This patch reintroduces the USB device locking code we tried out
earlier.  As before, it solves the problem of effectively locking all
the devices while drivers are registered and unregistered by introducing
an rwsem.  Unlike the earlier attempt, this version does not ever try to
acquire a lock re-entrantly.  I trust that will eliminate the races and
hang-ups you observed with the earlier version.  There are also copious
comments explaining exactly how things should work.

The patch interacts slightly with the locktree() code introduced by
David for suspend/resume support.  It doesn't change the functionality
at all; it just updates the routine to follow the new locking rules.
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>