Version: 2.6.18-1.2747.el5

Is seems that fix 5ecfbae093f0c37311e89b29bfc0c9d586eace87 aka

    [PATCH] fix zap_thread's ptrace related problems
    
    1. The tracee can go from ptrace_stop() to do_signal_stop()
       after __ptrace_unlink(p).
    
    2. It is unsafe to __ptrace_unlink(p) while p->parent may wait
       for tasklist_lock in ptrace_detach().

somehow not working anymore.

Attached program will create unkillable processes, but won't do it
on 2.6.20-rc7.