stacked unionfs freeze and crash FreeBSD

Hi,

The attached script, that uses stacked unionfs, causes FreeBSD-9 (cvsup 
yesterday) to freeze and FreeBSD-8 (cvsup two days ago) to crash:  

Fatal double fault
rip-0xffffffff81e4c1
rsp=0xffffff80b133ef50
rbp=0xffffff80b133f150
cupid = 2; apic id = 02
panic: double fault
cpuid = 2
uptime: 1h44m35s
cannot dump.  Device not defined or unavailable


Both systems use the stock GENERIC kernel, i.e. -9 had full diagnostics built 
in (and was run under VirtualBox) and -8 had no diagnostics (and was run 
native).  

A LOR is produces prior to freezing under -9 (quiet a time prior).  See 
kern/141950.  

The script uses unionfs to build ports (in an attempt to create a tinderbox 
without the need to delete and/or extract packages).  To use the script to:

# mkdir /tmp/localbase /tmp/builddir
# env LOCALBASE=/tmp/localbase BUILDDIR=/tmp/builddir ./ports-union-builder.sh


This will try build everything for x11/xorg.  

Is it possible that VirtualBox is interfering in getting usable diagnostics 
for -9 and how can I setup a dump device for -8.  Currently I have:

# swapinfo
Device          1K-blocks     Used    Avail Capacity
/dev/ad4s1b       8388608        0  8388608     0%
/dev/ad8s1b       8388608        0  8388608     0%
Total            16777216        0 16777216     0%


Any advice?

Regards