[CentOS] nfs stuck, don't know what processes to kill

My NFS server is up and other clients can access x. One particular client
can't. I tried to unmount the NFS share:

[root at nfsclient ~]#  umount -f /disk/x
umount2: Device or resource busy
umount.nfs: /disk/x: device is busy
umount2: Device or resource busy
umount.nfs: /disk/x: device is busy

If I use df or lsof to try to figure out what process to kill, they hang. I
am reluctant to just reboot, as many other users are getting stuff done.
dmesg doesn't show anything useful.

How to get unstuck?

thanks,
Dave