Mutual exclusion through RFS or for Un*x files in general?

[Tim C. Hall]

  Hi all,
	Got a question for all you RFS gurus out there.  We have a   
  file mounted with read/write access across three ATT 3B2/600 machines 
  through RFS.  Is there a way to provide mutual exclusion for the
  file?  (Let as many people as necessary read it, but only allow one
  to be updating).  I've messed with the "lock" bit on files, but that
  doesn't seem to work in this case.  Oh yea, we're running 3.2.2
  System V with version 3 of both RFS and TCP/IP WIN.  Any hints
  or explanations of this subject, or mutual exclusion procedures for 
  files under Un*x in general would be vastly appreciated.  Please send
  all replies by e-mail to thall%spacecom.spacenet-gw.af.mil at server.af.mil

  Thanks in advance.

                               Mr. Tim C. Hall, System Administrator
		               Air Force Space Command
      		               thall%spacecom.spacenet-gw.af.mil at server.af.mil