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=-8.4 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=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 6CF94C34022 for ; Mon, 17 Feb 2020 21:11:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 40C32207FD for ; Mon, 17 Feb 2020 21:11:38 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="Pzz6Zgxb" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728444AbgBQVLi (ORCPT ); Mon, 17 Feb 2020 16:11:38 -0500 Received: from mail-qk1-f193.google.com ([209.85.222.193]:45742 "EHLO mail-qk1-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727300AbgBQVLh (ORCPT ); Mon, 17 Feb 2020 16:11:37 -0500 Received: by mail-qk1-f193.google.com with SMTP id a2so17497907qko.12 for ; Mon, 17 Feb 2020 13:11:37 -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=FxRvJ7r8F0ZhX0NHb83GmNQC4jmvCqMGLG+tYQsonZM=; b=Pzz6Zgxbs2KU70lU07hCHuYB+qSwV2OzlgI3vnRxXKSknajbzUMUz9POTYrINqVTxS Q4v+CqY7C/FMizwmjXn3j6MRDtN5/l3Ah0YAxZNd7CKo63eb+UYaLAr+A5NlItB3SXoF Ytq8dEvLHRhR0hxyGKXmWQFFIocQ96jLq3lkE= 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=FxRvJ7r8F0ZhX0NHb83GmNQC4jmvCqMGLG+tYQsonZM=; b=PODpKrdEgatmz3FOT+5uszpxEdew9XP777Tr9h3zxos63t8K9zdUmlc7Bp/O/Q9gXK lavsGAnGYpqLTkWC5ddP8oxwcYhBo8Rp7uDb0A7YUZT4jtYGw3iTwdZ5/4/HYT+ZbLPA db3YgDwxaL12oKzNOnT9F05o2t7U1l6JaQI4yt2P29ODs0dBlVSiEL033QORczI8uzTt gHBlHjNRYCl8ndnVbOphGlAOhjiMlnJW9PUiiy2dku6DdtWeCXwiPm0lM9wMakRlzKUl U72xAsgGFSk2T5dNg19gwd/D3McxbJvbyhYfGHufQHCUS74jL78nXX70EPWzr4RKe4T6 PYqw== X-Gm-Message-State: APjAAAUNeEnW2TUXiQSi0Z+na4Swb8E5ys6ibRANlMKenFDYgD6/A/5T BoCbSDleKWw18xClM1DiL9dyhw== X-Google-Smtp-Source: APXvYqygX7dLAl4Fy2BYXdFwY91MRzUP3Zm9j4CbyODQwFddsNhaRmCqH65KiokFA+D4qv930fnjMA== X-Received: by 2002:a37:9e0f:: with SMTP id h15mr15770994qke.287.1581973896659; Mon, 17 Feb 2020 13:11:36 -0800 (PST) Received: from localhost ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id 132sm429238qkn.109.2020.02.17.13.11.35 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 17 Feb 2020 13:11:35 -0800 (PST) Date: Mon, 17 Feb 2020 16:11:35 -0500 From: Joel Fernandes To: "Paul E. McKenney" Cc: Steven Rostedt , 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, peterz@infradead.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, Linus Torvalds Subject: Re: [PATCH tip/core/rcu 06/30] rcu: Add WRITE_ONCE to rcu_node ->exp_seq_rq store Message-ID: <20200217211135.GA207704@google.com> References: <20200214235536.GA13364@paulmck-ThinkPad-P72> <20200214235607.13749-6-paulmck@kernel.org> <20200214224743.280772a7@oasis.local.home> <20200215105803.GY2935@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200215105803.GY2935@paulmck-ThinkPad-P72> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Sat, Feb 15, 2020 at 02:58:03AM -0800, Paul E. McKenney wrote: > On Fri, Feb 14, 2020 at 10:47:43PM -0500, Steven Rostedt wrote: > > On Fri, 14 Feb 2020 15:55:43 -0800 > > paulmck@kernel.org wrote: > > > > > From: "Paul E. McKenney" > > > > > > The rcu_node structure's ->exp_seq_rq field is read locklessly, so > > > this commit adds the WRITE_ONCE() to a load in order to provide proper > > > documentation and READ_ONCE()/WRITE_ONCE() pairing. > > > > > > This data race was reported by KCSAN. Not appropriate for backporting > > > due to failure being unlikely. > > > > > > Signed-off-by: Paul E. McKenney > > > --- > > > kernel/rcu/tree_exp.h | 2 +- > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h > > > index d7e0484..85b009e 100644 > > > --- a/kernel/rcu/tree_exp.h > > > +++ b/kernel/rcu/tree_exp.h > > > @@ -314,7 +314,7 @@ static bool exp_funnel_lock(unsigned long s) > > > sync_exp_work_done(s)); > > > return true; > > > } > > > - rnp->exp_seq_rq = s; /* Followers can wait on us. */ > > > + WRITE_ONCE(rnp->exp_seq_rq, s); /* Followers can wait on us. */ > > > > Didn't Linus say this is basically bogus? > > > > Perhaps just using it as documenting that it's read locklessly, but is > > it really needed? > > Yes, Linus explicitly stated that WRITE_ONCE() is not required in > this case, but he also said that he was OK with it being there for > documentation purposes. Just to add, PeterZ does approve of WRITE_ONCE() to prevent store-tearing where applicable. And I have reproduced Will's example [1] with the arm64 Clang compiler shipping with the latest Android NDK just now. It does break up stores when writing zeroes to 64-bit valyes, this is a real problem which WRITE_ONCE() resolves. And I've verified GCC on arm64 does break up 64-bit immediate value writes without WRITE_ONCE(). thanks, - Joel [1] https://lore.kernel.org/lkml/20190821103200.kpufwtviqhpbuv2n@willie-the-truck/ > And within RCU, I -do- need it because I absolutely need to see if a > given patch introduced new KCSAN reports. So I need it for the same > reason that I need the build to proceed without warnings. > > Others who are working with less concurrency-intensive code might quite > reasonably make other choices, of course. And my setting certain KCSAN > config options in my own builds doesn't inconvenience them, so we should > all be happy, right? :-) > > Thanx, Paul > > > -- Steve > > > > > > > > > spin_unlock(&rnp->exp_lock); > > > trace_rcu_exp_funnel_lock(rcu_state.name, rnp->level, > > > rnp->grplo, rnp->grphi, TPS("nxtlvl")); > >