From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752370AbdGEJqs (ORCPT ); Wed, 5 Jul 2017 05:46:48 -0400 Received: from foss.arm.com ([217.140.101.70]:54196 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751696AbdGEJqq (ORCPT ); Wed, 5 Jul 2017 05:46:46 -0400 Date: Wed, 5 Jul 2017 10:46:45 +0100 From: Will Deacon To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, ldr709@gmail.com, dhowells@redhat.com, peterz@infradead.org, corbet@lwn.net, stern@rowland.harvard.edu, parri.andrea@gmail.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr Subject: Re: [PATCH] doc: Update memory-barriers.txt for read-to-write dependencies Message-ID: <20170705094645.GC10112@arm.com> References: <20170630232810.GA28036@linux.vnet.ibm.com> <20170703130703.GD1573@arm.com> <20170703174106.GE2393@linux.vnet.ibm.com> <20170704163635.GK22175@arm.com> <20170704214651.GP2393@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170704214651.GP2393@linux.vnet.ibm.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 04, 2017 at 02:46:51PM -0700, Paul E. McKenney wrote: > On Tue, Jul 04, 2017 at 05:36:36PM +0100, Will Deacon wrote: > > On Mon, Jul 03, 2017 at 10:41:06AM -0700, Paul E. McKenney wrote: > > > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt > > > index 9d5e0f853f08..7be80911e502 100644 > > > --- a/Documentation/memory-barriers.txt > > > +++ b/Documentation/memory-barriers.txt > > > @@ -594,7 +594,23 @@ between the address load and the data load: > > > This enforces the occurrence of one of the two implications, and prevents the > > > third possibility from arising. > > > > > > -A data-dependency barrier must also order against dependent writes: > > > + > > > +[!] Note that this extremely counterintuitive situation arises most easily on > > > +machines with split caches, so that, for example, one cache bank processes > > > +even-numbered cache lines and the other bank processes odd-numbered cache > > > +lines. The pointer P might be stored in an odd-numbered cache line, and the > > > +variable B might be stored in an even-numbered cache line. Then, if the > > > +even-numbered bank of the reading CPU's cache is extremely busy while the > > > +odd-numbered bank is idle, one can see the new value of the pointer P (&B), > > > +but the old value of the variable B (2). > > > + > > > + > > > +A data-dependency barrier is not required to order dependent writes > > > +because the CPUs that the Linux kernel supports don't do writes until > > > +they are certain (1) that the write will actually happen, (2) of the > > > +location of the write, and (3) of the value to be written. But please > > > +carefully read the "CONTROL DEPENDENCIES" section: The compiler can > > > +and does break control dependencies in a great many situations. > > > > This makes it sound like only control dependencies are susceptible to > > being broken by compiler optimisations, so I'd drop the "control" and > > just say "The compiler can and does break dependencies in a great many > > situations". > > > > With that: > > > > Acked-by: Will Deacon > > Done! I also added a pointer to Documentation/RCU/rcu_dereference.txt, > which lists some of the vandalism^Woptimizations that modern compilers > can commit. Does the updated patch below capture it? Yes, thanks Paul. Will