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Description of problem:
I could not restart prefdm.
[root@dri//tmp]# initctl restart prefdm
initctl: Unknown instance:

Version-Release number of selected component (if applicable):
upstart-0.6.5-6.1.el6.x86_64
kdm-4.3.4-19.el6.x86_64

How reproducible:
always

Steps to Reproduce:
1. # initctl restart prefdm
  
Actual results:
initctl: Unknown instance:

Expected results:
xwindows restart

Additional info:
I use kdm as login manager

This issue has been proposed when we are only considering blocker
issues in the current Red Hat Enterprise Linux release.

** If you would still like this issue considered for the current
release, ask your support representative to file as a blocker on
your behalf. Otherwise ask that it be considered for the next
Red Hat Enterprise Linux release. **

It seems that none predfm instance isn't running. Try 'initctl status prefdm'. If it is 'stop/waiting' then you should use 'start prefdm'. This is documented in 'man restart' or initctl(8):

restart
              ...
              Note  that this command can only be used when there is an instance
              of JOB, if there is none then  it  returns  an  error  instead  of
              starting a new one.

After discussion with Petr I find that prefdm was not running and it was killed some time ago by kernel OOM killer.
I'm closing this one.