sched: Add missing rcu protection to wake_up_all_idle_cpus

Locklessly doing is_idle_task(rq->curr) is only okay because of
RCU protection.  The older variant of the broken code checked
rq->curr == rq->idle instead and therefore didn't need RCU.

Fixes: f6be8af1c95d ("sched: Add new API wake_up_if_idle() to wake up the idle cpu")
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Reviewed-by: Chuansheng Liu <chuansheng.liu@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/729365dddca178506dfd0a9451006344cd6808bc.1417277372.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/core.c

@@ -1628,8 +1628,10 @@ void wake_up_if_idle(int cpu)
 	struct rq *rq = cpu_rq(cpu);
 	unsigned long flags;
 
-	if (!is_idle_task(rq->curr))
-		return;
+	rcu_read_lock();
+
+	if (!is_idle_task(rcu_dereference(rq->curr)))
+		goto out;
 
 	if (set_nr_if_polling(rq->idle)) {
 		trace_sched_wake_idle_without_ipi(cpu);
@@ -1640,6 +1642,9 @@ void wake_up_if_idle(int cpu)
 		/* Else cpu is not in idle, do nothing here */
 		raw_spin_unlock_irqrestore(&rq->lock, flags);
 	}
+
+out:
+	rcu_read_unlock();
 }
 
 bool cpus_share_cache(int this_cpu, int that_cpu)