rcu_barrier — 
     Wait until all in-flight call_rcu callbacks complete.
 
| void rcu_barrier ( | void ); | 
   Note that this primitive does not necessarily wait for an RCU grace period
   to complete.  For example, if there are no RCU callbacks queued anywhere
   in the system, then rcu_barrier is within its rights to return
   immediately, without waiting for anything, much less an RCU grace period.