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=-8.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 8C124C3404D for ; Wed, 19 Feb 2020 00:16:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5D93424658 for ; Wed, 19 Feb 2020 00:16:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1582071401; bh=q8d6bMowdR3/X3NFByctGhiYj21CvwWmJenI2e7QtGE=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=hqo9IfA+OK5Q4dQZ48uQlth2chbQzaIhX5EZhWfSxXJrWUTNL+xwWzaGyqFC1fUGK VS6Dfa0EzprJHYgVXdcUf+fZItzNLHu5gTm8hSInLNnW0MCWeCe5CPAeNxsC7AM3tY 5xZw23/Ln8oy0Z05LAHxFtaH2ut6umG94360o4Kg= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726641AbgBSAQl (ORCPT ); Tue, 18 Feb 2020 19:16:41 -0500 Received: from mail.kernel.org ([198.145.29.99]:55664 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726521AbgBSAQl (ORCPT ); Tue, 18 Feb 2020 19:16:41 -0500 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 36C2F24656; Wed, 19 Feb 2020 00:16:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1582071400; bh=q8d6bMowdR3/X3NFByctGhiYj21CvwWmJenI2e7QtGE=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=uMnr8gEgC6TNTarUS/HBQim8Y/114l43sr66637H1i8gqjWrU/5cml4tYNG/osaLu GTas9R2yDJ5hfmw/uxlvQ15v9jjV8I9KQaDJA9Q27PpGP14aiCHbndhIBNeGbaK1hn +aYDZb6xHFAIkH439GfHdgNzlFW02dEyagGJ1hSo= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 0A9563520856; Tue, 18 Feb 2020 16:16:40 -0800 (PST) Date: Tue, 18 Feb 2020 16:16:40 -0800 From: "Paul E. McKenney" To: Joel Fernandes Cc: Steven Rostedt , Peter Zijlstra , rcu@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com Subject: Re: [PATCH tip/core/rcu 1/3] rcu-tasks: *_ONCE() for rcu_tasks_cbs_head Message-ID: <20200219001640.GP2935@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200215002520.15746-1-paulmck@kernel.org> <20200217123851.GR14914@hirez.programming.kicks-ass.net> <20200217181615.GP2935@paulmck-ThinkPad-P72> <20200218075648.GW14914@hirez.programming.kicks-ass.net> <20200218162719.GE2935@paulmck-ThinkPad-P72> <20200218201142.GF11457@worktop.programming.kicks-ass.net> <20200218202226.GJ2935@paulmck-ThinkPad-P72> <20200218174503.3d4e4750@gandalf.local.home> <20200218225455.GN2935@paulmck-ThinkPad-P72> <20200219000144.GA26663@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200219000144.GA26663@google.com> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Tue, Feb 18, 2020 at 07:01:44PM -0500, Joel Fernandes wrote: > On Tue, Feb 18, 2020 at 02:54:55PM -0800, Paul E. McKenney wrote: > > On Tue, Feb 18, 2020 at 05:45:03PM -0500, Steven Rostedt wrote: > > > On Tue, 18 Feb 2020 12:22:26 -0800 > > > "Paul E. McKenney" wrote: > > > > > > > On Tue, Feb 18, 2020 at 09:11:42PM +0100, Peter Zijlstra wrote: > > > > > On Tue, Feb 18, 2020 at 08:27:19AM -0800, Paul E. McKenney wrote: > > > > > > On Tue, Feb 18, 2020 at 08:56:48AM +0100, Peter Zijlstra wrote: > > > > > > > > > > > > I just took offence at the Changelog wording. It seems to suggest there > > > > > > > actually is a problem, there is not. > > > > > > > > > > > > Quoting the changelog: "Not appropriate for backporting due to failure > > > > > > being unlikely." > > > > > > > > > > That implies there is failure, however unlikely. > > > > > > > > > > In this particular case there is absolutely no failure, except perhaps > > > > > in KCSAN. This patch is a pure annotation such that KCSAN can understand > > > > > the code. > > > > > > > > > > Like said, I don't object to the actual patch, but I do think it is > > > > > important to call out false negatives or to describe the actual problem > > > > > found. > > > > > > > > I don't feel at all comfortable declaring that there is absolutely > > > > no possibility of failure. > > > > > > Perhaps wording it like so: > > > > > > "There's know known issue with the current code, but the *_ONCE() > > > annotations here makes KCSAN happy, allowing us to focus on KCSAN > > > warnings that can help bring about known issues in other code that we > > > can fix, without being distracted by KCSAN warnings that we do not see > > > a problem with." > > > > > > ? > > > > That sounds more like something I might put in rcutodo.html as a statement > > of the RCU approach to KCSAN reports. > > > > But switching to a different situation (for variety, if nothing else), > > what about the commit shown below? > > > > Thanx, Paul > > > > ------------------------------------------------------------------------ > > > > commit 35bc02b04a041f32470ae6d959c549bcce8483db > > Author: Paul E. McKenney > > Date: Tue Feb 18 13:41:02 2020 -0800 > > > > rcutorture: Mark data-race potential for rcu_barrier() test statistics > > > > The n_barrier_successes, n_barrier_attempts, and > > n_rcu_torture_barrier_error variables are updated (without access > > markings) by the main rcu_barrier() test kthread, and accessed (also > > without access markings) by the rcu_torture_stats() kthread. This of > > course can result in KCSAN complaints. > > > > Because the accesses are in diagnostic prints, this commit uses > > data_race() to excuse the diagnostic prints from the data race. If this > > were to ever cause bogus statistics prints (for example, due to store > > tearing), any misleading information would be disambiguated by the > > presence or absence of an rcutorture splat. > > > > This data race was reported by KCSAN. Not appropriate for backporting > > due to failure being unlikely and due to the mild consequences of the > > failure, namely a confusing rcutorture console message. > > > > Signed-off-by: Paul E. McKenney > > > > diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c > > index 5453bd5..b3301f3 100644 > > --- a/kernel/rcu/rcutorture.c > > +++ b/kernel/rcu/rcutorture.c > > @@ -1444,9 +1444,9 @@ rcu_torture_stats_print(void) > > atomic_long_read(&n_rcu_torture_timers)); > > torture_onoff_stats(); > > pr_cont("barrier: %ld/%ld:%ld\n", > > - n_barrier_successes, > > - n_barrier_attempts, > > - n_rcu_torture_barrier_error); > > + data_race(n_barrier_successes), > > + data_race(n_barrier_attempts), > > + data_race(n_rcu_torture_barrier_error)); > > Would it be not worth just fixing the data-race within rcutorture itself? I could use WRITE_ONCE() for updates and READ_ONCE() for statistics. However, my current rule is that diagnostic code that is not participating in the core synchronization uses data_race(). That way, if I do a typo and write to (say) n_barrier_attempts in some other thread, KCSAN will know to yell at me. Thanx, Paul