I can reproduce this with a LOCKDEBUG kernel, /usr/src on NFS, /mnt on
NFS, and then doing:

setenv DESTDIR /mnt
setenv INSTALLWORLDDIR /
setenv USETOOLS never
cd /usr/src
make installworld |& tee /tmp/log

As soon as pax starts to copy sets, I get:

panic: lockmgr: locking against myself
Stopped in pid 1269.1 (pax) at  netbsd:cpu_Debugger+0x6:        unlk    a6
db> bt                                                                    
cpu_Debugger(2,0,4f2edac,5bd2d54,5bd2d84) + 6
panic(187cb5,20002,5b91c7c,5,0) + 118        
_lockmgr(4f2edac,10002,4f2ed20,190e14,135) + c8a
genfs_lock(5bd2dac,19073c,4f2ed20,10002,5bd2dd0) + 20
VOP_LOCK(4f2ed20,10002) + 26                         
vn_lock(4f2ed20,20002) + 94 
namei(5bd2ec4) + a66       
sys___stat30(5b91c7c,5bd2f38,5bd2f78) + 30
syscall_plain(183,5b91c7c,5bd2fb4,ffffffff,ffffb627) + c0
syscall(183) + 62                                        
trap0() + e      

Chuck, is this related to your latest changes? Anything particular I should
dig?

Martin