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 60547C2BA83 for ; Sat, 15 Feb 2020 10:58:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 26B962072D for ; Sat, 15 Feb 2020 10:58:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1581764286; bh=8Vjv/lv2op4qINIa0AVJhDucTZu4NRTV0d1OgtzZmEM=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=dvrS/YUbxmhFcdRb8Aw2JVAG40VszzoTXN2z5rn3FJ1SeN5mkVp3+UyS1Ae7CH+Q7 kzTUO2EX+Yuz7RQq410POTfb898t4/w5fXZ563dcywiQ3f1o7mzpHR86r9A826d2aQ OHWRmvPj9KU/mgOppzgY8ERolJjxqU3kWXmgNkQg= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725971AbgBOK6F (ORCPT ); Sat, 15 Feb 2020 05:58:05 -0500 Received: from mail.kernel.org ([198.145.29.99]:33046 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725965AbgBOK6F (ORCPT ); Sat, 15 Feb 2020 05:58:05 -0500 Received: from paulmck-ThinkPad-P72.home (unknown [62.84.152.189]) (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 8FB4C20726; Sat, 15 Feb 2020 10:58:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1581764284; bh=8Vjv/lv2op4qINIa0AVJhDucTZu4NRTV0d1OgtzZmEM=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=ptCws8HtjKpK2tpsJ3AMQLf03GhR1BKeGrC2ISsKzG8QxU3fVhetQGNgHpi5rwQ7d gZ1GcYXOoH9jSQIDoK+0Esk40sDNOkptSitHND8ADkJNsUknIsbV7Vw3OSobQqhf9u 8jwxjxE6KTypK9A332F/eqV38X+aBnLqdrTjp2ZY= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 2ABAE3520CAA; Sat, 15 Feb 2020 02:58:03 -0800 (PST) Date: Sat, 15 Feb 2020 02:58:03 -0800 From: "Paul E. McKenney" To: Steven Rostedt Cc: 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, joel@joelfernandes.org, Linus Torvalds Subject: Re: [PATCH tip/core/rcu 06/30] rcu: Add WRITE_ONCE to rcu_node ->exp_seq_rq store Message-ID: <20200215105803.GY2935@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200214224743.280772a7@oasis.local.home> 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 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. 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")); >