From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: Compilers and RCU readers: Once more unto the breach! Date: Wed, 20 May 2015 08:12:00 -0700 Message-ID: <20150520151200.GS6776@linux.vnet.ibm.com> References: <20150520005510.GA23559@linux.vnet.ibm.com> <20150520024148.GD6776@linux.vnet.ibm.com> <20150520114745.GC11498@arm.com> <31547.1432127917@warthog.procyon.org.uk> <31805.1432129025@warthog.procyon.org.uk> <555C8FBE.4020505@arm.com> <20150520140343.GO6776@linux.vnet.ibm.com> <555C9714.6020001@arm.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from e35.co.us.ibm.com ([32.97.110.153]:41844 "EHLO e35.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752966AbbETPSd (ORCPT ); Wed, 20 May 2015 11:18:33 -0400 Received: from /spool/local by e35.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 20 May 2015 09:18:33 -0600 Content-Disposition: inline In-Reply-To: <555C9714.6020001@arm.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Ramana Radhakrishnan Cc: David Howells , Will Deacon , Linus Torvalds , Linux Kernel Mailing List , "c++std-parallel@accu.org" , "linux-arch@vger.kernel.org" , "gcc@gcc.gnu.org" , p796231 , "mark.batty@cl.cam.ac.uk" , Peter Zijlstra , Andrew Morton , Ingo Molnar , "michaelw@ca.ibm.com" On Wed, May 20, 2015 at 03:15:48PM +0100, Ramana Radhakrishnan wrote: > > > On 20/05/15 15:03, Paul E. McKenney wrote: > >On Wed, May 20, 2015 at 02:44:30PM +0100, Ramana Radhakrishnan wrote: > >> > >> > >>On 20/05/15 14:37, David Howells wrote: > >>>Paul E. McKenney wrote: > >>> > >>>>I was thinking of "y" as a simple variable, but if it is something more > >>>>complex, then the compiler could do this, right? > >>>> > >>>> char *x; > >>>> > >>>> y; > >>>> x = z; > >>> > >>>Yeah. I presume it has to maintain the ordering, though. > >> > >>The scheduler for e.g. is free to reorder if it can prove there is > >>no dependence (or indeed side-effects for y) between insns produced > >>for y and `x = z'. > > > >So for example, if y is independent of z, the compiler can do the > >following: > > > > char *x; > > > > x = z; > > y; > > > >But the dependency ordering is still maintained from z to x, so this > >is not a problem. > > > Well, reads if any of x (assuming x was initialized elsewhere) would > need to happen before x got assigned to z. Agreed, there needs to be a memory_order_consume load up there somewhere. (AKA rcu_dereference().) > I understood the original "maintain the ordering" as between the > statements `x = z' and `y'. Ah, I was assuming between x and z. David, what was your intent? ;-) > >Or am I missing something subtle here? > > No, it sounds like we are on the same page here. Whew! ;-) Thanx, Paul