This patch implements something we've been lacking for a long time: a way
to mark devices as USB_STATE_NOTATTACHED as soon as we know that they're
gone.  The usb_device->state member is no longer protected by the
->serialize semaphore; instead there's a new private spinlock.  Usbcore
routines should no longer set ->state directly; instead they should use
the new utility routine usb_set_device_state().  There are protections
against changing states while devices are being added or removed.

	Change assignments to udev->state into calls of
	usb_set_device_state().

	Add new private device_state_lock to the hub driver, along
	with usb_set_device_state() and recursively_mark_NOTATTACHED().

	Acquire the new spinlock while adding or removing children[]
	pointers.

	When disabling a port that has a child device, mark the child
	as NOTATTACHED.

You mentioned once having tried to do something like this and running into
trouble.  Take a good look and let me know if you see any difficulties
here.
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>