The relative performance of a lock'd instruction and an mfence will vary across processors and workloads. On my machine, simple peterson locking is ~10% faster when using an mfence as opposed to an xchgl. Maybe if the non-locked code benefited more from pipelining, the results would be different.

As an aside, g++ uses mfence for its C++11 atomics while clang uses xchgl; Does anyone know if Intel/AMD has ever guaranteed that all the usual ordering guarantees apply when mixing these? (Of course given their implementations it almost certainly will, but it would be nice to know this for a fact).