Re: is killing zombies possible w/o a reboot?

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To: linux-kernel@vger.kernel.org
Date:	Thu, 4 Nov 2004 19:29:13 -0500

On Wednesday 03 November 2004 15:48, Tom Felker wrote:
[...]
>> Isn't there some way to clean up a &^$#^#@)_ zombie?
>
>Ok, let me try to explain what probably happened.
>
>First, terminology. When one process wants to be come two
> processes, it fork()s. One process is the parent, and one it the
> child. The child usually exec()s to become a different program.
> The parent sometimes wants to know when the child ends and whether
> it succeeded. Thus, the wait() system calls. The parent can either
> check whether a child died, or go to sleep until one does. When
> the parent is awaken, it's told which child died and what the
> child's exit status was (usually 0 for success). But if the child
> dies before the parent wait()s, the kernel must keep a record of
> which child died and what its exit status was, and it can't
> reassign the late child's PID yet. This record is a "zombie," and
> shows up under top or ps with the 'Z' state. Zombies do _not_ hold
> open files, memory, or resources of any kind.
>
>That's the technical definition of a zombie, which I'm telling you
> because that's probably not your situation: I assume you used
> "zombie" as an informal term for a process that you can't kill.
> Your problem is a process in uninterruptible sleep (the "D" state).
>
>When a process executing in userspace wants information from a
> device, like a disk or TV capture card, it calls read(), and
> context switches into kernel space. Usually, it will take a moment
> for the data to be available from the device, so the process gets
> put on a wait queue so other processes can run. Obviously nothing
> is deallocated, because everyone expects the process will get it's
> data and proceed as normal. When the device has the data, it
> interrupts the CPU, and the kernel figures out who wanted the data
> and puts them on the run queue.
>
>When a process is on a wait queue waiting for data from a device
> (the D state), it's impossible to kill. This is because otherwise,
> when the interrupt did come, the structures associated with the
> process would have been freed, and the kernel would crash. It
> would require an incredible amount of innefficient bookkeeping to
> avoid this, and it's unnecessary because normally, the data request
> will finish (successfully or not), and the process will be woken
> up, or if it was sent SIGKILL, it will be killed.
>
>Long story short, what happened was, some faulty hardware or some
> buggy driver, probably associated with the capture card, had a
> problem and left the process in D state. Thus, it couldn't be
> killed, and since it had /dev/video open, tvtime couldn't run and
> failed gracefully, and because it held /dev/dsp open, and couldn't
> be killed as the init scripts would normally do in that situation,
> the audio drivers couldn't be unloaded and the boot process hung.
>
>So give us a bunch of information about what hardware you're using,
> output of dmesg, and steps to reproduce the driver bug (if it is
> that).

I cannot do that as it apparently was a transient thing. After the
reboot to the next kernel in the series, everythings has been working
as well as can be expected. I've listened to the radio for about 30
seconds, and the tv maybe 6 hours since.
Now that I know howto make the magic sysrq actually work and leave
meaningfull stuff in the logs, maybe I can report something that
might be constructive the next time it happens. Until then, I wait
for the other shoe I guess.

-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
99.28% setiathome rank, not too shabby for a WV hillbilly
Yahoo.com attorneys please note, additions to this message
by Gene Heskett are:
Copyright 2004 by Maurice Eugene Heskett, all rights reserved.
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• Previous message: Kyle Moffett: "Re: Possible GPL infringement in Broadcom-based routers"
• In reply to: Tom Felker: "Re: is killing zombies possible w/o a reboot?"
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