linux-kernel - Re: [PATCH v2 1/2] sched: fix init NOHZ

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Date:	Mon, 18 Feb 2013 15:33:26 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	linux-kernel@...r.kernel.org, linaro-dev@...ts.linaro.org,
	peterz@...radead.org, mingo@...nel.org,
	Mike Galbraith <efault@....de>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v2 1/2] sched: fix init NOHZ_IDLE flag

2013/2/8 Vincent Guittot <vincent.guittot@...aro.org>:
> On 8 February 2013 16:35, Frederic Weisbecker <fweisbec@...il.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
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