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

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Date:	Mon, 18 Feb 2013 16:40:40 +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

2013/2/18 Vincent Guittot <vincent.guittot@...aro.org>:
> On 18 February 2013 15:38, Frederic Weisbecker <fweisbec@...il.com> wrote:
>> 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

Ok but CPU 0 can assign NULL to the domain of cpu1 while CPU 1 is
already in the middle of nohz_kick_needed().
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