panic: lockmgr on FreeBSD 7.0-RELEASE-p4 amd64

[Jeff Wheelhouse]

Got the following panic overnight:

panic: lockmgr: thread 0xffffff0053cda680, not exclusive lock holder  
0xffffff002d7da680 unlocking
cpuid = 0
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2a
panic() at panic+0x17a
_lockmgr() at _lockmgr+0x872
VOP_UNLOCK_APV() at VOP_UNLOCK_APV+0x46
null_unlock() at null_unlock+0xff
VOP_UNLOCK_APV() at VOP_UNLOCK_APV+0x46
nullfs_mount() at nullfs_mount+0x244
vfs_donmount() at vfs_donmount+0xe4d
nmount() at nmount+0xa5
syscall() at syscall+0x254
Xfast_syscall() at Xfast_syscall+0xab
--- syscall (378, FreeBSD ELF64, nmount), rip = 0x206845ac, rsp =  
0x7fffffffdfb8, rbp = 0x7fffffffdfc0 ---

I've done some searches and "not exclusive lock holder" has been seen  
before, but I didn't find any previous reports related to nullfs with  
a stack trace at all like this on FreeBSD 7.

This machine is diskless and thus cannot store a kernel dump.  Ideas/ 
suggestions for fixes, causes or debugging steps?

The kernel is amd64, with config shown below.

Thanks,
Jeff

include GENERIC

device  carp
device  pf
device  pflog
device  pfsync

options SW_WATCHDOG
options DEVICE_POLLING

options         ALTQ
options         ALTQ_CBQ
options         ALTQ_RED
options         ALTQ_RIO
options         ALTQ_HFSC
options         ALTQ_PRIQ
options         ALTQ_NOPCC

options         KDB
options         KDB_UNATTENDED
options         KDB_TRACE
options         DDB
options         BREAK_TO_DEBUGGER