_cmpxchg
#define __cpu_fallback_try_cmpxchg(pcp, ovalp, nval, _cmpxchg) \
__val = _cmpxchg(pcp, __old, nval); \
} while (!_cmpxchg(16, mem1, &old, 0));
} while (!_cmpxchg(size, mem2 + offset, &old, new));