Hi Helge, In your x86 code you have: bool compare_exchange_strong(T &e, T d, memory_order order=memory_order_seq_cst) volatile { T prev=e; __asm__ __volatile__("lock cmpxchgb %1, %2\n" : "=a" (prev) : "q" (d), "m" (i), "a" (e) : "memory"); bool success=(prev==e); e=prev; return success; } Can you explain why 'e' is a reference and why you assign back to it? Maybe it would help if you could write out what that asm does in pseudo-code. The kernel_cmpxchg that I have does: if (*ptr == oldval) { *ptr = newval; return 0; } else { return !0; } I think I can just write bool compare_exchange_strong(T &e, T d, memory_order order=memory_order_seq_cst) volatile { return kernel_cmpxchg(e,d,&i) == 0; } but the extra stuff in your x86 version makes me suspect there is more to it. Phil.