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 10:45:40 -0700 Message-ID: <20200522174540.GK2869@paulmck-ThinkPad-P72> References: <20200522003850.GA32698@paulmck-ThinkPad-P72> <20200522094407.GK325280@hirez.programming.kicks-ass.net> <20200522105659.GH2869@paulmck-ThinkPad-P72> <20200522143609.GC32434@rowland.harvard.edu> 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]:52760 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726373AbgEVRpl (ORCPT ); Fri, 22 May 2020 13:45:41 -0400 Content-Disposition: inline In-Reply-To: <20200522143609.GC32434@rowland.harvard.edu> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Alan Stern 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 On Fri, May 22, 2020 at 10:36:09AM -0400, Alan Stern wrote: > 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. I suspect that you are right. I asked him if there were other accesses in my response to his ringbuffer (as opposed to litmus-test) patch: https://lore.kernel.org/bpf/20200522002502.GF2869@paulmck-ThinkPad-P72/ If there are other accesses requiring both, the litmus tests might need to be updated. Thanx, Paul