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>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d44d0c5..88f49bc 100644 (file)
--- a/kernel/sched/core.c
+++ b/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)