2012/07/12 21:41, Srivatsa S. Bhat wrote:
> 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 <isimatu.yasuaki@xxxxxxxxxxxxxx>
>>
>
> 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 <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
>
>> ---
>> 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"
Looks good to me. I'll update it.
Thanks,
Yasuaki Ishimatsu
>
> 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);
>>
>
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