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=-8.3 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 5A0E4C3404C for ; Wed, 19 Feb 2020 00:01:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 28FD2208E4 for ; Wed, 19 Feb 2020 00:01:52 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="M4Jm/WVI" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727890AbgBSABv (ORCPT ); Tue, 18 Feb 2020 19:01:51 -0500 Received: from mail-qk1-f195.google.com ([209.85.222.195]:39200 "EHLO mail-qk1-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727822AbgBSABs (ORCPT ); Tue, 18 Feb 2020 19:01:48 -0500 Received: by mail-qk1-f195.google.com with SMTP id a141so11534755qkg.6 for ; Tue, 18 Feb 2020 16:01:46 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=ZncEIyYi8WuUORtwhrTXqb5H8wAtob9Qk+8uAaIjx1I=; b=M4Jm/WVIOOrdqqA9r3dqNIkZ65s5JPx/OaYA/Z1C9ZmIbbyR1IAgkVtXwFEpy7YOr8 EDVyq0AWtnB/yInsz1kCPJeBIOx7GDOKgLBdM8XwY2k2e40ALUw3/F4Gc7KHG9HmzA/s /SRNBeyRSUYejg2W9qPrnqTqNz/yzVhBbP+f4= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=ZncEIyYi8WuUORtwhrTXqb5H8wAtob9Qk+8uAaIjx1I=; b=KxTvU4+ROJ7XkNi6KfJV3TswIGoyx/KxyrEVJ/6qlreYFLXJjXtRm5S18UDIf9UqMw q6QS3jrnJS5YDRbnFB30I/7knBM0E9i2m/+1iIYFAYQf6GFp3FhcmuU+ysBNb+8NZVSM XX8o/Lm/tqSlU7CXm1VXva/8qsUm9ImNjod+SS3tSu0q5KjWSofPaPW+NpGYjAwWsGzY W808KIU6GfOD8HTGbHV0r/MazlBiz94NhK8LL/Wd0J+iWf+zj/3MGyj3xIkfdk7Um5Bc RUSZgpKzX+6TS99TVyBR5KPeSLBk7ZVU4juavhd4aJJ4me0MeUl0/2yNidDVbIRzo0oJ Ouig== X-Gm-Message-State: APjAAAV3Reug3FW0gpM7z904Uiqh06F7ryd6c8s8MytdVbYrdGvWh5rP xpz7WK5TYrfWgpAT0OWEkMkp3g== X-Google-Smtp-Source: APXvYqw5t12g1wrtXskMvVyIqVO0btZ865Lf7DT68CyUl5rJs5IExjWAcjNd1r2JD94XwKTu2thuLQ== X-Received: by 2002:ae9:f016:: with SMTP id l22mr21209557qkg.101.1582070506240; Tue, 18 Feb 2020 16:01:46 -0800 (PST) Received: from localhost ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id y28sm142559qkj.44.2020.02.18.16.01.44 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 18 Feb 2020 16:01:45 -0800 (PST) Date: Tue, 18 Feb 2020 19:01:44 -0500 From: Joel Fernandes To: "Paul E. McKenney" 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: <20200219000144.GA26663@google.com> References: <20200215002446.GA15663@paulmck-ThinkPad-P72> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200218225455.GN2935@paulmck-ThinkPad-P72> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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? thanks, - Joel