"At the moment" means every kernel since something like 1.1.x or some
such ancient thing. And it probably means "until the end of time" (the
serialization right now is a global serialization of all devices, and it
_may_ become a per-device serialization in the future, but as a driver
writer you should more-or-less be able to depend on one particular
driver start_xmit() entrypoint always being serialized).
Linus