RFC: Deadlock in multithreaded application when doing IO and fork.

[Roland McGrath]

> The fix actually requires two things:
> (a) Run all atfork prepare handlers.
> (b) Lock list_all_lock to prevent more IO.
> (c) After (b) run malloc atfork handler e.g. run handler last.

I don't understand the (b) requirement.  It's not required to fix the
deadlock test case I described (there is no stdio use at all in that case).
Whatever it's about, it seems like it should be a follow-up change after
the basic change to run malloc atfork last.