Re: Killing a zombie process?

[Paul Goyette <paul%vps1.whooppee.com@localhost>]

On Wed, 30 Sep 2015, Robert Elz wrote:

>    Date:        Wed, 30 Sep 2015 15:55:04 +0800 (PHT)
>    From:        Paul Goyette <paul%vps1.whooppee.com@localhost>
>    Message-ID:  <Pine.NEB.4.64.1509301551450.12641%vps1.whooppee.com@localhost>
>
>  | So there must be some difference in how init(8) waits during normal
>  | operation and how it waits during the transition to single-user.
>
> Either that (which isn't really all that likely I'd guess) ...

Well, a quick read through sbin/init.c shows that sometimes it waits 
with WNOHANG and sometimes it doesn't.  I haven't figured out the actual 
code-flow yet, so I can't tell if this accounts for the steady-state vs 
transition-to-single-user difference or not.

> ... or perhaps
> the process is not yet linked to init, so can't be waited upon.   It
> needs to be on init's child queue for wait to find it, regardless of
> what the ppid has been set to.

Well, for the previous occurrence, I waited many hours, and the zombie 
was still there.  (It might even have been as much as a couple of days.) 
In today's event, the 'shutdown' transition was run less than one hour 
after the first notice, and at _that_ time the zombie was reaped.  It 
doesn't seem logical that the ppid gets set, but it gets enqueued only 
after starting a shutdown.

> I think I'd be checking out the sequence in the sys_exit() code, to see
> if there's anything that happens, or could happen, between setting
> the ppid to 1 and linking the process onto process 1's child list that
> could perhaps block and cause the zombie to just sit there (for this,
> once the process status is Z, you can't really trust some of the other
> ps output, pid and ppid should be correct, but whan is unlikely to have
> any meaning).

Yeah, I'll have a look at the sys_exit() code and see what I can find. 
If I get really brave, I might even use gdb to attach to init(8) and see 
which of the several waitpid() calls is active.  (I'd prefer to do this 
in a qemu VM, but then I'd need to reproduce the entire environment 
inside the VM.)



+------------------+--------------------------+-------------------------+
| Paul Goyette     | PGP Key fingerprint:     | E-mail addresses:       |
| (Retired)        | FA29 0E3B 35AF E8AE 6651 | paul at whooppee.com    |
| Kernel Developer | 0786 F758 55DE 53BA 7731 | pgoyette at netbsd.org  |
+------------------+--------------------------+-------------------------+