Most of the work has already been done by the project developers. The only major thing left to be implemented are thread safe hash tables. Since hash tables are used internally in CLOS, it is not viable to make them lockable. The current implementation should be replaced with lock-free open-addressing ones like described here. The hardest part in this reimplementation is integration with GC because of weak relations. For more information see doc/multithread.txt in the CLISP source distribution.