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Date: Tue, 10 Jul 2012 12:22:41 +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 10:44 AM, Yasuaki Ishimatsu wrote:
> Hi Srivatsa,
>
> 2012/07/10 9:13, 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,
>> Yasuaki Ishimatsu
>>
>>> 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.
>
> I forget to answer the question.
> As I answered in the above your first question, the fix is related to
> acpi_processor_handle_eject(). So the fix should be in the patch.
>
Yep, got it now. Thanks!
Regards,
Srivatsa S. Bhat
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