rcu: Add RCU-preempt check for waiting on newly onlined CPU

RCU should only be waiting on CPUs that were online at the time that the
current grace period started.  Failure to abide by this rule can result
in confusing splats during grace-period cleanup and initialization.
This commit therefore adds a check to RCU-preempt's preempted-task
queuing that checks for waiting on newly onlined CPUs.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree_plugin.h

@@ -183,6 +183,9 @@ static void rcu_preempt_ctxt_queue(struct rcu_node *rnp, struct rcu_data *rdp)
 	raw_lockdep_assert_held_rcu_node(rnp);
 	WARN_ON_ONCE(rdp->mynode != rnp);
 	WARN_ON_ONCE(!rcu_is_leaf_node(rnp));
+	/* RCU better not be waiting on newly onlined CPUs! */
+	WARN_ON_ONCE(rnp->qsmaskinitnext & ~rnp->qsmaskinit & rnp->qsmask &
+		     rdp->grpmask);
 
 	/*
 	 * Decide where to queue the newly blocked task.  In theory,