Bug 125510

Tested on at least two separate machines, a Shuttle with ICH6 and an
Intel Server with an ICH8.

Repeatedly plug and unplug a mass storage device. It doesn't seem to
matter what kind. The kernel will panic. There are multiple locations for
the panic, but I expect the system state that triggers the panic is
related: it's always a NULL dereference, and it's always the result of
plugging or unplugging the device, and the EIPs are relatively close.

I have two panics logged on the stock FreeBSD 7 kernel:

The first is a dereference off of NULL faulting address == 0:
EIP = 20:0xc04675b6
Supervisor write, page not present; trap 12 in proc 2 (g_event)

The second is a dereference 0x10 off of NULL, faulting address == 0x10:
EIP = 20:0xc04801e5
Supervisor write, page not present; trap 12 in proc 35 (usb2)

Likewise, if a mass storage device(s) is already plugged in and doing
I/O, and another device is plugged and unplugged repeatedly, the I/O
on the other device(s) will eventually stall, even if that original
device is connected through nested hubs. This can impact multiple devices
at once. The message is shown:

<dev>: BBB reset failed, IOERROR
<dev>: BBB bulk-in clear stall failed (TIMEOUT)
<dev>: BBB bulk-out clear stall failed (TIMEOUT)

The device is unresponsive until it is removed and reconnected.

Fix: 

Make sure NULL isn't dereferenced, to fix the first set of problems.
This may not be trivial. Apparently the USB driver is subtle and quick
to panic.
How-To-Repeat: To yield a panic, choose a mass storage device at random. Repeatedly
plug and unplug the device, especially prior to the system indicating
that the device has been detected.

To yield a stall, attach a mass storage device and start non-stop I/O
to it. You can also choose to select multiple mass storage devices and
have them all do I/O. Plug this device or hub into one port into the
EHCI host controller. Repeatedly plug and unplug another mass storage
device into an adjacent port. Other ports may work but I believe they
must be associated with the same host controller. Eventually I/O on one
or more of the connected devices will stall.