sched: Use WRITE_ONCE() for p->on_rq

Since RCU-tasks uses READ_ONCE(p->on_rq), ensure the write-side
matches with WRITE_ONCE().
Signed-off-by: "Paul E. McKenney" <paulmck@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/e4896e0b-eacc-45a2-a7a8-de2280a51ecc@paulmck-laptop

kernel/sched/core.c

@@ -2124,12 +2124,14 @@ void activate_task(struct rq *rq, struct task_struct *p, int flags)
 
 	enqueue_task(rq, p, flags);
 
-	p->on_rq = TASK_ON_RQ_QUEUED;
+	WRITE_ONCE(p->on_rq, TASK_ON_RQ_QUEUED);
+	ASSERT_EXCLUSIVE_WRITER(p->on_rq);
 }
 
 void deactivate_task(struct rq *rq, struct task_struct *p, int flags)
 {
-	p->on_rq = (flags & DEQUEUE_SLEEP) ? 0 : TASK_ON_RQ_MIGRATING;
+	WRITE_ONCE(p->on_rq, (flags & DEQUEUE_SLEEP) ? 0 : TASK_ON_RQ_MIGRATING);
+	ASSERT_EXCLUSIVE_WRITER(p->on_rq);
 
 	dequeue_task(rq, p, flags);
 }