sched, nohz: Fix missing RCU read lock

Yong Zhang reported:

 > [ INFO: suspicious RCU usage. ]
 > kernel/sched/fair.c:5091 suspicious rcu_dereference_check() usage!

This is due to the sched_domain stuff being RCU protected and
commit 0b005cf5 ("sched, nohz: Implement sched group, domain
aware nohz idle load balancing") overlooking this fact.

The sd variable only lives inside the for_each_domain() block,
so we only need to wrap that.
Reported-by: NYong Zhang <yong.zhang0@gmail.com>
Tested-by: NYong Zhang <yong.zhang0@gmail.com>
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
Link: http://lkml.kernel.org/r/1323264728.32012.107.camel@twinsSigned-off-by: NIngo Molnar <mingo@elte.hu>

kernel/sched/fair.c

@@ -5088,23 +5088,28 @@ static inline int nohz_kick_needed(struct rq *rq, int cpu)
 	if (rq->nr_running >= 2)
 		goto need_kick;
 
+	rcu_read_lock();
 	for_each_domain(cpu, sd) {
 		struct sched_group *sg = sd->groups;
 		struct sched_group_power *sgp = sg->sgp;
 		int nr_busy = atomic_read(&sgp->nr_busy_cpus);
 
 		if (sd->flags & SD_SHARE_PKG_RESOURCES && nr_busy > 1)
-			goto need_kick;
+			goto need_kick_unlock;
 
 		if (sd->flags & SD_ASYM_PACKING && nr_busy != sg->group_weight
 		    && (cpumask_first_and(nohz.idle_cpus_mask,
 					  sched_domain_span(sd)) < cpu))
-			goto need_kick;
+			goto need_kick_unlock;
 
 		if (!(sd->flags & (SD_SHARE_PKG_RESOURCES | SD_ASYM_PACKING)))
 			break;
 	}
+	rcu_read_unlock();
 	return 0;
+
+need_kick_unlock:
+	rcu_read_unlock();
 need_kick:
 	return 1;
 }