Ben,
         If the ip_rcv() and the inetdev_init() are on the same
interface in your stack backtrace, it's a certainty at that point
that the lock value is still 0ed, because none of the initialization
occurs until after it has returned from the function it interrupted
to do the receive.
        It'd have to be out of the register code and doing
ip_mc_init_dev() (after that call) to be a tight race with
lock creation.

                                        +-DLS