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. RCU read-side critical
sections are delimited by rcu_read_lock and rcu_read_unlock,
and may be nested.
If your read-side code is not protected by rcu_read_lock, do -not-
use synchronize_rcu.