rcu: Add test code for semaphore-like SRCU readers

This commit adds trivial test code for srcu_down_read() and
srcu_up_read().
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/update.c

@@ -555,9 +555,12 @@ struct early_boot_kfree_rcu {
 static void early_boot_test_call_rcu(void)
 {
 	static struct rcu_head head;
+	int idx;
 	static struct rcu_head shead;
 	struct early_boot_kfree_rcu *rhp;
 
+	idx = srcu_down_read(&early_srcu);
+	srcu_up_read(&early_srcu, idx);
 	call_rcu(&head, test_callback);
 	early_srcu_cookie = start_poll_synchronize_srcu(&early_srcu);
 	call_srcu(&early_srcu, &shead, test_callback);