From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752710AbbGNMpw (ORCPT ); Tue, 14 Jul 2015 08:45:52 -0400 Received: from e35.co.us.ibm.com ([32.97.110.153]:33813 "EHLO e35.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751935AbbGNMpu (ORCPT ); Tue, 14 Jul 2015 08:45:50 -0400 X-Helo: d03dlp02.boulder.ibm.com X-MailFrom: paulmck@linux.vnet.ibm.com X-RcptTo: linux-kernel@vger.kernel.org Date: Tue, 14 Jul 2015 05:45:40 -0700 From: "Paul E. McKenney" To: Will Deacon Cc: Peter Zijlstra , "linux-arch@vger.kernel.org" , "linux-kernel@vger.kernel.org" , Benjamin Herrenschmidt Subject: Re: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock_lock() Message-ID: <20150714124540.GC3717@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1436789704-10086-1-git-send-email-will.deacon@arm.com> <20150713131143.GY19282@twins.programming.kicks-ass.net> <20150713140915.GD2632@arm.com> <20150713142109.GE2632@arm.com> <20150713155447.GB19282@twins.programming.kicks-ass.net> <20150713175029.GO2632@arm.com> <20150713202032.GZ3717@linux.vnet.ibm.com> <20150713222346.GE19282@twins.programming.kicks-ass.net> <20150713230405.GB3717@linux.vnet.ibm.com> <20150714100429.GC15448@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150714100429.GC15448@arm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15071412-0013-0000-0000-0000140FEE1A Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 14, 2015 at 11:04:29AM +0100, Will Deacon wrote: > On Tue, Jul 14, 2015 at 12:04:06AM +0100, Paul E. McKenney wrote: > > On Tue, Jul 14, 2015 at 12:23:46AM +0200, Peter Zijlstra wrote: > > > If we look at the inside of the critical section again -- similar > > > argument as before: > > > > > > *A = a > > > smp_mb() > > > store M > > > load N > > > smp_mb() > > > *B = b > > > > > > A and B are fully ordered, and in this case even transitivity is > > > provided. > > > > > > I'm stating that the order of M and N don't matter, only the > > > load/stores that are inside the acquire/release are constrained. > > > > No argument here. > > > > > IOW, I think smp_mb__after_unlock_lock() already works as advertised > > > with all our acquire/release thingies -- as is stated by the > > > documentation. > > > > > > That said, I'm not aware of anybody but RCU actually using this, so its > > > not used in that capacity. > > > > OK, I might actually understand what you are getting at. And, yes, if > > someone actually comes up with a need to combine smp_mb__after_unlock_lock() > > with something other than locking, we should worry about it at that point. > > And probably rename smp_mb__after_unlock_lock() at that point, as well. > > Until then, why lock ourselves into semantics that no one needs, and > > that it is quite possible that no one will ever need? > > Given that RCU is currently the only user of this barrier, how would you > feel about making the barrier local to RCU and not part of the general > memory-barrier API? In theory, no objection. Your thought is to leave the definitions where they are, mark them as being used only by RCU, and removing mention from memory-barriers.txt? Or did you have something else in mind? > My main reason for proposing its removal is because I don't want to see > it being used (incorrectly) all over the place to order the new RELEASE > and ACQUIRE operations I posted separately, at which point we have to try > fixing up all the callers or retrofitting some semantics. It doesn't help > that memory-barriers.txt lumps things like LOCK and ACQUIRE together, > whereas this barrier is currently only intended to be used in conjunction > with the former. Heh! That lumping was considered to be a feature at the time. ;-) Thanx, Paul