cpu hotplug: adjust root-domain->online span in response to hotplug event

We currently set the root-domain online span automatically when the
domain is added to the cpu if the cpu is already a member of
cpu_online_map.

This was done as a hack/bug-fix for s2ram, but it also causes a problem
with hotplug CPU_DOWN transitioning.  The right way to fix the original
problem is to actually respond to CPU_UP events, instead of CPU_ONLINE,
which is already too late.

This solves the hung reboot regression reported by Andrew Morton and
others.
Signed-off-by: NGregory Haskins <ghaskins@novell.com>
Acked-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/sched.c

@@ -5813,6 +5813,13 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
 		/* Must be high prio: stop_machine expects to yield to it. */
 		rq = task_rq_lock(p, &flags);
 		__setscheduler(rq, p, SCHED_FIFO, MAX_RT_PRIO-1);
+
+		/* Update our root-domain */
+		if (rq->rd) {
+			BUG_ON(!cpu_isset(cpu, rq->rd->span));
+			cpu_set(cpu, rq->rd->online);
+		}
+
 		task_rq_unlock(rq, &flags);
 		cpu_rq(cpu)->migration_thread = p;
 		break;
@@ -5821,15 +5828,6 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
 	case CPU_ONLINE_FROZEN:
 		/* Strictly unnecessary, as first user will wake it. */
 		wake_up_process(cpu_rq(cpu)->migration_thread);
-
-		/* Update our root-domain */
-		rq = cpu_rq(cpu);
-		spin_lock_irqsave(&rq->lock, flags);
-		if (rq->rd) {
-			BUG_ON(!cpu_isset(cpu, rq->rd->span));
-			cpu_set(cpu, rq->rd->online);
-		}
-		spin_unlock_irqrestore(&rq->lock, flags);
 		break;
 
 #ifdef CONFIG_HOTPLUG_CPU
@@ -6105,8 +6103,6 @@ static void rq_attach_root(struct rq *rq, struct root_domain *rd)
 	rq->rd = rd;
 
 	cpu_set(rq->cpu, rd->span);
-	if (cpu_isset(rq->cpu, cpu_online_map))
-		cpu_set(rq->cpu, rd->online);
 
 	for (class = sched_class_highest; class; class = class->next) {
 		if (class->join_domain)