rcu: Prevent useless FQS scan after all CPUs have checked in

The force_qs_rnp() function checks for ->qsmask being all zero, that is,
all CPUs for the current rcu_node structure having already passed through
quiescent states.  But with RCU-preempt, this is not sufficient to report
quiescent states further up the tree, so there are further checks that
can initiate RCU priority boosting and also for races with CPU-hotplug
operations.  However, if neither of these further checks apply, the code
proceeds to carry out a useless scan of an all-zero ->qsmask.

This commit therefore adds code to release the current rcu_node
structure's lock and continue on to the next rcu_node structure, thereby
avoiding this useless scan.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.c

@@ -2672,6 +2672,8 @@ static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *rsp))
 				/* rcu_report_unblock_qs_rnp() rlses ->lock */
 				continue;
 			}
+			raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+			continue;
 		}
 		for_each_leaf_node_possible_cpu(rnp, cpu) {
 			unsigned long bit = leaf_node_cpu_bit(rnp, cpu);