[sparc64] [panic] mutex vm object not owned

Attempting to boot on (2-way SMP; SUN Fire V240) sparc64 a 9.0-CURRENT
kernel built on Jun 9 at 14:41, and fully csup'd before building (I don't
have the SVN revision number, sorry) yields, surprisingly late in the boot
process, this panic:

panic: mutex vm object not owned at /systank/src/sys/vm/vm_object.c:1692
cpuid = 0
KDB: stack backtrace:
panic() at panic+0x1c8
_mtx_assert() at _mtx_assert+0xb0
vm_object_collapse() at vm_object_collapse+0x28
vm_object_deallocate() at vm_object_deallocate+0x538
_vm_map_unlock() at _vm_map_unlock+0x64
vm_map_remove() at vm_map_remove+0x64
vmspace_exit() at vmspace_exit+0x100
exit1() at exit1+0x788
sys_exit() at sys_exit+0x10
syscallenter() at syscallenter+0x268
syscall() at syscall+0x74
-- syscall (1, FreeBSD ELF64, sys_exit) %o7=0x11980c --
userland() at 0x406fe8c8
user trace: trap %o7=0x11980c
pc 0x406fe8c8, sp 0x7fdffff7611
done
Uptime: 4m7s

The system was, at the time, attempting to bring up its jails.

Anything else that would be helpful to know?
Thanks.
--nwf;