[wplug] kill QUESTION -> mount question

[Michael Skowvron]

Robert E. Coutch wrote:

> Is there a way to get rid of a sleeping or zombie process other than by a 
> reboot?

Generally, no. However, zombie processes consume no resources. The 
zombie state is when a process has completely terminated and all 
resources have been freed except the processes entry in the process 
table. This is usually a very short term event, because on exit, the 
process sends a signal to the parent indicating that it has terminated 
and the parent processes frees the remaining resources.

If the parent process exits before the child, the child may become a 
zombie on exit. A process can fork a child and specify whether to 
catch or ignore the termination signal from the child. If the process 
is forked, SIGCLD is _not_ ignored, and then the parent exits; you've 
created a zombie.

Any way you look at it, it's a bug in the parent process. If the 
process was supposed to terminate before the child and didn't specify 
ignoring the SIGCLD -- it's a bug. Otherwise, the parent must have 
exited abmormally which would also be a bug.

You can ignore zombie processes as long as you want. They consume no 
resources other than the few bytes that are left in the process table. 
They cannot be scheduled, so they cannot consume any CPU.

Perpetually sleeping processes are often caused by bugs in the kernel. 
  A sleeping prcesses is awaiting for some action to happen in the 
kernel before it can proceed. Your kill signal gets posted alright, 
but the signal isn't seen by the process until it exits a kernel wait 
state. Since it's stuck in the kernel wait state it can never see the 
signal and can't be killed.

You have to reboot to clear this condition. Not rebooting may also 
lead to even more processes getting stuck.

Michael