acquiring duplicate lock when mounting nullfs

[Ulrich Spoerlein]

Hi,

this is on a RELENG_6 while mounting /usr/src and /usr/obj via nullfs
and doing 'make installkernel installworld'

It is similar to LOR #083, but not quite the same

acquiring duplicate lock of same type: "vnode interlock"
 1st vnode interlock @ /usr/src/sys/kern/vfs_vnops.c:806
 2nd vnode interlock @ /usr/src/sys/kern/vfs_subr.c:2036
KDB: stack backtrace:
kdb_backtrace(0,ff,c09816d0,c09816d0,c0907904,...) at kdb_backtrace+0x29
witness_checkorder(c30d56dc,9,c089bd90,7f4) at witness_checkorder+0x578
_mtx_lock_flags(c30d56dc,0,c089bd90,7f4,c218d830,...) at _mtx_lock_flags+0x78
vrefcnt(c30d5660) at vrefcnt+0x1d
null_checkvp(c2a8daa0,c08894b8,215) at null_checkvp+0x56
null_lock(cd689a80) at null_lock+0x62
VOP_LOCK_APV(c0900480,cd689a80) at VOP_LOCK_APV+0x87
vn_lock(c2a8daa0,1002,c27a3180,c2a8daa0,c31bbc2c,...) at vn_lock+0xa8
nullfs_root(c246d7c8,2,cd689af8,c27a3180,0,8,0,c09beca0,0,c089b632,3dd)
at nullfs_root+0x26
vfs_domount(c27a3180,c261c550,c28f7100,0,c239eb10,c09707e0,0,c089b632,2a3)
at vfs_domount+0x91d
vfs_donmount(c27a3180,0,c2a12e80,c2a12e80,0,...) at vfs_donmount+0x2ef
nmount(c27a3180,cd689d04) at nmount+0x8b
syscall(3b,3b,3b,bfbfe424,bfbfec7c,...) at syscall+0x25b
Xint0x80_syscall() at Xint0x80_syscall+0x1f
--- syscall (378, FreeBSD ELF32, nmount), eip = 0x280ba4d7, esp =
0xbfbfe3ac, ebp = 0xbfbfec28 ---

Uli