From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752744AbbJLXYY (ORCPT ); Mon, 12 Oct 2015 19:24:24 -0400 Received: from e32.co.us.ibm.com ([32.97.110.150]:49312 "EHLO e32.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751878AbbJLXYW (ORCPT ); Mon, 12 Oct 2015 19:24:22 -0400 X-IBM-Helo: d03dlp02.boulder.ibm.com X-IBM-MailFrom: paulmck@linux.vnet.ibm.com X-IBM-RcptTo: linux-kernel@vger.kernel.org Date: Mon, 12 Oct 2015 16:24:26 -0700 From: "Paul E. McKenney" To: Boqun Feng Cc: Peter Zijlstra , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, Ingo Molnar , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , Thomas Gleixner , Will Deacon , Waiman Long Subject: Re: [RFC v2 4/7] powerpc: atomic: Implement xchg_* and atomic{,64}_xchg_* variants Message-ID: <20151012232426.GJ3910@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> <20151012011749.GD27351@fixme-laptop.cn.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151012011749.GD27351@fixme-laptop.cn.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15101223-0005-0000-0000-000018E78402 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Oct 12, 2015 at 09:17:50AM +0800, Boqun Feng wrote: > Hi Paul, > > On Thu, Oct 01, 2015 at 11:03:01AM -0700, 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 > > Maybe it's good time for us to call it out which operation should be > a compiler barrier or a CPU barrier? > > I had something in my mind while I was working on this series, not > really sure whether it's correct, but probably a start point: > > All global and local atomic operations are at least atomic(no one can > observe the middle state) and volatile(compilers can't optimize out the > memory access). Based on this, there are four strictness levels, one > can rely on them: > > RELAXED: neither a compiler barrier or a CPU barrier > LOCAL: a compiler barrier > PARTIAL: both a compiler barrier and a CPU barrier but not transitive > FULL: both compiler barrier and a CPU barrier, and transitive. As Will noted, we have two types of transitive. The first type is that of release-acquire chains, where the transitivity is only observable within the chain. The second type is that of smp_mb(), where the transitivity is observable globally. Thanx, Paul > RELAXED includes all _relaxed variants and non-return atomics, LOCAL > includes all local atomics(local_* and {cmp}xchg_local), PARTIAL > includes _acquire and _release operations and FULL includes all fully > ordered global atomic operations. > > Thoughts? > > Regards, > Boqun > > > 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. ;-) > > > > Thanx, Paul > >