From: Andrey Borzenkov <arvidjaar@mail.ru>

The code that did proper check existed in 2.4 and was removed in 2.5 for
whatever reason. The patch restores it slightly modified as below.

2.4 code looks somewhat unclean in that

- it traverses task list without lock.

- it starts from current->real_parent but nothing prevents current be
  init_task itself.  This hung for me on 2.5 during boot.  May be 2.4 does
  something differently.