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=-7.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS 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 D2AFFC432BE for ; Fri, 23 Jul 2021 20:28:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B863F60F23 for ; Fri, 23 Jul 2021 20:28:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231396AbhGWTrj (ORCPT ); Fri, 23 Jul 2021 15:47:39 -0400 Received: from mail.kernel.org ([198.145.29.99]:33660 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229461AbhGWTri (ORCPT ); Fri, 23 Jul 2021 15:47:38 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 8B90E60E94; Fri, 23 Jul 2021 20:28:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1627072091; bh=oikyN+p4IRyE0L4SEjaNmzX3ZjbYgNMoIEpW+F4Yexk=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=AhlUrrG4qK0eAwXvQSfZoLjKNZzvbHjMTZJ/qPojYC+jkO/UKDQQ3Aw4cLWOGnHYg yE+TKyWn3lkswgVf7NQNcsh2wGz7c+xHKyYe/UNADDuqeYbSXHebZiPfOQH6osrmrt klDgxwLfBtFzP8JkR+klaMSwVFayNG1qityarIl9Ut9d9sKfXb7VuZFX4hpRb6ckzk iOklu3D7ifYwho3wbT9t6qCDY/5oKufA/d0vTD9jSh0mG+JeTiyXqmzjvba3bh/OmG O3znNSQGmB+nkI5XYd+xdaQeH7tniYINXo864QL0FN3iiYADutOL+JmwWG9t9sei/y 27rRqXO9C0gvw== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 5C78A5C068F; Fri, 23 Jul 2021 13:28:11 -0700 (PDT) Date: Fri, 23 Jul 2021 13:28:11 -0700 From: "Paul E. McKenney" To: Alan Stern 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, Manfred Spraul Subject: Re: [PATCH memory-model 2/4] tools/memory-model: Add example for heuristic lockless reads Message-ID: <20210723202811.GK4397@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20210721210726.GA828672@paulmck-ThinkPad-P17-Gen-1> <20210721211003.869892-2-paulmck@kernel.org> <20210723020846.GA26397@rowland.harvard.edu> <20210723162431.GF4397@paulmck-ThinkPad-P17-Gen-1> <20210723165947.GA46562@rowland.harvard.edu> <20210723173010.GI4397@paulmck-ThinkPad-P17-Gen-1> <20210723181138.GA48833@rowland.harvard.edu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210723181138.GA48833@rowland.harvard.edu> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jul 23, 2021 at 02:11:38PM -0400, Alan Stern wrote: > On Fri, Jul 23, 2021 at 10:30:10AM -0700, Paul E. McKenney wrote: > > On Fri, Jul 23, 2021 at 12:59:47PM -0400, Alan Stern wrote: > > > On Fri, Jul 23, 2021 at 09:24:31AM -0700, Paul E. McKenney wrote: > > > > On Thu, Jul 22, 2021 at 10:08:46PM -0400, Alan Stern wrote: > > > > > > > > > + void do_something_locked(struct foo *fp) > > > > > > + { > > > > > > + bool gf = true; > > > > > > + > > > > > > + /* IMPORTANT: Heuristic plus spin_lock()! */ > > > > > > + if (!data_race(global_flag)) { > > > > > > + spin_lock(&fp->f_lock); > > > > > > + if (!smp_load_acquire(&global_flag)) { > > > > > > > > > + void begin_global(void) > > > > > > + { > > > > > > + int i; > > > > > > + > > > > > > + spin_lock(&global_lock); > > > > > > + WRITE_ONCE(global_flag, true); > > > > > > > > > > Why does this need to be WRITE_ONCE? It still races with the first read > > > > > of global_flag above. > > > > > > > > But also with the smp_load_acquire() of global_flag, right? > > > > > > What I'm curious about is why, given these two races, you notate one of > > > them by changing a normal write to WRITE_ONCE and you notate the other > > > by changing a normal read to a data_race() read. Why not handle them > > > both the same way? > > > > Because the code can tolerate the first read returning complete nonsense, > > but needs the value from the second read to be exact at that point in > > time. > > 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. */ Thanx, Paul