[BUG] cpu-hotplug: Can't offline the CPU with naughty realtime processes

[Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>]

Hi,

I found a bug on 2.6.21 cpu-hotplug code.

When process A on CPU0 try to offline the CPU1 on which the process B,
realtime process (its task->policy == SCHED_FIFO or SCHED_RR) running
without sleep or yield, both CPU0 and CPU1 get hang. It's because of
the following code on __stop_machine_run().

struct task_struct *__stop_machine_run(int (*fn)(void *), void *data,
				       unsigned int cpu)
{
	...
	p = kthread_create(do_stop, &smdata, "kstopmachine");
	if (!IS_ERR(p)) {
		kthread_bind(p, cpu);
		wake_up_process(p);
		wait_for_completion(&smdata.done);
	}
	...
}

kstopmachine is created, bound to the CPU1, and woken up here, but
this process can't start to run because reschedule doesn't occur on
CPU1. Hence CPU0 also be able to run because it's waiting completion
of CPU1's offline work.

Thanks,

Sat