rcu: Make rcu_init_new_rnp() stop upon already-set bit

Currently, rcu_init_new_rnp() walks up the rcu_node combining tree,
setting bits in the ->qsmaskinit fields on the way up.  It walks up
unconditionally, regardless of the initial state of these bits.  This is
OK because only the corresponding RCU grace-period kthread ever tests
or sets these bits during runtime.  However, it is also pointless, and
it increases both memory and lock contention (albeit only slightly), so
this commit stops the walk as soon as an already-set bit is encountered.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.c

@@ -3479,9 +3479,10 @@ EXPORT_SYMBOL_GPL(rcu_barrier_sched);
 static void rcu_init_new_rnp(struct rcu_node *rnp_leaf)
 {
 	long mask;
+	long oldmask;
 	struct rcu_node *rnp = rnp_leaf;
 
-	raw_lockdep_assert_held_rcu_node(rnp);
+	raw_lockdep_assert_held_rcu_node(rnp_leaf);
 	WARN_ON_ONCE(rnp->wait_blkd_tasks);
 	for (;;) {
 		mask = rnp->grpmask;
@@ -3489,8 +3490,11 @@ static void rcu_init_new_rnp(struct rcu_node *rnp_leaf)
 		if (rnp == NULL)
 			return;
 		raw_spin_lock_rcu_node(rnp); /* Interrupts already disabled. */
+		oldmask = rnp->qsmaskinit;
 		rnp->qsmaskinit |= mask;
 		raw_spin_unlock_rcu_node(rnp); /* Interrupts remain disabled. */
+		if (oldmask)
+			return;
 	}
 }