From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753919AbdDMRKl (ORCPT ); Thu, 13 Apr 2017 13:10:41 -0400 Received: from merlin.infradead.org ([205.233.59.134]:53954 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752755AbdDMRKj (ORCPT ); Thu, 13 Apr 2017 13:10:39 -0400 Date: Thu, 13 Apr 2017 19:10:27 +0200 From: Peter Zijlstra To: "Paul E. McKenney" 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() Message-ID: <20170413171027.snjqn4u54t2kdzgx@hirez.programming.kicks-ass.net> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170413165755.GJ3956@linux.vnet.ibm.com> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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.