D33 broke the build on ARM pre-ARMv7, because there is no definition of store_load_barrier or load_load_barrier for those platforms. (Granted, the old fall-back behavior of doing nothing was almost certainly incorrect.)

I don't know whether we need CPU-level barriers here, or whether they are available. At a minimum, we need a compiler-level barrier. If we need a CPU-level barrier and it isn't provided by the instruction set, then I guess we should disable SMP for these platforms in mk/config.mk.in.