2); > > >>>> r2 = smp_load_acquire(y); > > >>>> r3 = READ_ONCE(*x); > > >>>> } > > >>>> > > >>>> exists (1:r1=1 /\ 1:r2=2 /\ 1:r3=0) > > >>>> > > >>>> C/C++ would call the atomic_xchg_relaxed part of a release sequence > > >>>> and hence would forbid this outcome. > > >>> > > >>> That's just weird. Either its _relaxed, or its _release. Making _relaxed > > >>> mean _release is just daft. > > >> > > >> The C11 memory model specifically allows atomic operations to be > > >> interspersed within a release sequence. But it doesn't say why. > > > > > > The use case put forward within the committee is for atomic quantities > > > with mode bits. The most frequent has the atomic quantity having > > > lock-like properties, in which case you don't want to lose the ordering > > > effects of the lock handoff just because a mode bit got set or cleared. > > > Some claim to actually use something like this, but details have not > > > been forthcoming. > > > > > > I confess to being a bit skeptical. If the mode changes are infrequent, > > > the update could just as well be ordered. > > > > Aren't reference counting implementations which use memory_order_relaxed > > for incrementing the count another important use case? Specifically, > > the synchronization between a memory_order_release decrement and the > > eventual memory_order_acquire/consume free shouldn't be interrupted by > > other (relaxed) increments and (release-only) decrements that happen in > > between. At least that's my understanding of this use case. I wasn't > > there when the C/C++ committee decided this. > > > > > That said, Daniel, the C++ memory model really does require that the > > > above litmus test be forbidden, my denigration of it notwithstanding. > > > > Yes I agree, that's why I'm curious what the Linux memory model has > > in mind here :) > > Bear in mind that the litmus test above uses xchg, not increment or > decrement. This makes a difference as far as the LKMM is concerned, > even if not for C/C++. Finally remembering this discussion... Yes, xchg is special. ;-) Will, are there plans to bring this sort of thing before the standards committee? > (Also, technically speaking, the litmus test doesn't have any release > operations, so no release sequence...) True! But if you translated it into C11, you would probably turn the smp_wmb() followed by write into a store release, which would get you a release sequence. Thanx, Paul