kernel/cpu.c: Add comment for priority in cpu_hotplug_pm_callback

cpu_hotplug_pm_callback should have higher priority than
bsp_pm_callback which depends on cpu_hotplug_pm_callback to disable cpu hotplug
to avoid race during bsp online checking.

This is to hightlight the priorities between the two callbacks in case people
may overlook the order.

Ideally the priorities should be defined in macro/enum instead of fixed values.
To do that, a seperate patchset may be pushed which will touch serveral other
generic files and is out of scope of this patchset.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Link: http://lkml.kernel.org/r/1352835171-3958-7-git-send-email-fenghua.yu@intel.com
Reviewed-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

kernel/cpu.c

@@ -601,6 +601,11 @@ cpu_hotplug_pm_callback(struct notifier_block *nb,
 
 static int __init cpu_hotplug_pm_sync_init(void)
 {
+	/*
+	 * cpu_hotplug_pm_callback has higher priority than x86
+	 * bsp_pm_callback which depends on cpu_hotplug_pm_callback
+	 * to disable cpu hotplug to avoid cpu hotplug race.
+	 */
 	pm_notifier(cpu_hotplug_pm_callback, 0);
 	return 0;
 }