From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758029Ab2GMPaO (ORCPT ); Fri, 13 Jul 2012 11:30:14 -0400 Received: from g5t0009.atlanta.hp.com ([15.192.0.46]:5131 "EHLO g5t0009.atlanta.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755841Ab2GMPaM (ORCPT ); Fri, 13 Jul 2012 11:30:12 -0400 Message-ID: <1342193136.19100.19.camel@misato.fc.hp.com> Subject: Re: [PATCH v4 2/3] acpi : prevent cpu from becoming online From: Toshi Kani To: Yasuaki Ishimatsu Cc: srivatsa.bhat@linux.vnet.ibm.com, lenb@kernel.org, linux-acpi@vger.kernel.org, linux-kernel@vger.kernel.org Date: Fri, 13 Jul 2012 09:25:36 -0600 In-Reply-To: <4FFFE182.2090601@jp.fujitsu.com> References: <4FFFE0DA.3050806@jp.fujitsu.com> <4FFFE182.2090601@jp.fujitsu.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.2.3 (3.2.3-1.fc16) Content-Transfer-Encoding: 7bit Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 2012-07-13 at 17:51 +0900, 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 | > > Reviewed-by: Srivatsa S. Bhat > Signed-off-by: Yasuaki Ishimatsu > > --- > 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 2012-07-13 17:31:37.799130100 +0900 > +++ linux-3.5-rc6/drivers/acpi/processor_driver.c 2012-07-13 17:39:47.727006338 +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(); > + pr_warn("Failed to remove CPU %d, ", pr->id); > + pr_warn("because other task brought the CPU back online\n"); Just a nit-pick, but you can simply do it like: pr_warn("Failed to remove CPU %d, because other task " "brought the CPU back online\n", pr->id); Other than that, Reviewed-by: Toshi Kani Thanks, -Toshi > + 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 2012-07-13 17:31:37.800130087 +0900 > +++ linux-3.5-rc6/kernel/cpu.c 2012-07-13 17:31:39.661106874 +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); >