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=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no 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 1A153C432BE for ; Fri, 23 Jul 2021 21:03:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0268C60F23 for ; Fri, 23 Jul 2021 21:03:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231637AbhGWUXR (ORCPT ); Fri, 23 Jul 2021 16:23:17 -0400 Received: from netrider.rowland.org ([192.131.102.5]:43335 "HELO netrider.rowland.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S231551AbhGWUXP (ORCPT ); Fri, 23 Jul 2021 16:23:15 -0400 Received: (qmail 53715 invoked by uid 1000); 23 Jul 2021 17:03:47 -0400 Date: Fri, 23 Jul 2021 17:03:47 -0400 From: Alan Stern To: "Paul E. McKenney" Cc: Manfred Spraul , 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 2/4] tools/memory-model: Add example for heuristic lockless reads Message-ID: <20210723210347.GA53526@rowland.harvard.edu> References: <20210721210726.GA828672@paulmck-ThinkPad-P17-Gen-1> <20210721211003.869892-2-paulmck@kernel.org> <20210723020846.GA26397@rowland.harvard.edu> <20210723130554.GA38923@rowland.harvard.edu> <20210723163008.GG4397@paulmck-ThinkPad-P17-Gen-1> <20210723170820.GB46562@rowland.harvard.edu> <20210723203248.GL4397@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210723203248.GL4397@paulmck-ThinkPad-P17-Gen-1> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jul 23, 2021 at 01:28:11PM -0700, Paul E. McKenney wrote: > On Fri, Jul 23, 2021 at 02:11:38PM -0400, Alan Stern wrote: > > In other words, if the second read races with the WRITE_ONCE, it needs to > > get either the value before the write or the value after the write; > > nothing else will do because it isn't a heuristic here. Fair point. > > > > > (If the value changes immediately after being read, the fact that > > > ->f_lock is held prevents begin_global() from completing.) > > > > This seems like something worth explaining in the document. That > > "IMPORTANT" comment doesn't really get the full point across. > > How about this comment instead? > > /* This works even if data_race() returns nonsense. */ That's somewhat better. On Fri, Jul 23, 2021 at 01:32:48PM -0700, Paul E. McKenney wrote: > On Fri, Jul 23, 2021 at 01:08:20PM -0400, Alan Stern wrote: > > This doesn't mention the reason for the acquire-release > > synchronization of global_flag. It's needed because work done between > > begin_global() and end_global() can affect a foo structure without > > holding its private f_lock member, and we want all such work to be > > visible to other threads when they call do_something_locked() later. > > Like this added paragraph at the end? > > The smp_load_acquire() and smp_store_release() are required > because changes to a foo structure between calls to begin_global() > and end_global() are carried out without holding that structure's > ->f_lock. The smp_load_acquire() and smp_store_release() > ensure that the next invocation of do_something() from the call > to do_something_locked() that acquires that ->f_lock will see > those changes. I'd shorten the last sentence: The smp_load_acquire() and smp_store_release() ensure that the next invocation of do_something() from do_something_locked() will see those changes. Alan