On Wed, Dec 16, 2009 at 4:24 PM, Sachin Sant <sachinp@xxxxxxxxxx> wrote:
> Xiaotian Feng wrote:
>>
>> Could follow be possible? We know there's cpu 0 and cpu 1,
>>
>> offline cpu1 > done
>> offline cpu0 > false
>>
>> consider this in cpu_down code,
>>
>>
>> int __ref cpu_down(unsigned int cpu)
>> {
>> <snip>
>> set_cpu_active(cpu, false); // here, we set cpu 0 to inactive
>>
>> synchronize_sched();
>>
>> err = _cpu_down(cpu, 0);
>> out:
>> <snip>
>> }
>>
>> Then in _cpu_down code:
>>
>> static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
>> {
>> <snip>
>> if (num_online_cpus() == 1) // if we're trying to
>> offline cpu0, num_online_cpus will be 1
>> return -EBUSY; // after return back
>> to cpu_down, we didn't change cpu 0 back to active
>>
>> if (!cpu_online(cpu))
>> return -EINVAL;
>>
>> if (!alloc_cpumask_var(&old_allowed, GFP_KERNEL))
>> return -ENOMEM;
>> <snip>
>> }
>>
>> Then cpu 0 is not active, but online, then we try to offline cpu1, .......
>> This can not be exposed because x86 does not have
>> /sys/devices/system/cpu0/online.
>> I guess following patch fixes this bug.
>>
>
> Just tested this one on the POWER box and the test passed.
> I did not observe the hang.
Thanks for confirm, I will send formatted patch to upstream then:-)
>
> Thanks
> -Sachin
>
>> ---
>> diff --git a/kernel/cpu.c b/kernel/cpu.c
>> index 291ac58..21ddace 100644
>> --- a/kernel/cpu.c
>> +++ b/kernel/cpu.c
>> @@ -199,14 +199,18 @@ static int __ref _cpu_down(unsigned int cpu, int
>> tasks_frozen)
>> .hcpu = hcpu,
>> };
>>
>> - if (num_online_cpus() == 1)
>> + if (num_online_cpus() == 1) {
>> + set_cpu_active(cpu, true);
>> return -EBUSY;
>> + }
>>
>> if (!cpu_online(cpu))
>> return -EINVAL;
>>
>> - if (!alloc_cpumask_var(&old_allowed, GFP_KERNEL))
>> + if (!alloc_cpumask_var(&old_allowed, GFP_KERNEL)) {
>> + set_cpu_active(cpu, true);
>> return -ENOMEM;
>> + }
>>
>> cpu_hotplug_begin();
>> err = __raw_notifier_call_chain(&cpu_chain, CPU_DOWN_PREPARE | mod,
>>
>>
>>
>>>
>>> Unless of course, I messed up, which appears to be rather likely given
>>> these problems ;-)
>>>
>>>
>>>
>>
>>
>
>
> --
>
> ---------------------------------
> Sachin Sant
> IBM Linux Technology Center
> India Systems and Technology Labs
> Bangalore, India
> ---------------------------------
>
>
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