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=-16.4 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 8607FC433FE for ; Fri, 17 Sep 2021 22:00:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6C71C61052 for ; Fri, 17 Sep 2021 22:00:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1344374AbhIQWBi (ORCPT ); Fri, 17 Sep 2021 18:01:38 -0400 Received: from mail.kernel.org ([198.145.29.99]:55260 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229589AbhIQWBg (ORCPT ); Fri, 17 Sep 2021 18:01:36 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 475B661052; Fri, 17 Sep 2021 22:00:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1631916013; bh=+TnN3rkhiKJL7FE663JtdQS0i9k7JNLeEkBjc2XurBA=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=cGoM7B/0gvT7Ldv7SGJa+EIIt3SzyBKqbHNemIc2aCLG5kR+Ze2fv1ZEJrHpWonYI H6ks9lYl+sQfSXf5oLUl+DXiOVq1or/UeQjExWAjdTi50keWwxsQ0nKMqZNmSvMydz 3nRSQXadC7+Twt9WHU7oGmvd2PHRW7dCu7xO5zFpxzZGBW+UR5kTaJs8JHxZcA6s43 k1Jc+rrCeCv+Y/wgpHEn3glvS3+3m3AgW3/TR60UVOLtNNq8G0hGPmFMAAvQFQkNct hKSjUrc9/jlxrvCmoBeU1PEysd9BAoS30NkRzT1xO40H42nakcMkeKKihh8BsURTEl NkVCIid8eEG9Q== Date: Sat, 18 Sep 2021 00:00:11 +0200 From: Frederic Weisbecker To: "Paul E. McKenney" Cc: LKML , rcu@vger.kernel.org, urezki@gmail.com, boqun.feng@gmail.com, Neeraj Upadhyay , joel@joelfernandes.org Subject: Re: [PATCH 2/4] rcu: Remove useless WRITE_ONCE() on rcu_data.exp_deferred_qs Message-ID: <20210917220011.GB48873@lothringen> References: <20210916121048.36623-1-frederic@kernel.org> <20210916121048.36623-3-frederic@kernel.org> <20210916164340.GF4156@paulmck-ThinkPad-P17-Gen-1> <20210916210514.GA40064@lothringen> <20210917181024.GS4156@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210917181024.GS4156@paulmck-ThinkPad-P17-Gen-1> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Sep 17, 2021 at 11:10:24AM -0700, Paul E. McKenney wrote: > On Thu, Sep 16, 2021 at 11:05:14PM +0200, Frederic Weisbecker wrote: > > On Thu, Sep 16, 2021 at 09:43:40AM -0700, Paul E. McKenney wrote: > > > On Thu, Sep 16, 2021 at 02:10:46PM +0200, Frederic Weisbecker wrote: > > > > This variable is never written nor read remotely. Remove this confusion. > > > > > > > > Signed-off-by: Frederic Weisbecker > > > > --- > > > > 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 f3947c49eee7..4266610b4587 100644 > > > > --- a/kernel/rcu/tree_exp.h > > > > +++ b/kernel/rcu/tree_exp.h > > > > @@ -255,7 +255,7 @@ static void rcu_report_exp_cpu_mult(struct rcu_node *rnp, > > > > */ > > > > static void rcu_report_exp_rdp(struct rcu_data *rdp) > > > > { > > > > - WRITE_ONCE(rdp->exp_deferred_qs, false); > > > > + rdp->exp_deferred_qs = false; > > > > > > Are you sure that this can never be invoked from an interrupt handler? > > > And that rdp->exp_deferred_qs is never read from an interrupt handler? > > > If either can happen, then the WRITE_ONCE() does play a role, right? > > > > Well, the only effect I can imagine is that it can partly prevent from an > > interrupt to report concurrently the quiescent state during the few > > instructions before we mask interrupts and lock the node. > > > > That's a micro performance benefit that avoid a second call to > > rcu_report_exp_cpu_mult() with the extra locking and early exit. > > I am not claiming that current compilers would mess this up, though I > have learned to have great respect for what future compilers might do... :) > > > But then that racy interrupt can still happen before we clear exp_deferred_qs. > > In this case __this_cpu_cmpxchg() would have been more efficient. > > Except that __this_cpu_cmpxchg() would have a possibility of failure, > and thus an extra branch not needed by WRITE_ONCE(). Or am I missing > your point here? Right, but an extra branch that could spare a call to rcu_report_exp_cpu_mult(). Anyway I don't mind the WRITE_ONCE(), but you know how ordering (whether compiler or CPU) makes me anxious when undocumented or not self-explanatory, (although arguably the latter can vary depending on the reviewer :) Thanks.