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SubjectRe: [PATCH v2 1/2] sched: fix init NOHZ_IDLE flag
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On 18 February 2013 15:38, Frederic Weisbecker <fweisbec@gmail.com> wrote:
> 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

We can clear the idle flag only in the nohz_kick_needed which will not
be called if the sched_domain is NULL so the sequence 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);
}
dom =
rcu_dereference(cpu1_dom)
//dom == NULL, return

rcu_assign_pointer(cpu1_dom, dom);

dom =
rcu_dereference(cpu1_dom)
//dom != NULL,
nohz_kick_needed {

set_idle(CPU 1)
dom
= rcu_dereference(cpu1_dom)

//dec nr_cpus_busy,
}

Vincent


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