panic when kldunloading

I did a "kldload ipl && kldunload ipl", which produces following panic:

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panic: page fault
panic messages:
---
Fatal trap 12: page fault while in kernel mode
fault virtual address   = 0x4
fault code              = supervisor read, page not present
instruction pointer     = 0x8:0xc01c7223
stack pointer           = 0x10:0xceb61c5c
frame pointer           = 0x10:0xceb61c5c
code segment            = base 0x0, limit 0xfffff, type 0x1b
                        = DPL 0, pres 1, def32 1, gran 1
processor eflags        = interrupt enabled, resume, IOPL = 0
current process         = 61515 (kldunload)
trap number             = 12
panic: page fault

syncing disks, buffers remaining... 2222 2222 2220 2220 2220 2220 2220 2220 2220
 2220 2220 2220 2220 2220 2220 2220 2220 2220 2220 2220 2220 2220
giving up on 635 buffers
Uptime: 14h57m46s
Dumping 255 MB
ata0: resetting devices ..
done
 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240
---
Reading symbols from /usr/obj/usr/src/sys/CHIHIRO/modules/usr/src/sys/modules/ac
pi/acpi.ko.debug...done.
Loaded symbols for /usr/obj/usr/src/sys/CHIHIRO/modules/usr/src/sys/modules/acpi
/acpi.ko.debug
Reading symbols from /boot/kernel/ng_socket.ko...done.
Loaded symbols for /boot/kernel/ng_socket.ko
Reading symbols from /boot/kernel/ng_iface.ko...done.
Loaded symbols for /boot/kernel/ng_iface.ko
Reading symbols from /boot/kernel/ng_ppp.ko...done.
Loaded symbols for /boot/kernel/ng_ppp.ko
Reading symbols from /boot/kernel/ng_bpf.ko...done.
Loaded symbols for /boot/kernel/ng_bpf.ko
Reading symbols from /boot/kernel/ng_vjc.ko...done.
Loaded symbols for /boot/kernel/ng_vjc.ko
Reading symbols from /boot/kernel/ng_pppoe.ko...done.
Loaded symbols for /boot/kernel/ng_pppoe.ko
Reading symbols from /boot/kernel/ipl.ko...done.
Loaded symbols for /boot/kernel/ipl.ko
#0  doadump () at /usr/src/sys/kern/kern_shutdown.c:238
238             dumping++;
(kgdb)
#0  doadump () at /usr/src/sys/kern/kern_shutdown.c:238
#1  0xc01bee1d in boot (howto=256) at /usr/src/sys/kern/kern_shutdown.c:370
#2  0xc01bf174 in panic () at /usr/src/sys/kern/kern_shutdown.c:543
#3  0xc02d629e in trap_fatal (frame=0xceb61c1c, eva=0)
    at /usr/src/sys/i386/i386/trap.c:837
#4  0xc02d5fb2 in trap_pfault (frame=0xceb61c1c, usermode=0, eva=4)
    at /usr/src/sys/i386/i386/trap.c:751
#5  0xc02d5b75 in trap (frame=
      {tf_fs = -1058471912, tf_es = 16, tf_ds = -1016070128, tf_edi = 0, tf_esi
= -1016010752, tf_ebp = -826925988, tf_isp = -826926008, tf_ebx = -1015220652, t
f_edx = 0, tf_ecx = -1015216448, tf_eax = -1070322800, tf_trapno = 12, tf_err =
0, tf_eip = -1071877597, tf_cs = 8, tf_eflags = 66051, tf_esp = -826925948, tf_s
s = -1071980710}) at /usr/src/sys/i386/i386/trap.c:433
#6  0xc02c6a38 in calltrap () at {standard input}:96
#7  0xc01adf5a in linker_file_unregister_sysctls (lf=0x0)
    at /usr/src/sys/kern/kern_linker.c:267
#8  0xc01ae73e in linker_file_unload (file=0xc370e800)
    at /usr/src/sys/kern/kern_linker.c:527
#9  0xc01aee73 in kldunload (td=0xc370e800, uap=0x0)
    at /usr/src/sys/kern/kern_linker.c:812
#10 0xc02d65ba in syscall (frame=
      {tf_fs = 47, tf_es = 47, tf_ds = 47, tf_edi = -1077937202, tf_esi = -10779
37460, tf_ebp = -1077937504, tf_isp = -826925708, tf_ebx = 0, tf_edx = 134563352
---Type <return> to continue, or q <return> to quit---
, tf_ecx = 0, tf_eax = 305, tf_trapno = 12, tf_err = 2, tf_eip = 134514247, tf_c
s = 31, tf_eflags = 582, tf_esp = -1077938628, tf_ss = 47})
    at /usr/src/sys/i386/i386/trap.c:1024
#11 0xc02c6a8d in Xint0x80_syscall () at {standard input}:138
---Can't read userspace from dump, or kernel process---
(kgdb) up 11
#11 0xc02c6a8d in Xint0x80_syscall () at {standard input}:138
138     {standard input}: No such file or directory.
        in {standard input}
Current language:  auto; currently asm


Also, sometimes I cannot do "kldload ipl", it complains about "No such file or
directory". What am I missing here?

Jiawei Ye
-- 
"Without the userland, the kernel is useless."
                                     --inspired by The Tao of Programming