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=-17.4 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,URIBL_BLOCKED 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 D68D9C07E96 for ; Tue, 6 Jul 2021 12:40:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B731C61C7A for ; Tue, 6 Jul 2021 12:40:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234661AbhGFMmr (ORCPT ); Tue, 6 Jul 2021 08:42:47 -0400 Received: from mail.kernel.org ([198.145.29.99]:48286 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S238114AbhGFMdk (ORCPT ); Tue, 6 Jul 2021 08:33:40 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id A9581619C6; Tue, 6 Jul 2021 12:31:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1625574661; bh=YX2FqAvReZaprtAKcAtlTHtMbKsoJC8hMnxjJo8l1TY=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=OcaMqQtwpOFDwUl/FzAnEYR4xzzL1zHG3qKygyj4wUr0MZztGHN8WDkxBJkMiMuKu q6NghdCk/pVGtxHWJavSyRhj0fLW434Zlh5s5GP7k8Pf1oUaD3uweuJ++U7STfH3cR 2TATd0TITCfpuXRFWe8I/q4FaXjoHRVFgbRX42LFLjEzV7m4xNVsKGXN8sz73tSgWC YmAU6PMkPKgD+gPuXbeM4/vS0fFYGXp1UCavHw8Ytewl9N3h+VV5jy4nAuUXAp1YSC KFYZT11Ydqi7pxTvqMrYgbJJrOtvPTI6OJ/FxDfhavfbL7TXh+BPjQ/XBF0Z4Zl1fW /3j/V8s+IEQpQ== Date: Tue, 6 Jul 2021 14:30:58 +0200 From: Frederic Weisbecker To: Peter Zijlstra Cc: "Paul E . McKenney" , LKML , Uladzislau Rezki , Boqun Feng , Ingo Molnar , Neeraj Upadhyay , Joel Fernandes Subject: Re: [PATCH 2/2] rcu: Remove needless preemption disablement in rcu_all_qs() Message-ID: <20210706123058.GB107277@lothringen> References: <20210705234344.104239-1-frederic@kernel.org> <20210705234344.104239-2-frederic@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 06, 2021 at 09:51:01AM +0200, Peter Zijlstra wrote: > On Tue, Jul 06, 2021 at 01:43:44AM +0200, Frederic Weisbecker wrote: > > The preemption is already disabled when we write rcu_data.rcu_urgent_qs. > > We can use __this_cpu_write() directly, although that path is mostly > > used when CONFIG_PREEMPT=n. > > > > Signed-off-by: Frederic Weisbecker > > Cc: Neeraj Upadhyay > > Cc: Joel Fernandes > > Cc: Uladzislau Rezki > > Cc: Boqun Feng > > --- > > kernel/rcu/tree_plugin.h | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > > index 27b74352cccf..38b3d01424d7 100644 > > --- a/kernel/rcu/tree_plugin.h > > +++ b/kernel/rcu/tree_plugin.h > > @@ -871,7 +871,7 @@ void rcu_all_qs(void) > > preempt_enable(); > > return; > > } > > - this_cpu_write(rcu_data.rcu_urgent_qs, false); > > + __this_cpu_write(rcu_data.rcu_urgent_qs, false); > > There's another subtle difference between this_cpu_write() and > __this_cpu_write() aside from preempt. this_cpu_write() is also > IRQ-safe, while __this_cpu_write() is not. > > I've not looked at the usage here to see if that is relevant, but the > Changelog only mentioned the preempt side of things, and that argument > is incomplete in general. You're right, I missed that. I see this rcu_urgent_qs is set by RCU TASKS from rcu_tasks_wait_gp() (did I missed another path?). Not sure if this is called from IRQ nor if it actually matters to protect against IRQs for that single write. I'm not quite used to rcu_tasks. Paul?