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=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 DB5A5C3402A for ; Mon, 17 Feb 2020 21:37:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B064D207FD for ; Mon, 17 Feb 2020 21:37:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1581975421; bh=Z3MjIfs82PU+SZJqaYgP0md1kb2zmhXzQIJP97cDfys=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=gt6u3qXNjempvhhewLnVuA2mhEUvqM5t68y6bQ5humTSW2Up1xdeOrjqYW4Fo92K2 JBy7gizRL5YK1RIRHqmLiIsP+vYnRBP6N8A8G/vUPHMkpX4zxZPJPKF+Yzq5910xBk ZqXy1Z+i2HUPYp49G1MdHlh8LtXPWjTlyxoSHHkY= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729958AbgBQVg4 (ORCPT ); Mon, 17 Feb 2020 16:36:56 -0500 Received: from mail.kernel.org ([198.145.29.99]:40994 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728935AbgBQVg4 (ORCPT ); Mon, 17 Feb 2020 16:36:56 -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 525922070B; Mon, 17 Feb 2020 21:36:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1581975415; bh=Z3MjIfs82PU+SZJqaYgP0md1kb2zmhXzQIJP97cDfys=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=eHuj2IElQrDCVE7vAgW19UJLaZkXkMyPRzO4U9ORoKQ2UMtGeMN72/kbMMZoK2Uss T1XRnD1rHjTzfMvX2Kp7JQJURbb+MiTBtngOspjcQyKmeBzdF5IvXqgucsaRfeLXr+ gqtkmgcUeRpVjKOYe9PBTD5ubpurcXhK5OXlOpUY= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 2B7C735227A8; Mon, 17 Feb 2020 13:36:55 -0800 (PST) Date: Mon, 17 Feb 2020 13:36:55 -0800 From: "Paul E. McKenney" To: Joel Fernandes 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: <20200217213655.GX2935@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200214235536.GA13364@paulmck-ThinkPad-P72> <20200214235607.13749-6-paulmck@kernel.org> <20200214224743.280772a7@oasis.local.home> <20200215105803.GY2935@paulmck-ThinkPad-P72> <20200217211135.GA207704@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200217211135.GA207704@google.com> 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 Mon, Feb 17, 2020 at 04:11:35PM -0500, Joel Fernandes wrote: > 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(). That does sounds a bit on the required side! ;-) Thanx, Paul > 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")); > > >