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=-2.3 required=3.0 tests=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 B6093C433DF for ; Fri, 22 May 2020 14:36:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8B1DC2225E for ; Fri, 22 May 2020 14:36:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730019AbgEVOgL (ORCPT ); Fri, 22 May 2020 10:36:11 -0400 Received: from netrider.rowland.org ([192.131.102.5]:53065 "HELO netrider.rowland.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1729879AbgEVOgK (ORCPT ); Fri, 22 May 2020 10:36:10 -0400 Received: (qmail 1718 invoked by uid 1000); 22 May 2020 10:36:09 -0400 Date: Fri, 22 May 2020 10:36:09 -0400 From: Alan Stern To: "Paul E. McKenney" Cc: Peter Zijlstra , parri.andrea@gmail.com, will@kernel.org, boqun.feng@gmail.com, npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr, akiyks@gmail.com, dlustig@nvidia.com, joel@joelfernandes.org, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, andriin@fb.com Subject: Re: Some -serious- BPF-related litmus tests Message-ID: <20200522143609.GC32434@rowland.harvard.edu> References: <20200522003850.GA32698@paulmck-ThinkPad-P72> <20200522094407.GK325280@hirez.programming.kicks-ass.net> <20200522105659.GH2869@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200522105659.GH2869@paulmck-ThinkPad-P72> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, May 22, 2020 at 03:56:59AM -0700, Paul E. McKenney wrote: > On Fri, May 22, 2020 at 11:44:07AM +0200, Peter Zijlstra wrote: > > On Thu, May 21, 2020 at 05:38:50PM -0700, Paul E. McKenney wrote: > > > Hello! > > > > > > Just wanted to call your attention to some pretty cool and pretty serious > > > litmus tests that Andrii did as part of his BPF ring-buffer work: > > > > > > https://lore.kernel.org/bpf/20200517195727.279322-3-andriin@fb.com/ > > > > > > Thoughts? > > > > I find: > > > > smp_wmb() > > smp_store_release() > > > > a _very_ weird construct. What is that supposed to even do? > > Indeed, and I asked about that in my review of the patch containing the > code. It -could- make sense if there is a prior read and a later store: > > r1 = READ_ONCE(a); > WRITE_ONCE(b, 1); > smp_wmb(); > smp_store_release(&c, 1); > WRITE_ONCE(d, 1); > > So a->c and b->c is smp_store_release() and b->d is smp_wmb(). But if > there were only stores, the smp_wmb() would suffice. And if there wasn't > the trailing store, smp_store_release() would suffice. But that wasn't the context in the litmus test. The context was: smp_wmb(); smp_store_release(); spin_unlock(); smp_store_release(); That certainly looks like a lot more ordering than is really needed. Alan