cp -R from the mounted ufs disk image hangs in DL+ vnread

[Yuri]

I have an 82GB UFS image file (ufs-snapshot) mounted on some directory 
ufs-snapshot.mount. (mount /dev/`mdconfig -a -t vnode -f ufs-snapshot` 
ufs-snapshot.mount)

Command 'cp -R ufs-snapshot.mount/usr other-dir/' hanged in the middle 
with DL+ status:
$ ps ax | grep cp
73635  10  DL+     0:12.19 cp -R ufs-snapshot.mount/usr other-dir/
'top' shows it in vnread state:
73635 root            1  20    0 10084K  2672K vnread  1   0:12  0.00% cp

When I ran 'ls' in the same mounted directory it hanged too with D+ status:
$ ps ax | grep ls
75882   2  D+      0:00.00 ls ufs-snapshot.mount/

What is happening? Why cp and ls hanged?
I think, cp -R hanged first and later ls is waiting on some op initiated 
by cp -R.
Somehow, cp -R managed to hang itself.

How can I find out what cp is waiting on?

9.0-STABLE amd64

Yuri