release_task can sleep.  Sleeping allows a CPU to go down underneath you.
release_task removes you from the tasklist, so you don't get migrated off the
CPU: BUG() in sched.c.

In last week's episode, our dashing hero (Ingo Molnar) solved this for
self-reaping tasks by grabbing the hotplug cpu lock to prevent this. 
However, in an unexpected twist, the problem remains for tasks whose
parents call release_task on them: the zombies are off the task list, and
lurk on the dead CPU.

Fortunately, the comedic sidekick (Rusty Russell) has an answer: let's make
the hotplug callback walk the runqueue of the dead CPU as well, taking care
of the zombies.

1) Restore exit.c to its former form.  The comment is incorrect: sched.c
   checks PF_DEAD, not the state, to decide to do the final
   put_task_struct(), and it does it for all tasks, self-reaping or no.

2) Implement migrate_dead_tasks() in the sched.c hotplug CPU callback.

3) Rename migrate_all_tasks() to migrate_live_tasks().
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>