Hi Toshi, 2012/07/13 1:49, Toshi Kani wrote:
On Thu, 2012-07-12 at 20:40 +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 | 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 2012-07-12 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);pr_warn() should be used per the recent checkpatch change.
O.K. I'll update it. Thanks, Yasuaki Ishimatsu
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 2012-07-12 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);-- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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