NFS server panic on OpenBSD diskless client shutdown

["John D. Baker" <jdbaker%mylinuxisp.com@localhost>]

I've been ignoring this for a while due to lack of time to properly
document it, but I've noticed the following problem with diskless NFS
clients running OpenBSD:

NFS server:
  NetBSD-6.0BETA2/amd64 or NetBSD-6.99.x/amd64

NFS diskless client:
  OpenBSD-5.1/amd64 or OpenBSD-5.1/i386


Server console info:

$ uname -a
NetBSD shadow.bozonet.ntc 6.0_BETA2 NetBSD 6.0_BETA2 (SHADOW) #42: Mon Jul 30 
13:20:55 CDT 2012  
sysop%shadow.bozonet.ntc@localhost:/d0/nbsd/netbsd-6/obj/amd64/sys/arch/amd64/compile/SHADOW
 amd64

$ showmount -a
All mount points on localhost:
drmemory.bozonet.ntc:/d0
drmemory.bozonet.ntc:/d0/diskless/drmemory-obsd
drmemory.bozonet.ntc:/d0/diskless/drmemory-obsd/swap


If anything causes the client to disconnect, whether explicitly attempted
shutdown/poweroff or a crash, the server panics as follows:

uvm_fault(0xfffffe813decc8d8, 0x0, 1) -> e
fatal page fault in supervisor mode 
trap type 6 code 0 rip ffffffff805571d8 cs 8 rflags 10286 cr2  7c cpl 0 rsp fffffe800179f460
kernel: page fault trap, code=0
Stopped in pid 280.3 (nfsd) at  netbsd:VOP_ABORTOP+0x1c:        testb   $0x10,7c
(%rdi) 
db{1}> bt
VOP_ABORTOP() at netbsd:VOP_ABORTOP+0x1c
nfsrv_rename() at netbsd:nfsrv_rename+0x5af
nfssvc_nfsd() at netbsd:nfssvc_nfsd+0x1ce 
sys_nfssvc() at netbsd:sys_nfssvc+0x22d 
syscall() at netbsd:syscall+0xc4 
db{1}> sync 
_

(The attempted sync (or reboot) never happens.  The cursor just sits on
the next line and the only way out is power-cycle.)

Complete dmesg.boot for server example above is at:

  http://bobdbob.com/~jdbaker/nbsd-debug/dmesg-shadow.txt

So far: 
I don't have examples for -current as the machine I have running it has
no serial port.  I have tried no other ports for the server.  I have not
tried "simple" OpenBSD clients (boot/run from own disk, but simply mount
remote filesystems either explicitly or via amd).  I have tried no other
OpenBSD client platforms neither simple nor diskless.

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