panic: Duplicate free of item 0xc5862800 from zone 0xc55f8580(NAMEI)

Got the following panic while doing a buildworld on a dual xeon (quad
logical processor) system today, using 6-CURRENT source from today. 

Robert N M Watson

Slab at 0xc5857200, freei 2 = 0.
panic: Duplicate free of item 0xc5862800 from zone 0xc55f8580(NAMEI)

cpuid = 1
KDB: enter: panic
[thread pid 10534 tid 100207 ]
Stopped at      kdb_enter+0x2b: nop     
db> trace
Tracing pid 10534 tid 100207 td 0xc5e27000
kdb_enter(c08317e5) at kdb_enter+0x2b
panic(c084b549,c5862800,c55f8580,c0839cfb,c084b52d) at panic+0x127
uma_dbg_free(c55f8580,0,c5862800) at uma_dbg_free+0x110
uma_zfree_arg(c55f8580,c5862800,0) at uma_zfree_arg+0xf4
namei(ebb3fbdc,c0626abe,c5e27000,c5e27000,c5e27000) at namei+0x245
vn_open_cred(ebb3fbdc,ebb3fcdc,1a4,c5ddec80,3) at vn_open_cred+0x278
vn_open(ebb3fbdc,ebb3fcdc,1a4,3,c0626abe) at vn_open+0x1e
kern_open(c5e27000,8418900,0,8001,1b6) at kern_open+0xb6
open(c5e27000,ebb3fd14,3,b,202) at open+0x1a
syscall(2f,2f,bfbf002f,0,0) at syscall+0x213
Xint0x80_syscall() at Xint0x80_syscall+0x1f
--- syscall (5, FreeBSD ELF32, open), eip = 0x82d0e27, esp = 0xbfbfe2fc,
ebp = 0xbfbfe318 ---
db> show lockedvnods
Locked vnodes

0xc61819fc: tag ufs, type VREG
    usecount 1, writecount 0, refcount 4 mountedhere 0
    flags ()
    v_object 0xc6174738 ref 0 pages 5
     lock type ufs: EXCL (count 1) by thread 0xc5e27a10 (pid 10541)
        ino 2028972, on dev ar0s1e

0xc6b5fd50: tag ufs, type VREG
    usecount 1, writecount 0, refcount 3 mountedhere 0
    flags ()
    v_object 0xc76d3528 ref 0 pages 1
     lock type ufs: EXCL (count 1) by thread 0xc624aa10 (pid 10538)
        ino 2028317, on dev ar0s1e