infrastructure -must- respect grace periods, and -must- invoke callbacks
from a known environment in which no locks are held.
-It -is- safe for synchronize_sched() and synchronize_rcu_bh() to return
-immediately on an UP system. It is also safe for synchronize_rcu()
-to return immediately on UP systems, except when running preemptable
-RCU.
+Note that it -is- safe for synchronize_rcu() to return immediately on
+UP systems, including !PREEMPT SMP builds running on UP systems.
Quick Quiz #3: Why can't synchronize_rcu() return immediately on
UP systems running preemptable RCU?