Cannot unmount nullfs in current

Hi,

I am trying to build some software which uses
nanobsd, and mounts/unmounts many nullfs mounts
while it runs.  I am hitting failures where
I cannot unmount nullfs file systems.  I cannot figure out why.

Here is more info.

SYSTEM
======
I am running amd64, current build at this revision:

10.0-CURRENT FreeBSD 10.0-CURRENT #0 r249181: Sat Apr  6 03:07:32 UTC 2013
amd64

STEPS TO REPRODUCE
===============

(1)  Create a directory, /opt2/branches.  Make sure that /opt2/branches
     is on ZFS

(2)
       mkdir -p /opt2/branches/freenas
       mkdir -p /opt2/branches/freenas-cache

(3)

git clone git://github.com/freenas/freenas.git /opt2/branches/freenas
git clone git://github.com/freenas/ports.git/opt2/branches/freenas-cache/ports
git clone git://github.com/trueos/trueos.git/opt2/branches/freenas-cache/trueos

(4)  sudo to root

(5)  cd /opt2/branches/freenas

(6)
script build.log env GIT_REPO=/opt2/branches/freenas-cache/trueos \
        GIT_PORTS_REPO=/opt2/branches/freenas-cache/ports \
        sh build/do_build.sh


The build cranks for a while, and then I get this error:

00:02:37 ### log:
/opt2/branches/freenas/os-base/amd64/_.cust.add_pkg_archivers_lzo2
do_build.sh: ERROR: FreeNAS /opt2/branches/freenas/nanobsd/os-base build
FAILED; please check above log for more details



If I look in .cust.add_pkg_archivers_lzo2, I see this error:

+ umount /opt2/branches/freenas/os-base/amd64/_.w/usr/ports/distfiles
umount: unmount of
/opt2/branches/freenas/os-base/amd64/_.w/usr/ports/distfiles failed: Device
busy


If I try to do this manually:

# umount /opt2/branches/freenas/os-base/amd64/_.w/usr/ports/distfiles
umount: unmount of
/opt2/branches/freenas/os-base/amd64/_.w/usr/ports/distfiles failed: Device
busy


I can't figure out why this mount is busy.
If I do:

umount -f /opt2/branches/freenas/os-base/amd64/_.w/usr/ports/distfiles

it unmounts, but I don't like using the '-f' flag to force the unmount.

Any ideas?  I am attaching some of my logs.

--
Craig