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SubjectRe: [PATCH 1/2] acpi : cpu hot-remove returns error number when cpu_down() fails
On 07/09/2012 08:01 AM, Yasuaki Ishimatsu wrote:
> 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.
>

You are right. Sorry, I overlooked that.

> + 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)) {

How about this:
if (unlikely(cpu_online(pr->id)) {
since the probability of this happening is quite small...
> + put_online_cpus();
> + goto retry;
> + }
> arch_unregister_cpu(pr->id);
> acpi_unmap_lsapic(pr->id);
> + put_online_cpus();
> return ret;
> }

This retry logic doesn't look elegant, but I don't see any better method :-(

> #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;
> + }
> +

Firstly, why is this change needed?
Secondly, if the change is indeed an improvement, then why is it
in _this_ patch? IMHO, in that case it should be part of a separate patch.

Coming back to my first point, I don't see why this hunk is needed. We
already take the cpu_add_remove_lock (cpu_maps_update_begin/end) before
checking the status of the cpu (online or present). And all hotplug
operations (cpu_up/cpu_down/disable|enable_nonboot_cpus) go through that
lock. Isn't that enough? Or am I missing something?
> idle = idle_thread_get(cpu);
> if (IS_ERR(idle)) {
> ret = PTR_ERR(idle);
>

Regards,
Srivatsa S. Bhat



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