synchronize_rcu — wait until a grace period has elapsed.
| void synchronize_rcu ( | void ); | 
   Control will return to the caller some time after a full grace
   period has elapsed, in other words after all currently executing RCU
   read-side critical sections have completed.  Note, however, that
   upon return from synchronize_rcu, the caller might well be executing
   concurrently with new RCU read-side critical sections that began while
   synchronize_rcu was waiting.  RCU read-side critical sections are
   delimited by rcu_read_lock and rcu_read_unlock, and may be nested.
   
   See the description of synchronize_sched for more detailed information
   on memory ordering guarantees.