From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758897Ab2GLMnL (ORCPT ); Thu, 12 Jul 2012 08:43:11 -0400 Received: from e28smtp01.in.ibm.com ([122.248.162.1]:34405 "EHLO e28smtp01.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752853Ab2GLMnI (ORCPT ); Thu, 12 Jul 2012 08:43:08 -0400 Message-ID: <4FFEC60B.7040800@linux.vnet.ibm.com> Date: Thu, 12 Jul 2012 18:11:47 +0530 From: "Srivatsa S. Bhat" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:13.0) Gecko/20120605 Thunderbird/13.0 MIME-Version: 1.0 To: Yasuaki Ishimatsu CC: linux-acpi@vger.kernel.org, linux-kernel@vger.kernel.org, lenb@kernel.org, toshi.kani@hp.com Subject: Re: [PATCH v3 2/3 RESEND] acpi : prevent cpu from becoming online References: <4FFEB35A.6030500@jp.fujitsu.com> <4FFEB4BA.8010800@jp.fujitsu.com> <4FFEB7BA.6050505@jp.fujitsu.com> In-Reply-To: <4FFEB7BA.6050505@jp.fujitsu.com> Content-Type: text/plain; charset=ISO-2022-JP Content-Transfer-Encoding: 7bit x-cbid: 12071212-4790-0000-0000-000003A83F2E Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/12/2012 05:10 PM, Yasuaki Ishimatsu wrote: > Even if acpi_processor_handle_eject() offlines cpu, there is a chance > to online the cpu after that. So the patch closes the window by using > get/put_online_cpus(). > > Why does the patch change _cpu_up() logic? > > The patch cares the race of hot-remove cpu and _cpu_up(). If the patch > does not change it, there is the following race. > > hot-remove cpu | _cpu_up() > ------------------------------------- ------------------------------------ > call acpi_processor_handle_eject() | > call cpu_down() | > call get_online_cpus() | > | call cpu_hotplug_begin() and stop here > call arch_unregister_cpu() | > call acpi_unmap_lsapic() | > call put_online_cpus() | > | start and continue _cpu_up() > return acpi_processor_remove() | > continue hot-remove the cpu | > > So _cpu_up() can continue to itself. And hot-remove cpu can also continue > itself. If the patch changes _cpu_up() logic, the race disappears as below: > > hot-remove cpu | _cpu_up() > ----------------------------------------------------------------------- > call acpi_processor_handle_eject() | > call cpu_down() | > call get_online_cpus() | > | call cpu_hotplug_begin() and stop here > call arch_unregister_cpu() | > call acpi_unmap_lsapic() | > cpu's cpu_present is set | > to false by set_cpu_present()| > call put_online_cpus() | > | start _cpu_up() > | check cpu_present() and return -EINVAL > return acpi_processor_remove() | > continue hot-remove the cpu | > > Signed-off-by: Yasuaki Ishimatsu > Please consider fixing the grammar issue below (since it is a user-visible print statement). Other than that, everything looks fine. Reviewed-by: Srivatsa S. Bhat > --- > drivers/acpi/processor_driver.c | 14 ++++++++++++++ > kernel/cpu.c | 8 +++++--- > 2 files changed, 19 insertions(+), 3 deletions(-) > > Index: linux-3.5-rc6/drivers/acpi/processor_driver.c > =================================================================== > --- linux-3.5-rc6.orig/drivers/acpi/processor_driver.c 2012-07-12 20:34:29.438289841 +0900 > +++ linux-3.5-rc6/drivers/acpi/processor_driver.c 2012-07-12 20:39:29.190542257 +0900 > @@ -850,8 +850,22 @@ static int acpi_processor_handle_eject(s > return ret; > } > > + get_online_cpus(); > + /* > + * The cpu might become online again at this point. So we check whether > + * the cpu has been onlined or not. If the cpu became online, it means > + * that someone wants to use the cpu. So acpi_processor_handle_eject() > + * returns -EAGAIN. > + */ > + if (unlikely(cpu_online(pr->id))) { > + put_online_cpus(); > + printk(KERN_WARNING "Failed to remove CPU %d, " > + "since someone onlines the cpu\n" , pr->id); How about: "Failed to remove CPU %d, because some other task brought the CPU back online\n" Regards, Srivatsa S. Bhat > + return -EAGAIN; > + } > arch_unregister_cpu(pr->id); > acpi_unmap_lsapic(pr->id); > + put_online_cpus(); > return ret; > } > #else > Index: linux-3.5-rc6/kernel/cpu.c > =================================================================== > --- linux-3.5-rc6.orig/kernel/cpu.c 2012-07-12 20:34:29.438289841 +0900 > +++ linux-3.5-rc6/kernel/cpu.c 2012-07-12 20:34:35.040219535 +0900 > @@ -343,11 +343,13 @@ static int __cpuinit _cpu_up(unsigned in > unsigned long mod = tasks_frozen ? CPU_TASKS_FROZEN : 0; > struct task_struct *idle; > > - if (cpu_online(cpu) || !cpu_present(cpu)) > - return -EINVAL; > - > cpu_hotplug_begin(); > > + if (cpu_online(cpu) || !cpu_present(cpu)) { > + ret = -EINVAL; > + goto out; > + } > + > idle = idle_thread_get(cpu); > if (IS_ERR(idle)) { > ret = PTR_ERR(idle); >