samba - Oct 2002 - postexec umount /cdrom doesn't work

Hi, Dear All

This is my section of [CDROM] in smb.conf:
[cdrom]
     comment = Samba server's CD-ROM
     writable = no
     locking = no
     path = /cdrom
     public = yes
;
; The next two parameters show how to auto-mount a CD-ROM when the
;       cdrom share is accesed. For this to work /etc/fstab must contain
;       an entry like this:
;
;       /dev/scd0   /cdrom  iso9660 defaults,noauto,ro,user   0 0
;
; The CD-ROM gets unmounted automatically after the connection to the
;
; If you don't want to use auto-mounting/unmounting make sure the CD
;       is mounted on /cdrom
;
     preexec = /bin/mount /cdrom
     postexec = /bin/umount /cdrom

When I open the service `cdrom' in a Windows box, /cdrom can be correctly 
mounted.  But after I close the Explore window, waiting for /cdrom being 
umounted, it never umounted.  I run `lsof /cdrom':

COMMAND  PID USER   FD   TYPE DEVICE SIZE   NODE NAME
smbd    2161 root  cwd    DIR   22,0 2048 665600 /cdrom

smbd is still using /cdrom.  In fact, it never quits.  I have to kill smbd 
manually.

My samba version is:
Version 2.2.3a-6 for Debian

Any idea?  Thank you in advance.

Best regards,
Dai Yuwen