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>
Reviewed-by: Toshi Kani <toshi.kani@xxxxxx>
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-3.6-rc7/drivers/acpi/processor_driver.c
===================================================================
--- linux-3.6-rc7.orig/drivers/acpi/processor_driver.c 2012-09-28 19:16:33.207858261 +0900
+++ linux-3.6-rc7/drivers/acpi/processor_driver.c 2012-09-28 19:23:05.129858476 +0900
@@ -857,8 +857,22 @@ 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, because other task "
+ "brought the CPU back online\n", pr->id);
+ return -EAGAIN;
+ }
arch_unregister_cpu(pr->id);
acpi_unmap_lsapic(pr->id);
+ put_online_cpus();
return ret;
}
#else
Index: linux-3.6-rc7/kernel/cpu.c
===================================================================
--- linux-3.6-rc7.orig/kernel/cpu.c 2012-09-24 10:10:57.000000000 +0900
+++ linux-3.6-rc7/kernel/cpu.c 2012-09-28 19:19:32.321858402 +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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