panic: vput: negative ref cnt

Hi,
   Getting a reproducable panic during backups:

panic: vput: negative ref cnt
cpuid = 0
KDB: enter: panic
[thread pid 3521 tid 100374 ]
Stopped at	kdb_enter+0x3d: movq	$0,0x654104(%rip)
db> bt
Tracing pid 3521 tid 100374 0xffffff008c79e380
kdb_enter() at kdb_enter+0x3d
panic() at panic+0x176
vput() at vput+0x10c
dounmount() at dounmount0x421
unmount() at unmount+0x24b
syscall() at syscall+0x1bf
Xfast_syscall() at Xfast_syscall+0xab
--- syscall (22, FreeBSD ELF64, unmount), rip = 0x800695d7c, rsp = 
0x7fffffffe218, rbp = 0 ---

My script takes a zfs snapshot, mounts the snapshot, backs it up using 
star, then umounts the snapshot. The panic seems to occur on the umount.
   Cheers.

-- 
Mark Powell - UNIX System Administrator - The University of Salford
Information & Learning Services, Clifford Whitworth Building,
Salford University, Manchester, M5 4WT, UK.
Tel: +44 161 295 6843  Fax: +44 161 295 5888  www.pgp.com for PGP key