Xref: utzoo comp.unix.questions:6796 comp.unix.wizards:8158 comp.unix.microport:565
Path: utzoo!mnetor!uunet!husc6!bloom-beacon!gatech!udel!princeton!mccc!pjh
From: pjh@mccc.UUCP (Pete Holsberg)
Newsgroups: comp.unix.questions,comp.unix.wizards,comp.unix.microport
Subject: Re: Trouble killing processes in SysV/AT
Message-ID: <608@mccc.UUCP>
Date: 30 Apr 88 15:05:30 GMT
References: <3950@killer.UUCP> <3951@killer.UUCP> <468@micropen>
Reply-To: pjh@mccc.UUCP (Pete Holsberg)
Organization: The College On The Other Side of Route 1
Lines: 31

In article <468@micropen> dave@micropen (David F. Carlson) writes:
...In article <3951@killer.UUCP>, wnp@killer.UUCP (Wolf Paul) writes:
...> Can anyone enlighten me as to what causes a process to become "immortal"
...> in System VR2,  or Microport UNIX System V/AT, to be more specific?
...> 
...This "prblem" is not a Micrport issue at all: it is UNIX all the way.
...
...> I have encountered this a number of times, where it would be impossible
...> even for root to kill a process; if the parent process of the "immortal"
...> process is killed, the child attaches itself to init, PID 1.
...> What causes a process to refuse to die? I thought signal 9 (kill) could
...> not be intercepted or ignored?
...
...If you are technically minded and want a real answer read:
...	"The Design of the UNIX Operating System" by Maurice Bach.
...
...The quick answer is that any process that is in the kernel with a WCHAN
...will not go back to user mode until that channel is awoken.  Who will
...awaken it?  Two choices:  a device driver interrupt or a kernel timer
...interrupt.  In all likelihood your ill-behaved process is waiting in
...a poorly written device driver close().  No close should allow a process
...to wait forever on a event that may not come.  Signals (kill -9) are
...delivered when a process in kernel mode re-enters user mode.  However,
...you process is waiting in kernel mode and won't get those signals til
...its done: NEVER! (or until the long sought interrupt allows it's WCHAN
...to go again.

This happens frequently on my 3B2/400 when it gets into a deadly embrace
with my modem:  I cannot kill -9 any of the processes associated with
that port!  It takes toggling the modem's ON/OFF switch to break the
embrace.  Surely there must be a better way!  ??