On Mon, Aug 13, 2012 at 5:52 AM, Huacai Chen [off-list ref] wrote:

When poweroff machine, kernel_power_off() call disable_nonboot_cpus().
And if we have HOTPLUG_CPU configured, disable_nonboot_cpus() is not an
empty function but attempt to actually disable the nonboot cpus. Since
system state is SYSTEM_POWER_OFF, play_dead() won't be called and thus
disable_nonboot_cpus() hangs. Therefore, we make this patch to avoid
poweroff failure.

I have seen the same problem; sometimes it causes a kernel oops too.

 #ifdef CONFIG_HOTPLUG_CPU
-               if (!cpu_online(cpu) && !cpu_isset(cpu, cpu_callin_map) &&
-                   (system_state == SYSTEM_RUNNING ||
-                    system_state == SYSTEM_BOOTING))
+               if (!cpu_online(cpu) && !cpu_isset(cpu, cpu_callin_map))
                        play_dead();

This fix works for me.

Acked-by: Kevin Cernekee <cernekee@gmail.com>

I do see SMP boot problems ("Attempted to kill the idle task!" panic)
on a 4-way box if the !cpu_isset() check is removed.