Re: Anomalies while handling p_nstopchild count

[Taylor R Campbell <campbell+netbsd-tech-kern%mumble.net@localhost>]

   Date: Sat, 10 Oct 2015 16:50:42 +0800 (PHT)
   From: Paul Goyette <paul%vps1.whooppee.com@localhost>

   While continuing to track down the zombie-that-would-not-die I managed
   to find two more places where a process's p_stat and its parent's count
   of children to wait for (p_nstopchild) get out of sync.  The additional
   issues are documented in PR kern/50308 and kern/50318.

   With fixes for all four of these PRs in my local kernel, the zombie
   problem seems to have disappeared, and no other ill effects have been
   seen.  I have confirmed that at least kern/50300 was being seen in my
   local system, and correlated with the appearance of the long-lived
   zombie;  kern/50298 and kern/50308 have not been specifically observed.

Based on the analysis I just sent to one of PR 50318 (not noticing
until I was done that it applied to all four of them), the four
patches look good to me.  Please commit them separately, with a brief
analysis and PR reference in each one, so we have a chance of
bisection if anything goes wrong.

We also ought to add automatic tests for proc.12.stop{exec,exit,fork},
since the code for them looks fishy and is likely seldom exercised.