unionfs related panic on 10.0-ALPHA4

Hi all,
I've got a panic on my 10.0 FreeBSD.
This system was building port with poudriere and generate special
nanobsd images (with unionfs usage) when it panic.
I didn't have enough swap space for a full dump, I've just have a text dump:

root_at_orange:/var/crash # cat info.last
Dump header from device /dev/gpt/swap0
  Architecture: amd64
  Architecture Version: 1
  Dump Length: 30720B (0 MB)
  Blocksize: 512
  Dumptime: Mon Oct  7 15:16:55 2013
  Hostname: orange.bsdrp.net
  Magic: FreeBSD Text Dump
  Version String: FreeBSD 10.0-ALPHA4 #15 r256062: Sat Oct  5 20:51:12 CEST 2013
    root_at_orange.bsdrp.net:/usr/obj/usr/local/BSDRP/BSDRPcur/FreeBSD/src/sys/PROD
  Panic String: __lockmgr_args: downgrade a recursed lockmgr zfs _at_
/usr/local/BSDRP/BSDRPcur/FreeBSD/src/sys/modules/unionfs/../../fs/unionfs/union_vnops.c:1906

  Dump Parity: 1116491583
  Bounds: 1
  Dump Status: good

root_at_orange:/var/crash # cat version.txt
FreeBSD 10.0-ALPHA4 #15 r256062: Sat Oct  5 20:51:12 CEST 2013
    root_at_orange.bsdrp.net:/usr/obj/usr/local/BSDRP/BSDRPcur/FreeBSD/src/sys/PROD

root_at_orange:/var/crash # cat msgbuf.txt
...
panic: __lockmgr_args: downgrade a recursed lockmgr zfs _at_
/usr/local/BSDRP/BSDRPcur/FreeBSD/src/sys/modules/unionfs/../../fs/unionfs/union_vnops.c:1906

cpuid = 4
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe095d7d5480
kdb_backtrace() at kdb_backtrace+0x39/frame 0xfffffe095d7d5530
panic() at panic+0x155/frame 0xfffffe095d7d55b0
__lockmgr_args() at __lockmgr_args+0xdd3/frame 0xfffffe095d7d56c0
vop_stdlock() at vop_stdlock+0x3c/frame 0xfffffe095d7d56e0
VOP_LOCK1_APV() at VOP_LOCK1_APV+0x9d/frame 0xfffffe095d7d5710
unionfs_lock() at unionfs_lock+0x2bc/frame 0xfffffe095d7d57a0
VOP_LOCK1_APV() at VOP_LOCK1_APV+0x9d/frame 0xfffffe095d7d57d0
_vn_lock() at _vn_lock+0x43/frame 0xfffffe095d7d5830
unionfs_readdir() at unionfs_readdir+0x13f/frame 0xfffffe095d7d5980
VOP_READDIR_APV() at VOP_READDIR_APV+0x98/frame 0xfffffe095d7d59b0
kern_getdirentries() at kern_getdirentries+0x20b/frame 0xfffffe095d7d5ab0
sys_getdirentries() at sys_getdirentries+0x28/frame 0xfffffe095d7d5ae0
amd64_syscall() at amd64_syscall+0x357/frame 0xfffffe095d7d5bf0
Xfast_syscall() at Xfast_syscall+0xfb/frame 0xfffffe095d7d5bf0
--- syscall (196, FreeBSD ELF64, sys_getdirentries), rip =
0x8008adcea, rsp = 0x7fffffffcfd8, rbp = 0x7fffffffd210 ---
KDB: enter: panic

Regards,

Olivier