In reply to caowangyang:
The LRM states
“If task enable statements are nested (that is, one task enables another, and that one enables yet another), then disabling a task within the chain shall disable all tasks downward on the chain.”
and
“The disable fork statement terminates all descendants of the calling process as well as the descendants of the process’s descendants. In other words, if any of the child processes have descendants of their own, the disable fork statement shall terminate them as well.”.
I’d say the description is pretty clear.