Problem with FileLock

Hi experts,

We need to update a file (on a file server) from many different
machines. To synchromize the updates I am using FileLock.

Everything works as advertised (ie if a machine gets the lock,
it writes to the file while the other machines wait, everything is
written in the proper order). The same test, using Linux machines,
fails: the file is clobbered (meaning one update gets on top of
another). No Exceptions are thrown; in fact the debug statements
indicate that all the machines are acquiring the exclusive lock)

Anyone can shed light? Here is the critical method with some
debug statements:

    public void write (String s) throws Exception {
        ByteBuffer bb = stringToByteBuffer(s);

        //lock the file and wait till we can
        FileChannel channel = fos.getChannel();
        FileLock lock = null;
        try {
            while ((lock = channel.tryLock()) == null) {
                System.out.println (Utils.getHostname() + " Failed lock...wait");
                Thread.sleep(100);
            }

            System.out.println (Utils.getHostname() + " Locked:" + lock);
            System.out.println (Utils.getHostname() + " Lock type is "+
((lock.isShared())?"shared":"exclusive"));

            System.out.println (Utils.getHostname() + " Is lock valid: " +
lock.isValid());

         //write the title first if noone's done it
         //and they asked for one
         if (channel.size() <= 0 && bbTitle != null)
         channel.write (bbTitle);
         channel.write(bb);

         } catch (Exception e) {
         throw (e);
         } finally {
         if (lock != null) {
         System.out.println (Utils.getHostname() + " Releasing lock");
         lock.release();
         }
         }

    }