Hi Alan, Oleg.

I changed the patch the way you asked.
It now consumes much fewer per-thread bytes
(2 bytes + padding), and does much fewer suspicious
things on the hot path. And no zombies with ppid=1.

The attached patch adds do_signal_parent() function.
It is the same as do_notify_parent(), but can be used
when the child is not terminating, but (for example)
detaches with PR_DETACH.