stacked NFS mounts

Suppose I have an NFS mount on /foo and another on top of that, say /foo/bar.
Now, when the NFS server dies, I can't umount /foo because of the /foo/bar 
mount. But I also cannot umount /foo/bar because that apparently tries to 
access /foo/bar (the bar directory in the /foo NFS mount) and that failes.

Is that intentional? Any way to get out short of rebooting?

(In the real case, the NFS server didn't die, it was updated from -6 to -8 
and the NFS handles seem to be incompatible, probably because of the 
struct ufid change.)