A long time ago there was thread group code that at exit time tried to
reparent a task to another task in the thread group.  I discovered a major
race condition in this code, and submitted a patch that removed it.  This
patch was accepted in, I think, 2.4.12.  The code reappeared in 2.4.18 and
sometime in the 2.5 tree before 2.5.15, breaking applications that use
thread groups.

As part of chasing this down, I figured out a way to remove the race
condition while still preserving this behavior.  I've attached a patch
against 2.5.15 that fixes it.