Signal Request Not Reaching Child

[Kurtis D. Rader]

In article <1990Nov26.214748.5977 at mdivax1.uucp> mdivax1!rmosher () writes:
%
%A process (P) generates (and can kill) child processes.  Children are
%normally given a signal (kill -3) by P and kill themselves after cleaning
%up and acknowledging P's request.
%
%The problem is that (very rarely) a child process refuses to die.  No
%ack is sent to P.  When in this state, even kill -9 will not kill the
%child process.  However, the child process handles terminal i/o and if
%the terminal is shut off and on the child can be aborted.  This makes
%me think that the child is in a state where it cannot collect a software
%signal, perhaps because it is in the middle of a screen write or some
%terminal i/o.  Could this be the case?  What else could cause this?
%Work around solutions would also be appreciated.  Note: the terminal is
%set up in BLOCK mode, if that makes any difference.

About once every two weeks a customer will call me to report they have
a process that won't respond to a "kill -9".  It always turns out that
the process is in a "fast wait"; i.e., it's waiting for an event at a
non-interruptable priority.  The most common cause is a printer or
terminal that has sent an x-off to the host and was then turned off.
What happens is that the "kill -?" (HUP, QUIT, whatever) causes the
kernel to try closing the serial port.  The close however is not
allowed to complete until the pending output is flushed.  This in
turn can't occur because the port has been blocked by the x-off.  At
this point the process is waiting on the vnode for the device at a
priority less than PZERO for an event that won't occur anytime soon
(if ever).  Turning the device back on and doing whatever is necessary
to cause a x-on to be sent clears things up.  If it's a parallel
printer, tape drive or similar device a reboot is usually required.
--
Kurtis D. Rader, Technical Support Engineer    voice:    503/578-3714
Service Hotline, Sequent Computer Systems      fax:      503/578-3731
15450 SW Koll Parkway, M/S UMP2-502            UUCP:     ...uunet!sequent!krader
Beaverton, OR  97006-6063                      internet: krader at sequent.com