BETA4 kernel panic on boot with USB devices

Hi,

I found 5.3-BETA4 kernel hangs on boot sequence.

======================================================================
(stuff deleted)
atkbdc0: <Keyboard controller (i8042)> at port 0x64,0x60 on isa0
atkbd0: <AT Keyboard> flags 0x1 irq 1 on atkbdc0
device_attach: atkbd0 attach returned 6
fdc0: ready for input in output
fdc0: cmd 3 failed at out byte 1 of 3
sc0: <System console> at flags 0x100 on isa0
sc0: VGA <16 virtual consoles, flags=0x300>
vga0: <Generic ISA VGA> at port 0x3c0-0x3df iomem 0xa0000-0xbffff
on isa0
Timecounter "TSC" frequency 1808812226 Hz quality 800
Timecounters tick every 1.000 msec
ipfw2 initialized, divert enabled, rule-based forwarding disabled,
 default to deny, logging unlimited
ad0: 117246MB <Maxtor 4R120L0/RAMB1TU0> [238216/16/63] at ata0-master
UDMA133
uhub2: at uhub1 port 3 (addr 2) disconnected
ukbd0: detached
uhub2: at uhub1 port 1 (addr 2) disconnected
=====================================================================
after this message, kernel panic occurs.

This occurs with the following revisions of drivers:
src/sys/dev/usb/usb_subr.c: 1.69.2.1
src/sys/dev/usb/uhub.c: 1.62.2.1

When I replace these codes backward to 1.69 and 1.62, respectively,
and reconfig kernel, it boots up successfully.
# In this case, I still need to set hint.atkbd.0.flags=0x1 (without it
# I cannot use USB keyboard in single-user mode). Also, I don't know
# why keyboard and mouse are automatically detached/attached in boot
# sequence and multi-user mode (periodically).

Output of usbdevs:
======================================================================
Controller /dev/usb0:
addr 1: full speed, self powered, config 1, OHCI root hub(0x0000),
nVidia(0x0000), rev 1.00
 port 1 powered
 port 2 powered
 port 3 powered
 port 4 powered
Controller /dev/usb1:
addr 1: full speed, self powered, config 1, OHCI root hub(0x0000),
nVidia(0x0000), rev 1.00
 port 1 powered
 port 2 powered
 port 3 addr 2: full speed, power 90 mA, config 1, Generic USB
Hub(0x0065), Chicony(0x0472), rev 1.00
  port 1 addr 3: full speed, self powered, config 1, PFU-65 USB
Keyboard(0x0065), Chicony(0x0472), rev 1.00
  port 2 powered
  port 3 addr 4: low speed, power 50 mA, config 1, USB-PS/2
Trackball(0xc401), Logitech(0x046d), rev 2.10
 port 4 powered
Controller /dev/usb2:
addr 1: high speed, self powered, config 1, EHCI root hub(0x0000),
nVidia(0x0000), rev 1.00
 port 1 powered
 port 2 powered
 port 3 powered
 port 4 powered
 port 5 powered
 port 6 powered
 port 7 powered
 port 8 powered
======================================================================

-- 
FUJIMOTO Kou; Tokyo Denki University