Go - mutual exclusion

• Ex. Binary semaphore
• Race Condition
  
• Semaphore
  
• Ex. sync.Mutex
  Mutex.Unlock happens before Mutex.Lock
  
• Every Lock needs Unlock, can use defer to prevent missing Unlock
  
• Go's mutex is not is not re-entrant, so can't lock a mutext which is already locked.
  To do that may cause deadlock. ex: lock in outer method and try to lock again in inner method
• In order to prevent deadlock, need have two kinds of method. Exported method acquire lock, private method assume mutex already locked