kern/47730: umount -f nfsdir; mount nfsdir => kernel panic (layerfs_statvfs)

[cheusov%tut.by@localhost]

>Number:         47730
>Category:       kern
>Synopsis:       umount -f nfsdir; mount nfsdir => kernel panic 
>(layerfs_statvfs)
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Apr 08 10:55:00 +0000 2013
>Originator:     Aleksey Cheusov
>Release:        NetBSD 6.0.1_PATCH
>Organization:
>Environment:
System: NetBSD cheusov.imb.invention.com 6.0.1_PATCH NetBSD 6.0.1_PATCH 
(GENERIC) #4: Wed Mar 13 17:27:04 FET 2013 
cheusov%cheusov.imb.invention.com@localhost:/srv/obj/sys/arch/i386/compile/GENERIC
 i386
Architecture: i386
Machine: i386
>Description:
My 6.0.1_PATCH system paniced immediately after

   umount -f /nfs/dir
   mount /nfs/dir

nfs mount option (if it matters) are tcp,intr.

#0  0xc05afd78 in maybe_dump (howto=260)
    at /srv/src_netbsd6/sys/arch/i386/i386/machdep.c:878
#1  cpu_reboot (howto=260, bootstr=0x0)
    at /srv/src_netbsd6/sys/arch/i386/i386/machdep.c:899
#2  0xc0788d4b in vpanic (fmt=0xc0ba72f2 "trap",
    ap=0xdc2c7a24 "Äz,ÜÄz,Ü\310\240\207\300\b")
    at /srv/src_netbsd6/sys/kern/subr_prf.c:308
#3  0xc0788def in panic (fmt=0xc0ba72f2 "trap")
    at /srv/src_netbsd6/sys/kern/subr_prf.c:205
#4  0xc07cee5b in trap (frame=0xdc2c7ac4)
    at /srv/src_netbsd6/sys/arch/i386/i386/trap.c:396
#5  0xc010cfb8 in ?? ()
#6  0xc0569ee7 in layerfs_statvfs (mp=0xc5d13008, sbp=0xc5654008)
    at /srv/src_netbsd6/sys/miscfs/genfs/layer_vfsops.c:160
#7  0xc087a0cb in VFS_STATVFS (mp=0xc5d13008, a=0xc5654008)
    at /srv/src_netbsd6/sys/kern/vfs_subr.c:1036
#8  0xc087c460 in dostatvfs (mp=0xc5d13008, sp=0xc5654008, l=0xce2d9020, 
flags=1, root=0)
    at /srv/src_netbsd6/sys/kern/vfs_syscalls.c:1030
#9  0xc087c777 in do_sys_getvfsstat (l=0xce2d9020, sfsp=0xbb9173b0, 
bufsize=220572,
    flags=1, copyfn=0xc0100820, entry_sz=2228, retval=0xdc2c7d1c)
    at /srv/src_netbsd6/sys/kern/vfs_syscalls.c:1184
#10 0xc087c92b in sys_getvfsstat (l=0xce2d9020, uap=0xdc2c7cf4, 
retval=0xdc2c7d1c)
    at /srv/src_netbsd6/sys/kern/vfs_syscalls.c:1236
#11 0xc07a18ad in sy_call (rval=0xdc2c7d1c, uap=0xdc2c7cf4, l=0xce2d9020, 
sy=0xc0c2b9d0)
    at /srv/src_netbsd6/sys/sys/syscallvar.h:61
#12 syscall (frame=0xdc2c7d48) at 
/srv/src_netbsd6/sys/arch/x86/x86/syscall.c:179
#13 0xc010056d in ?? ()

>How-To-Repeat:
Unsure, but maybe it can be reproducible, see above

>Fix:

Unknown