slab: start_cpu_timer/cache_reap CONFIG_HOTPLUG_CPU problems

[Oleg Nesterov <oleg@tv-sign.ru>]

For the beginning, about another (but related) minor problem,

	debug_smp_processor_id:

		/*
		 * Kernel threads bound to a single CPU can safely use
		 * smp_processor_id():
		 */

This is only true without CONFIG_HOTPLUG_CPU. Otherwise CPU can go away when
the task takes a preemption or sleeps. I think we need #ifndef here.


Now,
	static void __devinit start_cpu_timer(int cpu)
	{
		struct delayed_work *reap_work = &per_cpu(reap_work, cpu);

		if (keventd_up() && reap_work->work.func == NULL) {
			init_reap_node(cpu);
			INIT_DELAYED_WORK(reap_work, cache_reap);
			schedule_delayed_work_on(cpu, reap_work,
						__round_jiffies_relative(HZ, cpu));
		}
	}

This is wrong. Suppose we have a CPU_UP,CPU_DOWN,CPU_UP sequence. The last
CPU_UP will not restart a per-cpu "cache_reap timer".


With or without recent changes, it is possible that work->func() will run on
another CPU (not that to which it was submitted) if CPU goes down. In fact,
this can happen while work->func() is running, so even smp_processor_id()
is not safe to use in work->func().

However, cache_reap() seems to wrongly assume that smp_processor_id() is stable,
this is the second problem.

Is my understanding correct?

Oleg.