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SubjectRe: [PATCH v2 1/2] sched: fix init NOHZ_IDLE flag
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2013/2/18 Frederic Weisbecker <fweisbec@gmail.com>:
> 2013/2/8 Vincent Guittot <vincent.guittot@linaro.org>:
>> On 8 February 2013 16:35, Frederic Weisbecker <fweisbec@gmail.com> wrote:
>>> What if the following happen (inventing function names but you get the idea):
>>>
>>> CPU 0 CPU 1
>>>
>>> dom = new_domain(...) {
>>> nr_cpus_busy = 0;
>>> set_idle(CPU 1); old_dom =get_dom()
>>> clear_idle(CPU 1)
>>> }
>>> rcu_assign_pointer(cpu1_dom, dom);
>>>
>>>
>>> Can this scenario happen?
>>
>> This scenario will be:
>>
>> CPU 0 CPU 1
>>
>> detach_and_destroy_domain {
>> rcu_assign_pointer(cpu1_dom, NULL);
>> }
>>
>> dom = new_domain(...) {
>> nr_cpus_busy = 0;
>> set_idle(CPU 1); old_dom =get_dom()
>> old_dom is null
>> //clear_idle(CPU
>> 1) can't happen because a null domain is attached so we will never
>> call nohz_kick_needed which is the only place where we can clear_idle
>> }
>> rcu_assign_pointer(cpu1_dom, dom);
>
> So is the following possible?
>
> = CPU 0 = = CPU 1=
>
> detach_and_destroy_domain {
> rcu_assign_pointer(cpu1_dom, NULL);
> }
>
> dom = new_domain(...) {
> nr_cpus_busy = 0;
> set_idle(CPU 1);
> }
>
> clear_idle(CPU 1)
>
> dom = rcu_dereference(cpu1_dom)
>
> //dom == NULL, return
>
> rcu_assign_pointer(cpu1_dom, NULL);
>
>
> set_idle(CPU 1)
>
> dom = rcu_dereference(cpu1_dom)
>
> //dec nr_cpus_busy, making it negative

Sorry, gmail messed up as usual.

I pasted the original at: http://pastebin.com/DMm5U8J8


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