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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 3670AC433DF for ; Fri, 22 May 2020 17:45:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0B6E420738 for ; Fri, 22 May 2020 17:45:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1590169544; bh=HnkRVWiR8hMQlze8/0Lt2vEZ/kFWm+ECVepsC/iwB/I=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=VISSUOhJovu62mRiWhXC4Ic1UO3DNtxnersL79KuEheR+kDnpZKSPsAtWoqO5/cMr dmWhO3WL7r1wUtQi0qCU3RL+5DNZOh8R9e3qp/kBi/735kOL466Rvez1/y27/qFi9l yNlwnII7ORXadP067O4ZpK3eHmAC7vihsnfykaB0= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730832AbgEVRpm (ORCPT ); Fri, 22 May 2020 13:45:42 -0400 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 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 6F88220738; Fri, 22 May 2020 17:45:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1590169540; bh=HnkRVWiR8hMQlze8/0Lt2vEZ/kFWm+ECVepsC/iwB/I=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=QHZuNFYt/X65UhTVIVQIRysFvy211kKktmaSSQ019cEbdRxMP/A4mnF1fwBFATER7 W6I2lKHvbb4vf71cou0J9nYAwc2DolcBwZkzC62z7zLVIvyClDrDihBysgb0yP+GOW H+I5UPS359gaQnGUw7mSZjQAUm1sLQbeSR3BI0wY= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 5C5E73522E41; Fri, 22 May 2020 10:45:40 -0700 (PDT) Date: Fri, 22 May 2020 10:45:40 -0700 From: "Paul E. McKenney" 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 Subject: Re: Some -serious- BPF-related litmus tests Message-ID: <20200522174540.GK2869@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200522003850.GA32698@paulmck-ThinkPad-P72> <20200522094407.GK325280@hirez.programming.kicks-ass.net> <20200522105659.GH2869@paulmck-ThinkPad-P72> <20200522143609.GC32434@rowland.harvard.edu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200522143609.GC32434@rowland.harvard.edu> User-Agent: Mutt/1.9.4 (2018-02-28) 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 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