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Date: Tue, 10 Jul 2012 12:21:24 +0530
From: "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To: Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
CC: linux-acpi@...r.kernel.org, lenb@...nel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] acpi : cpu hot-remove returns error number when cpu_down()
fails
On 07/10/2012 05:43 AM, Yasuaki Ishimatsu wrote:
> Hi Srivatsa,
>
> 2012/07/09 20:25, Srivatsa S. Bhat wrote:
>> 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@...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...
>
> Thanks. I'll update it.
>
>>> + 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?
>
> I cared the race of hot-remove cpu and _cpu_up(). If I do 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 I change it, I think 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 |
>
> Thus I think the change is necessary.
>
Thanks for the detailed explanation. I had missed this race condition.
Now I see why all the changes in your patch are needed.
Regards,
Srivatsa S. Bhat
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