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 EA60DC433EF for ; Fri, 17 Sep 2021 18:10:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D1F7B61164 for ; Fri, 17 Sep 2021 18:10:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S245253AbhIQSLt (ORCPT ); Fri, 17 Sep 2021 14:11:49 -0400 Received: from mail.kernel.org ([198.145.29.99]:42672 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233022AbhIQSLr (ORCPT ); Fri, 17 Sep 2021 14:11:47 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id C1B8661029; Fri, 17 Sep 2021 18:10:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1631902224; bh=zBWfmOM2pXUJCqk+liGb2j6sPPLvloaGWUXYS9v8Cxg=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=AEg+F2obGkSaa/ClmC4lWEh1MjekN6tRb228PkHsIehnULO0M+Fy2KaAjwZmIWGH1 4sSHQkVyPHAUnYPlxRoh1uIHOHOqMTSDX4Np1YG2f0EtFcs3yHUhkGzqb4ogu8ph89 fLbto8z6SKNhygbAQM0jX7ZIE8t8J/MBkLJGWY/o5/RQxrAjEa5wpcp9WzmUDamOWa zCeI62zZfWSBauoljI0VPc8Kp1B5YquH6CEhReOV86BZuli/Jmu1w907w4+1HOeW8Z xdANa90Qz0kfHEWQ2KDMM3iIkEPYjlV+1FKD6uDqbH2nFWu7YIzAH+tafDKIDFsYVu uN8MsQNp2iv6A== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 9441C5C014A; Fri, 17 Sep 2021 11:10:24 -0700 (PDT) Date: Fri, 17 Sep 2021 11:10:24 -0700 From: "Paul E. McKenney" To: Frederic Weisbecker 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: <20210917181024.GS4156@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20210916121048.36623-1-frederic@kernel.org> <20210916121048.36623-3-frederic@kernel.org> <20210916164340.GF4156@paulmck-ThinkPad-P17-Gen-1> <20210916210514.GA40064@lothringen> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210916210514.GA40064@lothringen> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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? I should hasten to add that getting rid of ->exp_deferred_qs is quite attractive! Thanx, Paul > > > rcu_report_exp_cpu_mult(rdp->mynode, rdp->grpmask, true); > > > } > > > > > > -- > > > 2.25.1 > > >