nfsd panic, _rw_rlock()

FreeBSD 8-CURRENT, built from sources around 27/02/2009:

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Fatal trap 9: general protection fault while in kernel mode
cpuid = 3; apic id = 03
instruction pointer     = 0x8:0xffffffff80541391
stack pointer           = 0x10:0xfffffffe9df97650
frame pointer           = 0x10:0xfffffffe9df97680
code segment            = base 0x0, limit 0xfffff, type 0x1b
                       = DPL 0, pres 1, long 1, def32 0, gran 1
processor eflags        = interrupt enabled, resume, IOPL = 0
current process         = 2921 (nfsd)
[thread pid 2921 tid 102235 ]
Stopped at      _rw_rlock+0x11: movq    0x18(%rdi),%rax

_rw_rlock() at _rw_rlock+0x11
vfs_stdcheckexp() at vfs_stdcheckexp+0xb4
nfsrv_fhtovp() at nfsrv_fhtovp+0xd3
nfsrv_read() at nfsrv_read+0x2a7
nfssvc() at nfssvc+0x4af
syscall() at syscall+0x1e7
Xfast_syscall() at Xfast_syscall+0xab
--- syscall (155, FreeBSD ELF64, nfssvc), rip = 0x800695c4c, rsp = 0x7fffffffe8e8, rbp = 0 ---
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The filesystem being exported over NFS is ZFS.

After running for a week, I just caught three of these.  Anyone else
seeing this?

Thanks,

Matthew