Determining which lights are occluded is non-trivial and can't be done on at a low level (too many checks) thus you need to precompute it on some level, but if

Determining which lights are occluded is non-trivial and can't be done on at a low level (too many checks) thus you need to precompute it on some level, but if different parts of the scene have different sets of lights visible you need to somehow abstract that to keep your parallel computations going. All very doable but are the hard problems with the tech that should be solved before we say that ray tracing is solved.