* rename file to make it more explicit
* thread has list for iteration for its child thread and reference to its root thread
 - to make iterate all the system thread easy
 - enhance error handling stability
  -> in some critical error, process and all of its child threads will be killed
   - based on thought
     threads within a process usually dependent on each other
     so any single thread can not keep operating properly, it's better to kill
     all relevant thread instead of just one