From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: Some -serious- BPF-related litmus tests Date: Fri, 22 May 2020 03:56:59 -0700 Message-ID: <20200522105659.GH2869@paulmck-ThinkPad-P72> References: <20200522003850.GA32698@paulmck-ThinkPad-P72> <20200522094407.GK325280@hirez.programming.kicks-ass.net> Reply-To: paulmck@kernel.org Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from mail.kernel.org ([198.145.29.99]:56712 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728371AbgEVK5A (ORCPT ); Fri, 22 May 2020 06:57:00 -0400 Content-Disposition: inline In-Reply-To: <20200522094407.GK325280@hirez.programming.kicks-ass.net> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Peter Zijlstra Cc: stern@rowland.harvard.edu, 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 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 would at least want a comment, in my opinion. ;-) Thanx, Paul