rcu: Convert ->need_future_gp[] array to boolean

There is no longer any need for ->need_future_gp[] to count the number of
requests for future grace periods, so this commit converts the additions
to assignments to "true" and reduces the size of each element to one byte.
While we are in the area, fix an obsolete comment.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>

kernel/rcu/tree.c

@@ -1709,7 +1709,7 @@ rcu_start_future_gp(struct rcu_node *rnp, struct rcu_data *rdp,
 	 * current grace period, we don't need to explicitly start one.
 	 */
 	if (rnp->gpnum != rnp->completed) {
-		need_future_gp_element(rnp, c)++;
+		need_future_gp_element(rnp, c) = true;
 		trace_rcu_future_gp(rnp, rdp, c, TPS("Startedleaf"));
 		goto out;
 	}
@@ -1741,7 +1741,7 @@ rcu_start_future_gp(struct rcu_node *rnp, struct rcu_data *rdp,
 	}
 
 	/* Record the need for the future grace period. */
-	need_future_gp_element(rnp_root, c)++;
+	need_future_gp_element(rnp_root, c) = true;
 
 	/* If a grace period is not already in progress, start one. */
 	if (rnp_root->gpnum != rnp_root->completed) {
@@ -1769,7 +1769,7 @@ static bool rcu_future_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
 	bool needmore;
 	struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
 
-	need_future_gp_element(rnp, c) = 0;
+	need_future_gp_element(rnp, c) = false;
 	needmore = need_any_future_gp(rnp);
 	trace_rcu_future_gp(rnp, rdp, c,
 			    needmore ? TPS("CleanupMore") : TPS("Cleanup"));

kernel/rcu/tree.h

@@ -150,7 +150,7 @@ struct rcu_node {
 	struct swait_queue_head nocb_gp_wq[2];
 				/* Place for rcu_nocb_kthread() to wait GP. */
 #endif /* #ifdef CONFIG_RCU_NOCB_CPU */
-	int need_future_gp[4];	/* Counts of upcoming no-CB GP requests. */
+	u8 need_future_gp[4];	/* Counts of upcoming GP requests. */
 	raw_spinlock_t fqslock ____cacheline_internodealigned_in_smp;
 
 	spinlock_t exp_lock ____cacheline_internodealigned_in_smp;