Re: [Queue-developers] [patch] Same job launched on multiple hosts, unlink errors

[Mike C. <da...@ix...>]

On Thu, May 03, 2001 at 11:10:10PM -0700, Cyril Bortolato wrote:
> I came up with the patch to queued.c below. In startjob() I check if
> there's already an efmXXX file for the job passed to startjob(). If
> there is, it means that job is already running on some host and we'd
> better not start it again. So I set the job's pid accordingly and

There is still a race condition here, unfortunately.  The efm file could
still show up after you look for it but before you create it.  You've
reduced the window, but not eliminated it.

One solution might be from the linux open(2) man page:

       O_EXCL When  used with O_CREAT, if the file already exists
              it is an error and the open will fail.   O_EXCL  is
              broken  on NFS file systems, programs which rely on
              it for performing locking tasks will contain a race
              condition.  The solution for performing atomic file
              locking using a lockfile is to create a unique file
              on  the  same  fs (e.g., incorporating hostname and
              pid), use link(2) to make a link to  the  lockfile.
              If  link() returns 0, the lock is successful.  Oth<AD>
              erwise, use stat(2) on the unique file to check  if
              its  link  count  has increased to 2, in which case
              the lock is also successful.

We can't necessarily rely on flock working (not everyone has a working
lockd).  We could build in a lock protocol into queue, but there are so
many other things broken with queue right now it's not even funny. (I've
pretty much given up on queue for now and wrote a few cheesy shell scripts
that work much better.)

> return ALREADY_LOCKED.

mrc
-- 
       Mike Castle       Life is like a clock:  You can work constantly
  da...@ix...  and be right all the time, or not work at all
www.netcom.com/~dalgoda/ and be right at least twice a day.  -- mrc
    We are all of us living in the shadow of Manhattan.  -- Watchmen