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SubjectRe: [PATCH 1/2] acpi : cpu hot-remove returns error number when cpu_down() fails
Hi Srivatsa,

Thank you for your reviewing.

2012/07/06 18:51, Srivatsa S. Bhat wrote:
> On 07/06/2012 08:46 AM, Yasuaki Ishimatsu wrote:
>> Even if cpu_down() fails, acpi_processor_remove() continues to remove the cpu.
>
> Ouch!
>
>> But in this case, it should return error number since some process may run on
>> the cpu. If the cpu has a running process and the cpu is turned the power off,
>> the system cannot work well.
>>
>> Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
>>
>> ---
>> drivers/acpi/processor_driver.c | 18 ++++++++++++------
>> 1 file changed, 12 insertions(+), 6 deletions(-)
>>
>> Index: linux-3.5-rc4/drivers/acpi/processor_driver.c
>> ===================================================================
>> --- linux-3.5-rc4.orig/drivers/acpi/processor_driver.c 2012-06-25 04:53:04.000000000 +0900
>> +++ linux-3.5-rc4/drivers/acpi/processor_driver.c 2012-07-05 21:02:58.711285382 +0900
>> @@ -610,7 +610,7 @@ err_free_pr:
>> static int acpi_processor_remove(struct acpi_device *device, int type)
>> {
>> struct acpi_processor *pr = NULL;
>> -
>> + int ret;
>>
>> if (!device || !acpi_driver_data(device))
>> return -EINVAL;
>> @@ -621,8 +621,9 @@ static int acpi_processor_remove(struct
>> goto free;
>>
>> if (type == ACPI_BUS_REMOVAL_EJECT) {
>> - if (acpi_processor_handle_eject(pr))
>> - return -EINVAL;
>> + ret = acpi_processor_handle_eject(pr);
>> + if (ret)
>> + return ret;
>> }
>>
>> acpi_processor_power_exit(pr, device);
>> @@ -841,12 +842,17 @@ static acpi_status acpi_processor_hotadd
>>
>> static int acpi_processor_handle_eject(struct acpi_processor *pr)
>> {
>> - if (cpu_online(pr->id))
>> - cpu_down(pr->id);
>> + int ret;
>> +
>> + if (cpu_online(pr->id)) {
>> + ret = cpu_down(pr->id);
>> + if (ret)
>> + return ret;
>> + }
>>
>
> Strictly speaking, this is not thorough enough. What prevents someone
> from onlining that same cpu again, at this point?
> So, IMHO, you need to wrap the contents of this function inside a
> get_online_cpus()/put_online_cpus() block, to prevent anyone else
> from messing with CPU hotplug at the same time.

If I understand your comment by mistake, please let me know.
If the contents is wrapped a inside get_online_cpus()/put_online_cpus() block
as below, cpu_down() will stop since cpu_down() calls cpu_hotplug_begin() and
cpu_hotplug_begin() waits for cpu_hotplug.refcount to become 0.

+ get_online_cpus()
+ if (cpu_online(pr->id)) {
+ ret = cpu_down(pr->id);
+ if (ret)
+ return ret;
+ }
+ put_online_cpus()
I think following patch can prevent it correctly. How about the patch?

---
drivers/acpi/processor_driver.c | 12 ++++++++++++
kernel/cpu.c | 8 +++++---
2 files changed, 17 insertions(+), 3 deletions(-)
Index: linux-3.5-rc4/drivers/acpi/processor_driver.c
===================================================================
--- linux-3.5-rc4.orig/drivers/acpi/processor_driver.c 2012-07-09 09:59:01.280211202 +0900
+++ linux-3.5-rc4/drivers/acpi/processor_driver.c 2012-07-09 11:05:34.559859236 +0900
@@ -844,14 +844,26 @@ static int acpi_processor_handle_eject(s
{
int ret;

+retry:
if (cpu_online(pr->id)) {
ret = cpu_down(pr->id);
if (ret)
return ret;
}

+ get_online_cpus();
+ /*
+ * Someone might online the cpu again at this point. So we check that
+ * cpu has been onlined or not. If cpu is online, we try to offline
+ * the cpu again.
+ */
+ if (cpu_online(pr->id)) {
+ put_online_cpus();
+ goto retry;
+ }
arch_unregister_cpu(pr->id);
acpi_unmap_lsapic(pr->id);
+ put_online_cpus();
return ret;
}
#else
Index: linux-3.5-rc4/kernel/cpu.c
===================================================================
--- linux-3.5-rc4.orig/kernel/cpu.c 2012-07-09 09:59:01.280211202 +0900
+++ linux-3.5-rc4/kernel/cpu.c 2012-07-09 09:59:02.903190965 +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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