sched: Fix sched_domain iterations vs. RCU

Vladis Kletnieks reported a new RCU debug warning in the scheduler.

Since commit dce840a0 ("sched: Dynamically allocate sched_domain/
sched_group data-structures") the sched_domain trees are protected by
RCU instead of RCU-sched.

This means that we need to include rcu_read_lock() protection when we
iterate them since disabling preemption doesn't suffice anymore.

Reported-by: Valdis.Kletnieks@vt.edu
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1302882741.2388.241.camel@twinsSigned-off-by: NIngo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -1208,11 +1208,17 @@ int get_nohz_timer_target(void)
 	int i;
 	struct sched_domain *sd;
 
+	rcu_read_lock();
 	for_each_domain(cpu, sd) {
-		for_each_cpu(i, sched_domain_span(sd))
-			if (!idle_cpu(i))
-				return i;
+		for_each_cpu(i, sched_domain_span(sd)) {
+			if (!idle_cpu(i)) {
+				cpu = i;
+				goto unlock;
+			}
+		}
 	}
+unlock:
+	rcu_read_unlock();
 	return cpu;
 }
 /*
@@ -2415,12 +2421,14 @@ ttwu_stat(struct task_struct *p, int cpu, int wake_flags)
 		struct sched_domain *sd;
 
 		schedstat_inc(p, se.statistics.nr_wakeups_remote);
+		rcu_read_lock();
 		for_each_domain(this_cpu, sd) {
 			if (cpumask_test_cpu(cpu, sched_domain_span(sd))) {
 				schedstat_inc(sd, ttwu_wake_remote);
 				break;
 			}
 		}
+		rcu_read_unlock();
 	}
 #endif /* CONFIG_SMP */