rcu: Advance outgoing CPU's callbacks before migrating them

It is possible that the outgoing CPU is unaware of recent grace periods,
and so it is also possible that some of its pending callbacks are actually
ready to be invoked.  The current callback-migration code would needlessly
force these callbacks to pass through another grace period.  This commit
therefore invokes rcu_advance_cbs() on the outgoing CPU's callbacks in
order to give them full credit for having passed through any recent
grace periods.

This also fixes an odd theoretical bug where there are no callbacks in
the system except for those on the outgoing CPU, none of those callbacks
have yet been associated with a grace-period number, there is never again
another callback registered, and the surviving CPU never again takes a
scheduling-clock interrupt, never goes idle, and never enters nohz_full
userspace execution.  Yes, this is (just barely) possible.  It requires
that the surviving CPU be a nohz_full CPU, that its scheduler-clock
interrupt be shut off, and that it loop forever in the kernel.  You get
bonus points if you can make this one happen!  ;-)
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.c

@@ -3926,6 +3926,7 @@ static void rcu_migrate_callbacks(int cpu, struct rcu_state *rsp)
 	struct rcu_data *my_rdp;
 	struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
 	struct rcu_node *rnp = rdp->mynode;  /* Outgoing CPU's rdp & rnp. */
+	struct rcu_node *rnp_root = rcu_get_root(rdp->rsp);
 
 	if (rcu_is_nocb_cpu(cpu) || rcu_segcblist_empty(&rdp->cblist))
 		return;  /* No callbacks to migrate. */
@@ -3936,7 +3937,11 @@ static void rcu_migrate_callbacks(int cpu, struct rcu_state *rsp)
 		local_irq_restore(flags);
 		return;
 	}
-	raw_spin_lock(&rsp->orphan_lock); /* irqs already disabled. */
+	raw_spin_lock_rcu_node(rnp_root); /* irqs already disabled. */
+	rcu_advance_cbs(rsp, rnp_root, rdp); /* Leverage recent GPs. */
+	raw_spin_unlock_rcu_node(rnp_root);
+
+	raw_spin_lock(&rsp->orphan_lock);
 	rcu_send_cbs_to_orphanage(cpu, rsp, rnp, rdp);
 	rcu_adopt_orphan_cbs(rsp, flags);
 	raw_spin_unlock_irqrestore(&rsp->orphan_lock, flags);