rewrite the waiting/synchronous mechanisms: - [x] workqueue - [ ] completion - [ ] thread - [ ] rcu --- reference - rcu: [1](https://github.com/urcu/userspace-rcu/blob/master/include/urcu/static/urcu-memb.h#L128), [2](https://github.com/urcu/userspace-rcu/blob/master/src/urcu.c)
