sched domains bringup race?

[Dave Hansen <haveblue@us.ibm.com>]

I keep getting oopses for the non-boot CPU in find_busiest_group(). 
This occurs the first time that the CPU goes idle.  Those groups are set
up in sched_init_smp(), which is called after smp_init():

static int init(void * unused)
{
	...
        fixup_cpu_present_map();
        smp_init();
        sched_init_smp();

But, the idle threads for the secondary CPUs are initialized in
smp_init().  So, what happens when a CPU tries to schedule (using sched
domains) before sched_init_smp() completes?  I think it goes boom! :)

Anyway, I was thinking that we should just hold the runqueue lock on the
non-boot CPUs until the sched domain init code is done.  Does that sound
feasible?

-- Dave