Q: force_quiescent_state && cpu_online_map

[Oleg Nesterov <oleg@redhat.com>]

I don't think this matters, but still...

	force_quiescent_state:

			 * cpu_online_map is updated by the _cpu_down()
			 * using __stop_machine(). Since we're in irqs disabled
			 * section, __stop_machine() is not exectuting, hence
			 * the cpu_online_map is stable.
			 *
			 * However,  a cpu might have been offlined _just_ before
			 * we disabled irqs while entering here.
			 * And rcu subsystem might not yet have handled the CPU_DEAD
			 * notification, leading to the offlined cpu's bit
			 * being set in the rcp->cpumask.
			 *
			 * Hence cpumask = (rcp->cpumask & cpu_online_map) to prevent
			 * sending smp_reschedule() to an offlined CPU.
			 */
			cpus_and(cpumask, rcp->cpumask, cpu_online_map);
			cpu_clear(rdp->cpu, cpumask);
			for_each_cpu_mask_nr(cpu, cpumask)
				smp_send_reschedule(cpu);

However,

	// called by __stop_machine take_cpu_down()
	arch/x86/kernel/smpboot.c:cpu_disable_common()

		/*
		 * HACK:
		 * Allow any queued timer interrupts to get serviced
		 * This is only a temporary solution until we cleanup
		 * fixup_irqs as we do for IA64.
		 */
		local_irq_enable();
		mdelay(1);
		local_irq_disable();
		...
		remove_cpu_from_maps(cpu);

So it is possible to send the ipi to the dying CPU. I know nothing
about this low-level irq code, most probably this is harmless. We
already did clear_local_APIC(), but I don't understand what it does.

Oleg.