From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752874AbbJFAyM (ORCPT ); Mon, 5 Oct 2015 20:54:12 -0400 Received: from e32.co.us.ibm.com ([32.97.110.150]:34209 "EHLO e32.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751370AbbJFAyK (ORCPT ); Mon, 5 Oct 2015 20:54:10 -0400 X-IBM-Helo: d03dlp01.boulder.ibm.com X-IBM-MailFrom: paulmck@linux.vnet.ibm.com X-IBM-RcptTo: linux-kernel@vger.kernel.org Date: Mon, 5 Oct 2015 09:57:03 -0700 From: "Paul E. McKenney" To: Will Deacon Cc: Peter Zijlstra , Boqun Feng , "linux-kernel@vger.kernel.org" , "linuxppc-dev@lists.ozlabs.org" , Ingo Molnar , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , Thomas Gleixner , Waiman Long Subject: Re: [RFC v2 4/7] powerpc: atomic: Implement xchg_* and atomic{,64}_xchg_* variants Message-ID: <20151005165703.GD4487@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1442418575-12297-1-git-send-email-boqun.feng@gmail.com> <1442418575-12297-5-git-send-email-boqun.feng@gmail.com> <20151001122440.GP2881@worktop.programming.kicks-ass.net> <20151001150909.GC4043@linux.vnet.ibm.com> <20151001171304.GX3816@twins.programming.kicks-ass.net> <20151001180301.GJ4043@linux.vnet.ibm.com> <20151005144407.GI8818@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151005144407.GI8818@arm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15100600-0005-0000-0000-000018B822F3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Oct 05, 2015 at 03:44:07PM +0100, Will Deacon wrote: > On Thu, Oct 01, 2015 at 07:03:01PM +0100, Paul E. McKenney wrote: > > On Thu, Oct 01, 2015 at 07:13:04PM +0200, Peter Zijlstra wrote: > > > On Thu, Oct 01, 2015 at 08:09:09AM -0700, Paul E. McKenney wrote: > > > > On Thu, Oct 01, 2015 at 02:24:40PM +0200, Peter Zijlstra wrote: > > > > > > > > I must say I'm somewhat surprised by this level of relaxation, I had > > > > > expected to only loose SMP barriers, not the program order ones. > > > > > > > > > > Is there a good argument for this? > > > > > > > > Yes, when we say "relaxed", we really mean relaxed. ;-) > > > > > > > > Both the CPU and the compiler are allowed to reorder around relaxed > > > > operations. > > > > > > Is this documented somewhere, because I completely missed this part. > > > > Well, yes, these need to be added to the documentation. I am assuming > > that Will is looking to have the same effect as C11 memory_order_relaxed, > > which is relaxed in this sense. If he has something else in mind, > > he needs to tell us what it is and why. ;-) > > I was treating them purely as being single-copy atomic and not providing > any memory ordering guarantees (much like the non *_return atomic operations > that we already have). I think this lines up with C11, minus the bits > about data races which we don't call out anyway. As long as it is single-copy atomic and not multi-copy atomic, I believe we are on the saem page. We have slowly been outlawing some sorts of data races over the past few years, and I would guess that this will continue, expecially if good tooling emerges (and KTSAN is showing some promise from what I can see). Thanx, Paul