From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-11.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id C78F3C55178 for ; Fri, 6 Nov 2020 20:40:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 75A08208C7 for ; Fri, 6 Nov 2020 20:40:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728467AbgKFUka (ORCPT ); Fri, 6 Nov 2020 15:40:30 -0500 Received: from netrider.rowland.org ([192.131.102.5]:54119 "HELO netrider.rowland.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1728413AbgKFUka (ORCPT ); Fri, 6 Nov 2020 15:40:30 -0500 Received: (qmail 55677 invoked by uid 1000); 6 Nov 2020 15:40:08 -0500 Date: Fri, 6 Nov 2020 15:40:08 -0500 From: Alan Stern To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org, parri.andrea@gmail.com, will@kernel.org, peterz@infradead.org, boqun.feng@gmail.com, npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr, akiyks@gmail.com Subject: Re: [PATCH memory-model 5/8] tools/memory-model: Add a glossary of LKMM terms Message-ID: <20201106204008.GA55521@rowland.harvard.edu> References: <20201105215953.GA15309@paulmck-ThinkPad-P72> <20201105220017.15410-5-paulmck@kernel.org> <20201106165930.GC47039@rowland.harvard.edu> <20201106180445.GX3249@paulmck-ThinkPad-P72> <20201106192351.GA53131@rowland.harvard.edu> <20201106195912.GA3249@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201106195912.GA3249@paulmck-ThinkPad-P72> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org On Fri, Nov 06, 2020 at 11:59:12AM -0800, Paul E. McKenney wrote: > On Fri, Nov 06, 2020 at 02:23:51PM -0500, Alan Stern wrote: > > On Fri, Nov 06, 2020 at 10:04:46AM -0800, Paul E. McKenney wrote: > > > On Fri, Nov 06, 2020 at 11:59:30AM -0500, Alan Stern wrote: > > > > > + See also "Control Dependency". > > > > > > > > There should also be an entry for "Data Dependency", linked from here > > > > and from Control Dependency. > > > > > > > > > +Marked Access: An access to a variable that uses an special function or > > > > > + macro such as "r1 = READ_ONCE()" or "smp_store_release(&a, 1)". > > > > > > > > How about "r1 = READ_ONCE(x)"? > > > > > > Good catches! I am planning to squash the commit below into the > > > original. Does that cover it? > > > > No, because you didn't add a glossary entry for "Data Dependency" and > > there's no link from "Control Dependency" to "Data Dependency". > > Sigh. I was thinking "entry in the list", and didn't even thing to > check for an entry in the glossary as a whole. With the patch below > (on top of the one sent earlier), are we good? > > Thanx, Paul > > ------------------------------------------------------------------------ > > commit 5a49c32551e83d30e304d6c3fbb660737ba2654e > Author: Paul E. McKenney > Date: Fri Nov 6 11:57:25 2020 -0800 > > fixup! tools/memory-model: Add a glossary of LKMM terms > > Signed-off-by: Paul E. McKenney > > diff --git a/tools/memory-model/Documentation/glossary.txt b/tools/memory-model/Documentation/glossary.txt > index 471bf13..b2da636 100644 > --- a/tools/memory-model/Documentation/glossary.txt > +++ b/tools/memory-model/Documentation/glossary.txt > @@ -64,7 +64,7 @@ Control Dependency: When a later store's execution depends on a test > fragile, and can be easily destroyed by optimizing compilers. > Please see control-dependencies.txt for more information. > > - See also "Address Dependency". > + See also "Address Dependency" and "Data Dependency". > > Cycle: Memory-barrier pairing is restricted to a pair of CPUs, as the > name suggests. And in a great many cases, a pair of CPUs is all > @@ -85,6 +85,23 @@ Cycle: Memory-barrier pairing is restricted to a pair of CPUs, as the > > See also "Pairing". > > +Data Dependency: When the data written by a later store is computed based > + on the value returned by an earlier load, a "data dependency" > + extends from that load to that later store. For example: > + > + 1 r1 = READ_ONCE(x); > + 2 WRITE_ONCE(y, r1 + 1); > + > + In this case, the data dependency extends from the READ_ONCE() > + on line 1 to the WRITE_ONCE() on line 2. Data dependencies are > + fragile and can be easily destroyed by optimizing compilers. > + Because optimizing compilers put a great deal of effort into > + working out what values integer variables might have, this is > + especially true in cases where the dependency is carried through > + an integer. > + > + See also "Address Dependency" and "Control Dependency". > + > From-Reads (fr): When one CPU's store to a given variable happened > too late to affect the value returned by another CPU's > load from that same variable, there is said to be a from-reads Yes, this is better. Is it really true that data dependencies are so easily destroyed? I would expect that a true "semantic" dependency (i.e., one where the value written really does vary according to the value read) would be rather hard to second guess. Alan