speedtouch: turn a micro race into a nano race.  The race is that an ATM device can
  be used the moment atm_dev_register returns, but you only get to fill out the
  atm_dev structure after atm_dev_register returns (this is a design flaw in the
  ATM layer).  Thus there is a small window during which you can be called with an
  incompletely set up data structure.  Workaround this by causing all ATM callbacks
  to fail if the dev_data field has not been set.  There is still a nano race if
  writing/reading the dev_data field is not atomic.  Is it atomic on all architectures?