From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755109AbdDMRkC (ORCPT ); Thu, 13 Apr 2017 13:40:02 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:58059 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752537AbdDMRj7 (ORCPT ); Thu, 13 Apr 2017 13:39:59 -0400 Date: Thu, 13 Apr 2017 10:39:51 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com, dvyukov@google.com, will.deacon@arm.com Subject: Re: [PATCH tip/core/rcu 07/13] rcu: Add smp_mb__after_atomic() to sync_exp_work_done() Reply-To: paulmck@linux.vnet.ibm.com References: <20170412165441.GA17149@linux.vnet.ibm.com> <1492016149-18834-7-git-send-email-paulmck@linux.vnet.ibm.com> <20170413091832.phnfppqjjy6sislo@hirez.programming.kicks-ass.net> <20170413161042.GA3956@linux.vnet.ibm.com> <20170413162409.q5gsqfytjyirgfep@hirez.programming.kicks-ass.net> <20170413165755.GJ3956@linux.vnet.ibm.com> <20170413171027.snjqn4u54t2kdzgx@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170413171027.snjqn4u54t2kdzgx@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17041317-0008-0000-0000-000001F4328F X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00006930; HX=3.00000240; KW=3.00000007; PH=3.00000004; SC=3.00000208; SDB=6.00847083; UDB=6.00417896; IPR=6.00625517; BA=6.00005288; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00015033; XFM=3.00000013; UTC=2017-04-13 17:39:56 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17041317-0009-0000-0000-0000349F1A21 Message-Id: <20170413173951.GM3956@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-04-13_14:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=0 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1702020001 definitions=main-1704130148 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 13, 2017 at 07:10:27PM +0200, Peter Zijlstra wrote: > On Thu, Apr 13, 2017 at 09:57:55AM -0700, Paul E. McKenney wrote: > > On Thu, Apr 13, 2017 at 06:24:09PM +0200, Peter Zijlstra wrote: > > > On Thu, Apr 13, 2017 at 09:10:42AM -0700, Paul E. McKenney wrote: > > > > On Thu, Apr 13, 2017 at 11:18:32AM +0200, Peter Zijlstra wrote: > > > > > On Wed, Apr 12, 2017 at 09:55:43AM -0700, Paul E. McKenney wrote: > > > > > > However, a little future-proofing is a good thing, > > > > > > especially given that smp_mb__before_atomic() is only required to > > > > > > provide acquire semantics rather than full ordering. This commit > > > > > > therefore adds smp_mb__after_atomic() after the atomic_long_inc() > > > > > > in sync_exp_work_done(). > > > > > > > > > > Oh!? As far as I'm away the smp_mb__{before,after}_atomic() really must > > > > > provide full MB, no confusion about that. > > > > > > > > > > We have other primitives for acquire/release. > > > > > > > > Hmmm... Rechecking atomic_ops.txt, it does appear that you are quite > > > > correct. Adding Will and Dmitry on CC, but dropping this patch for now. > > > > > > I'm afraid that document is woefully out dated. I'm surprised it says > > > anything on the subject. > > > > And there is some difference of opinion. Some believe that the > > smp_mb__before_atomic() only guarantees acquire and smp_mb__after_atomic() > > only guarantees release, but all current architectures provide full > > ordering, as you noted and as stated in atomic_ops.txt. > > Which 'some' think it only provides acquire/release ? > > I made very sure -- when I renamed/audited/wrote all this -- that they > indeed do a full memory barrier. > > > How do we decide? > > I say its a full mb, always was. > > People used it to create acquire/release _like_ constructs, because we > simply didn't have anything else. > > Also, I think Linus once opined that acquire/release is part of a > store/load (hence smp_store_release/smp_load_acquire) and not a barrier. > > > Once we do decide, atomic_ops.txt of course needs to be updated accordingly. > > There was so much missing there that I didn't quite know where to start. Well, if there are no objections, I will fix up the smp_mb__before_atomic() and smp_mb__after_atomic() pieces. I suppose that one alternative is the new variant of kerneldoc, though very few of these functions have comment headers, let alone kerneldoc headers. Which reminds me, the question of spin_unlock_wait() and spin_is_locked() semantics came up a bit ago. Here is what I believe to be the case. Does this match others' expectations? o spin_unlock_wait() semantics: 1. Any access in any critical section prior to the spin_unlock_wait() is visible to all code following (in program order) the spin_unlock_wait(). 2. Any access prior (in program order) to the spin_unlock_wait() is visible to any critical section following the spin_unlock_wait(). o spin_is_locked() semantics: Half of spin_unlock_wait(), but only if it returns false: 1. Any access in any critical section prior to the spin_unlock_wait() is visible to all code following (in program order) the spin_unlock_wait(). Thanx, Paul