From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S966043AbdDSXZI (ORCPT ); Wed, 19 Apr 2017 19:25:08 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:56232 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965976AbdDSXZC (ORCPT ); Wed, 19 Apr 2017 19:25:02 -0400 Date: Wed, 19 Apr 2017 16:24:57 -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> <20170413173951.GM3956@linux.vnet.ibm.com> <20170413175136.5qnzvqrmzyuvlqsj@hirez.programming.kicks-ass.net> <20170413175907.hcrlh2z55zivs4v4@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170413175907.hcrlh2z55zivs4v4@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17041923-0048-0000-0000-000001623F7F X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00006941; HX=3.00000240; KW=3.00000007; PH=3.00000004; SC=3.00000208; SDB=6.00849811; UDB=6.00419661; IPR=6.00628444; BA=6.00005304; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00015099; XFM=3.00000013; UTC=2017-04-19 23:25:00 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17041923-0049-0000-0000-000040A246B5 Message-Id: <20170419232457.GD3956@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-04-19_16:,, 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=8.0.1-1703280000 definitions=main-1704190194 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 13, 2017 at 07:59:07PM +0200, Peter Zijlstra wrote: > On Thu, Apr 13, 2017 at 07:51:36PM +0200, Peter Zijlstra wrote: > > > > 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(). > > > > Urgh.. yes those are pain. The best advise is to not use them. > > > > 055ce0fd1b86 ("locking/qspinlock: Add comments") > > The big problem with spin_unlock_wait(), aside from the icky barrier > semantics, is that it tends to end up prone to starvation. So where > spin_lock()+spin_unlock() have guaranteed fwd progress if the lock is > fair (ticket,queued,etc..) spin_unlock_wait() must often lack that > guarantee. > > Equally, spin_unlock_wait() was intended to be 'cheap' and be a > read-only loop, but in order to satisfy the barrier requirements, it > ends up doing stores anyway (see for example the arm64 and ppc > implementations). Good points, and my proposed patch includes verbiage urging the use of something else to get the job done. Does that work? Thanx, Paul