How do you force a CIFS connection to unmount

I use lazy unmount: umount -l (that's a lowercase L)

Lazy unmount. Detach the filesystem from the filesystem hierarchy now, and cleanup all references to the filesystem as soon as it is not busy anymore. (Requires kernel 2.4.11 or later.)

umount -a -t cifs -l

worked like a charm for me on CentOS 6.3. It saved me a server reboot.

On RHEL 6 this worked:

umount -f -a -t cifs -l 

This works for me (Ubuntu 13.10 Desktop to an Ubuntu 14.04 Server) :-

 sudo umount -f /mnt/my_share

Mounted with

 sudo mount -t cifs -o username=me,password=mine //192.168.0.111/serv_share /mnt/my_share

where serv_share is that set up and pointed to in the smb.conf file.

I had this issue for a day until I found the real resolution. Instead of trying to force unmount an smb share that is hung, mount the share with the "soft" option. If a process attempts to connect to the share that is not available it will stop trying after a certain amount of time.

soft Make the mount soft. Fail file system calls after a number of seconds.

mount -t smbfs -o soft //username@server/share /users/username/smb/share

stat /users/username/smb/share/file
stat: /users/username/smb/share/file: stat: Operation timed out

May not be a real answer to your question but it is a solution to the problem