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 <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@xxxxxxxxxxxxxx>
>
> ---
> 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 <toshi.kani@xxxxxx>
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);
>
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