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=-11.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 070F6C433E6 for ; Thu, 28 Jan 2021 21:35:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BC7DC64DEB for ; Thu, 28 Jan 2021 21:35:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231300AbhA1VfX (ORCPT ); Thu, 28 Jan 2021 16:35:23 -0500 Received: from mail.kernel.org ([198.145.29.99]:35338 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229596AbhA1Ve4 (ORCPT ); Thu, 28 Jan 2021 16:34:56 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id 2A03364DE8; Thu, 28 Jan 2021 21:34:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1611869655; bh=wN2rpturJmJM7GiWuXK4eGf/ciS8iKIQ7U1Snpc7h5s=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=JlQ6Mob8h88NUFo2916XCe1b7Pg4y+SDol2GTmf927YbnmzDCGBET8sTUw4TztMgR SRxhfcpWttg2KHdNjs2ByrJf5CUinlE016Mma8lazXGhVuiaftS++DKM/Kc3oq6XU0 kuKiWPtXNQPpgNlzf7Pce08IRxlm3lukvwQ3qjhUzf4eed3Agt3bLDoluKxaQvk8x/ mYY0FJ4R28dtd7MjeMagjK48b0xCUYuhnOPjE+JlvfPjZ7as0d96FRnzy1+JSgKFhI lSuBcfZJ3T46Qh4dY2+mBE6l/tNSKPJ9RgErdkzAM7bFRmt/iHyrAA721QwG1apRvT jt2HZA989mKyw== Date: Thu, 28 Jan 2021 22:34:13 +0100 From: Frederic Weisbecker To: "Paul E. McKenney" Cc: LKML , Boqun Feng , Lai Jiangshan , Neeraj Upadhyay , Josh Triplett , Stable , Joel Fernandes Subject: Re: [PATCH 04/16] rcu/nocb: Only (re-)initialize segcblist when needed on CPU up Message-ID: <20210128213413.GC122776@lothringen> References: <20210128171222.131380-1-frederic@kernel.org> <20210128171222.131380-5-frederic@kernel.org> <20210128191228.GQ2743@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210128191228.GQ2743@paulmck-ThinkPad-P72> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 28, 2021 at 11:12:28AM -0800, Paul E. McKenney wrote: > On Thu, Jan 28, 2021 at 06:12:10PM +0100, Frederic Weisbecker wrote: > > Simply checking if the segcblist is enabled is enough to know if we > > need to initialize it or not. It's safe to check within hotplug > > machine. > > > > Signed-off-by: Frederic Weisbecker > > Cc: Josh Triplett > > Cc: Lai Jiangshan > > Cc: Joel Fernandes > > Cc: Neeraj Upadhyay > > Cc: Boqun Feng > > Hmmm... > > At the start of a CPU-hotplug operation, an incoming CPU's callback > list can be in a number of states: > > 1. Disabled and empty. This is the case when the boot CPU has > not done call_rcu(), when a non-boot CPU first comes online, > and when a non-offloaded CPU comes back online. In this case, > it is permissible to initialize ->cblist. Because either the > CPU is currently running with interrupts disabled (boot CPU) > or is not yet running at all (other CPUs), it is not necessary > to acquire ->nocb_lock. > > 2. Disabled and non-empty. This is the case when the boot CPU has > done call_rcu(). It is not permissible to initialize ->cblist > because doing so will leak any callbacks posted by early boot > invocations of call_rcu(). I don't think that's possible. In this case __call_rcu() has called rcu_segcblist_init() and has enabled the segcblist. > > Test for the possibility of leaking by building with > CONFIG_PROVE_RCU=y and booting with rcupdate.rcu_self_test=1. > > 3. Enabled, whether empty or not. This is the case when an > offloaded CPU comes back online. This is the only case where > the ->nocb_lock must be held to modify ->cblist. However, > it is not necessarily to modify ->cblist because the rcuoc > kthread is on the job. > > So I believe that it is necessary to check for both disabled and empty. > But don't take my word for it! Build with CONFIG_PROVE_RCU=y and boot > with rcupdate.rcu_self_test=1. ;-) I'm trying that :-)