I have a CIFS share mounted on a Linux machine. The CIFS server is down, or the internet connection is down, and anything that touches the CIFS mount now takes several minutes
I had a very similar problem with davfs. In the man page of umount.davfs, I found that the -f -l -n -r -v options are ignored by umount.davfs. To force-unmount my davfs mount, I had to use umount -i -f -l /media/davmount.