From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753887AbdDMQ6G (ORCPT ); Thu, 13 Apr 2017 12:58:06 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:55842 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751678AbdDMQ6D (ORCPT ); Thu, 13 Apr 2017 12:58:03 -0400 Date: Thu, 13 Apr 2017 09:57:55 -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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170413162409.q5gsqfytjyirgfep@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17041316-0056-0000-0000-0000033C0FF7 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00006930; HX=3.00000240; KW=3.00000007; PH=3.00000004; SC=3.00000208; SDB=6.00847069; UDB=6.00417887; IPR=6.00625503; 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 16:57:59 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17041316-0057-0000-0000-000007721B76 Message-Id: <20170413165755.GJ3956@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-04-13_11:,, 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-1704130142 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. How do we decide? Once we do decide, atomic_ops.txt of course needs to be updated accordingly. Thanx, Paul