Messages in this thread Patch in this message | | | Date | Thu, 12 Jul 2012 20:27:54 +0900 | From | Yasuaki Ishimatsu <> | Subject | [PATCH v3 2/3] acpi : prevent cpu from becoming online |
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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 <isimatu.yasuaki@jp.fujitsu.com>
--- 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 20:16:17.985934856 +0900 +++ linux-3.5-rc6/drivers/acpi/processor_driver.c 2012-07-12 20:16:20.122908531 +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("Failed to remove CPU %d, since someone onlines it\n" + , pr->id); + 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 20:16:17.985934856 +0900 +++ linux-3.5-rc6/kernel/cpu.c 2012-07-12 20:25:07.940309872 +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);
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