PROBLEM: (53741) (PATCH ID: OSF445-255)
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This patch fixes a kernel memory fault in pgrp_ref().
The stack trace is listed below:

>  0 thread_block() 
    1 thread_preempt 
    2 boot 
    3 panic 
    4 trap 
    5 _XentMM
    6 pgrp_ref 
    7 exit 
    8 rexit 
    9 syscall 
   10 _Xsyscall