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=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no 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 DB8AAC2D0CE for ; Tue, 21 Jan 2020 18:10:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id AFC1522522 for ; Tue, 21 Jan 2020 18:10:09 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=armlinux.org.uk header.i=@armlinux.org.uk header.b="CHjGe4+n" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729259AbgAUSKJ (ORCPT ); Tue, 21 Jan 2020 13:10:09 -0500 Received: from pandora.armlinux.org.uk ([78.32.30.218]:55058 "EHLO pandora.armlinux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728186AbgAUSKI (ORCPT ); Tue, 21 Jan 2020 13:10:08 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=armlinux.org.uk; s=pandora-2019; h=Sender:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=FTbjmTPewhmuZeI4RXiIzCWj8QRQqw5GhRQOtHVv+dc=; b=CHjGe4+nP671nkSnHLv1XH8kc t//WmmqJb7tD1MqKGKVq0gRrITiatPK5VqYD8ukB2rLZGmP2qmwe4HhQidekE2c5Mv2JJAbhIdBfv 5jV3xc8yRSsgiqmjGzMdjT7eykoxcGa2cSLhavfrMvYwzgN5iKiij1oWThcs7AEB4Z1wpc6X5Z5jw iU3sN/wYSVgi3wsYuTlD92rt2mYk802LhCk2kGEGHNQnEppUqYwrnRx5B1uiSaZLRcQPunKaVUgVj KoSFCXL78XQtQP8iqImB7pY1QCSpuruZLF/zhvke/hwYuwWwp77kb5G/lf4knIITcx2ijShNz2pdR ULStvBuCg==; Received: from shell.armlinux.org.uk ([2002:4e20:1eda:1:5054:ff:fe00:4ec]:37284) by pandora.armlinux.org.uk with esmtpsa (TLSv1.2:ECDHE-RSA-AES256-GCM-SHA384:256) (Exim 4.90_1) (envelope-from ) id 1itxy3-0006Q1-1Q; Tue, 21 Jan 2020 18:09:35 +0000 Received: from linux by shell.armlinux.org.uk with local (Exim 4.92) (envelope-from ) id 1itxxy-00042T-5z; Tue, 21 Jan 2020 18:09:30 +0000 Date: Tue, 21 Jan 2020 18:09:30 +0000 From: Russell King - ARM Linux admin To: Qais Yousef Cc: Thomas Gleixner , Greg Kroah-Hartman , Josh Poimboeuf , "Peter Zijlstra (Intel)" , Jiri Kosina , Nicholas Piggin , Daniel Lezcano , Ingo Molnar , Eiichi Tsukata , Zhenzhong Duan , Nadav Amit , "Rafael J. Wysocki" , Tony Luck , Fenghua Yu , Catalin Marinas , Will Deacon , linux-arm-kernel@lists.infradead.org, linux-ia64@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 01/14] smp: Create a new function to shutdown nonboot cpus Message-ID: <20200121180930.GJ25745@shell.armlinux.org.uk> References: <20191125112754.25223-1-qais.yousef@arm.com> <20191125112754.25223-2-qais.yousef@arm.com> <20200121170350.GC18808@shell.armlinux.org.uk> <20200121174751.5opyyjwxfnwdgcev@e107158-lin.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200121174751.5opyyjwxfnwdgcev@e107158-lin.cambridge.arm.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jan 21, 2020 at 05:47:52PM +0000, Qais Yousef wrote: > On 01/21/20 17:03, Russell King - ARM Linux admin wrote: > > On Mon, Nov 25, 2019 at 11:27:41AM +0000, Qais Yousef wrote: > > > +void smp_shutdown_nonboot_cpus(unsigned int primary_cpu) > > > +{ > > > + unsigned int cpu; > > > + > > > + if (!cpu_online(primary_cpu)) { > > > + pr_info("Attempting to shutdodwn nonboot cpus while boot cpu is offline!\n"); > > > + cpu_online(primary_cpu); > > Eh, that should be cpu_up(primary_cpu)! > > Which I have to say I'm not if is the right thing to do. > migrate_to_reboot_cpu() picks the first online cpu if reboot_cpu (assumed 0) is > offline > > migrate_to_reboot_cpu(): > 225 /* Make certain the cpu I'm about to reboot on is online */ > 226 if (!cpu_online(cpu)) > 227 cpu = cpumask_first(cpu_online_mask); > > > > + } > > > + > > > + for_each_present_cpu(cpu) { > > > + if (cpu == primary_cpu) > > > + continue; > > > + if (cpu_online(cpu)) > > > + cpu_down(cpu); > > > + } > > > > How does this avoid racing with userspace attempting to restart CPUs > > that have already been taken down by this function? > > This is meant to be called from machine_shutdown() only. > > But you've got a point. > > The previous logic that used disable_nonboot_cpus(), which in turn called > freeze_secondary_cpus() didn't hold hotplug lock. So I assumed the higher level > logic of machine_shutdown() ensures that hotplug lock is held to synchronize > with potential other hotplug operations. freeze_secondary_cpus() takes the CPU maps lock while it takes CPUs down, and then disables cpu hotplug by incrementing cpu_hotplug_disabled. Incrementing that prevents cpu_up() and cpu_down() being used, thereby preventing userspace from changing the online state of any CPU in the system. > But I can see now that it doesn't. > > With this series that migrates users to use device_{online,offline}, holding > the lock_device_hotplug() should protect against such races. > > Worth noting that this an existing problem in the code and not something > I introduced, of course it makes sense to fix it properly as part of this > series. > > I'm not sure how the other archs deal with this TBH. > > Thanks for having a look! > > Cheers > > -- > Qais Yousef > -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up According to speedtest.net: 11.9Mbps down 500kbps up