From mboxrd@z Thu Jan 1 00:00:00 1970 From: Joel Fernandes Subject: Re: Some -serious- BPF-related litmus tests Date: Thu, 28 May 2020 17:48:23 -0400 Message-ID: <20200528214823.GA211369@google.com> References: <20200522003850.GA32698@paulmck-ThinkPad-P72> <20200522094407.GK325280@hirez.programming.kicks-ass.net> <20200522143201.GB32434@rowland.harvard.edu> <20200522174352.GJ2869@paulmck-ThinkPad-P72> <006e2bc6-7516-1584-3d8c-e253211c157e@fb.com> <20200525145325.GB2066@tardis> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54170 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2436629AbgE1Vs0 (ORCPT ); Thu, 28 May 2020 17:48:26 -0400 Received: from mail-qk1-x731.google.com (mail-qk1-x731.google.com [IPv6:2607:f8b0:4864:20::731]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 6B6CDC08C5C6 for ; Thu, 28 May 2020 14:48:25 -0700 (PDT) Received: by mail-qk1-x731.google.com with SMTP id n11so417215qkn.8 for ; Thu, 28 May 2020 14:48:25 -0700 (PDT) Content-Disposition: inline In-Reply-To: Sender: linux-arch-owner@vger.kernel.org List-ID: To: Andrii Nakryiko Cc: Boqun Feng , Andrii Nakryiko , "Paul E . McKenney" , Alan Stern , Peter Zijlstra , parri.andrea@gmail.com, will@kernel.org, npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr, akiyks@gmail.com, dlustig@nvidia.com, open list , linux-arch@vger.kernel.org On Mon, May 25, 2020 at 11:38:23AM -0700, Andrii Nakryiko wrote: > On Mon, May 25, 2020 at 7:53 AM Boqun Feng wrote: > > > > Hi Andrii, > > > > On Fri, May 22, 2020 at 12:38:21PM -0700, Andrii Nakryiko wrote: > > > On 5/22/20 10:43 AM, Paul E. McKenney wrote: > > > > On Fri, May 22, 2020 at 10:32:01AM -0400, Alan Stern 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, it looks like one or the other of those is redundant (depending > > > > > on the context). > > > > > > > > Probably. Peter instead asked what it was supposed to even do. ;-) > > > > > > I agree, I think smp_wmb() is redundant here. Can't remember why I thought > > > that it's necessary, this algorithm went through a bunch of iterations, > > > starting as completely lockless, also using READ_ONCE/WRITE_ONCE at some > > > point, and settling on smp_read_acquire/smp_store_release, eventually. Maybe > > > there was some reason, but might be that I was just over-cautious. See reply > > > on patch thread as well ([0]). > > > > > > [0] https://lore.kernel.org/bpf/CAEf4Bza26AbRMtWcoD5+TFhnmnU6p5YJ8zO+SoAJCDtp1jVhcQ@mail.gmail.com/ > > > > > > > While we are at it, could you explain a bit on why you use > > smp_store_release() on consumer_pos? I ask because IIUC, consumer_pos is > > only updated at consumer side, and there is no other write at consumer > > side that we want to order with the write to consumer_pos. So I fail > > to find why smp_store_release() is necessary. > > > > I did the following modification on litmus tests, and I didn't see > > different results (on States) between two versions of litmus tests. > > > > This is needed to ensure that producer can reliably detect whether it > needs to trigger poll notification. Boqun's question is on the consumer side though. Are you saying that on the consumer side, the loads prior to the smp_store_release() on the consumer side should have been seen by the consumer? You are already using smp_load_acquire() so that should be satisified already because the smp_load_acquire() makes sure that the smp_load_acquire()'s happens before any future loads and stores. > Basically, consumer caught up at > about same time as producer commits new record, we need to make sure > that: > - either consumer sees updated producer_pos > consumer_pos, and thus > knows that there is more data to consumer (but producer might not send > notification of new data in this case); > - or producer sees that consumer already caught up (i.e., > consumer_pos == producer_pos before currently committed record), and > in such case will definitely send notifications. Could you set a variable on the producer side to emulate a notification, and check that in the conditions at the end? thanks, - Joel > > This is critical for correctness of epoll notifications. > Unfortunately, litmus tests don't test this notification aspect, as I > haven't originally figured out the invariant that can be defined to > validate this. I'll give it another thought, though, maybe this time > I'll come up with something. > > > Regards, > > Boqun > > > > [...]