From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752514AbdK2W7d (ORCPT ); Wed, 29 Nov 2017 17:59:33 -0500 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:44664 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752071AbdK2W7b (ORCPT ); Wed, 29 Nov 2017 17:59:31 -0500 Date: Wed, 29 Nov 2017 14:59:26 -0800 From: "Paul E. McKenney" To: Daniel Lustig Cc: Alan Stern , 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: <20171129194602.6zmjj7z5ih4ri25h@hirez.programming.kicks-ass.net> <20171129204220.GH3624@linux.vnet.ibm.com> <6b068a40-75bb-4152-b1ec-9ef3beacbdd5@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <6b068a40-75bb-4152-b1ec-9ef3beacbdd5@nvidia.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17112922-0044-0000-0000-000003B72B39 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00008124; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000242; SDB=6.00953102; UDB=6.00481547; IPR=6.00733181; BA=6.00005721; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00018258; XFM=3.00000015; UTC=2017-11-29 22:59:29 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17112922-0045-0000-0000-000007E65876 Message-Id: <20171129225926.GK3624@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-11-29_08:,, 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-1711290297 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Nov 29, 2017 at 02:18:48PM -0800, 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. Well, C++ release sequences will likely soon not order memory_order_consume loads: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0735r0.html So we were hoping to avoid release sequences entirely. But if someone in the kernel really is using them, we will have to model them, but only those interacting with acquire loads. > > 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 :) Read P0735R0 (the above URL) and then tell me with a straight face that you would not also have been tempted. ;-) Thanx, Paul