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=-5.5 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 6EF4AC433E0 for ; Sat, 18 Jul 2020 02:00:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 561EB20768 for ; Sat, 18 Jul 2020 02:00:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726923AbgGRCAJ (ORCPT ); Fri, 17 Jul 2020 22:00:09 -0400 Received: from mail110.syd.optusnet.com.au ([211.29.132.97]:55270 "EHLO mail110.syd.optusnet.com.au" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726665AbgGRCAJ (ORCPT ); Fri, 17 Jul 2020 22:00:09 -0400 Received: from dread.disaster.area (pa49-180-53-24.pa.nsw.optusnet.com.au [49.180.53.24]) by mail110.syd.optusnet.com.au (Postfix) with ESMTPS id 5990F105E9E; Sat, 18 Jul 2020 12:00:02 +1000 (AEST) Received: from dave by dread.disaster.area with local (Exim 4.92.3) (envelope-from ) id 1jwc8v-0002KH-1J; Sat, 18 Jul 2020 12:00:01 +1000 Date: Sat, 18 Jul 2020 12:00:01 +1000 From: Dave Chinner To: Alan Stern Cc: Eric Biggers , "Darrick J. Wong" , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, "Paul E . McKenney" , linux-fsdevel@vger.kernel.org, Akira Yokosawa , Andrea Parri , Boqun Feng , Daniel Lustig , David Howells , Jade Alglave , Luc Maranget , Nicholas Piggin , Peter Zijlstra , Will Deacon Subject: Re: [PATCH] tools/memory-model: document the "one-time init" pattern Message-ID: <20200718020001.GO5369@dread.disaster.area> References: <20200717044427.68747-1-ebiggers@kernel.org> <20200717205340.GR7625@magnolia> <20200718005857.GB2183@sol.localdomain> <20200718012555.GA1168834@rowland.harvard.edu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200718012555.GA1168834@rowland.harvard.edu> User-Agent: Mutt/1.10.1 (2018-07-13) X-Optus-CM-Score: 0 X-Optus-CM-Analysis: v=2.3 cv=LPwYv6e9 c=1 sm=1 tr=0 a=moVtWZxmCkf3aAMJKIb/8g==:117 a=moVtWZxmCkf3aAMJKIb/8g==:17 a=kj9zAlcOel0A:10 a=_RQrkK6FrEwA:10 a=7-415B0cAAAA:8 a=m2fqJy7gfEZGdrr6IoAA:9 a=uIZ0nfzhNTXwYeV5:21 a=TVXeNXAKodzJbbhi:21 a=CjuIK1q_8ugA:10 a=biEYGPWJfzWAr4FL6Ov7:22 Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org Archived-At: List-Archive: List-Post: On Fri, Jul 17, 2020 at 09:25:55PM -0400, Alan Stern wrote: > On Fri, Jul 17, 2020 at 05:58:57PM -0700, Eric Biggers wrote: > > On Fri, Jul 17, 2020 at 01:53:40PM -0700, Darrick J. Wong wrote: > > > > +There are also cases in which the smp_load_acquire() can be replaced by > > > > +the more lightweight READ_ONCE(). (smp_store_release() is still > > > > +required.) Specifically, if all initialized memory is transitively > > > > +reachable from the pointer itself, then there is no control dependency > > > > > > I don't quite understand what "transitively reachable from the pointer > > > itself" means? Does that describe the situation where all the objects > > > reachable through the object that the global struct foo pointer points > > > at are /only/ reachable via that global pointer? > > > > > > > The intent is that "transitively reachable" means that all initialized memory > > can be reached by dereferencing the pointer in some way, e.g. p->a->b[5]->c. > > > > It could also be the case that allocating the object initializes some global or > > static data, which isn't reachable in that way. Access to that data would then > > be a control dependency, which a data dependency barrier wouldn't work for. > > > > It's possible I misunderstood something. (Note the next paragraph does say that > > using READ_ONCE() is discouraged, exactly for this reason -- it can be hard to > > tell whether it's correct.) Suggestions of what to write here are appreciated. > > Perhaps something like this: > > Specifically, if the only way to reach the initialized memory > involves dereferencing the pointer itself then READ_ONCE() is > sufficient. This is because there will be an address dependency > between reading the pointer and accessing the memory, which will > ensure proper ordering. But if some of the initialized memory > is reachable some other way (for example, if it is global or > static data) then there need not be an address dependency, > merely a control dependency (checking whether the pointer is > non-NULL). Control dependencies do not always ensure ordering > -- certainly not for reads, and depending on the compiler, > possibly not for some writes -- and therefore a load-acquire is > necessary. Recipes are aimed at people who simply don't understand any of that goobledegook. This won't help them -write correct code-. > Perhaps this is more wordy than you want, but it does get the important > ideas across. You think they are important because you understand what those words mean. Large numbers of developers do not understand what they mean, nor how to put them into practise correctly. Seriously: if you want people to use this stuff correctly, you need to -dumb it down-, not make it even more challenging by explaining words people don't understand with yet more words they don't understand... This is the "curse of knowledge" cognative bias in a nutshell. Cheers, Dave. -- Dave Chinner david@fromorbit.com