correct use of mfence

Hi,

I am writting atomic inc/dec/cas operations in assembly.

increment64(__int64* value, int val)
{
__int64* temp = &val;
_asm
{
push edi
push ebx
mov edi, pValue
mov eax, [edi]
mov edx, [edi+4]
again:
mov ecx, edx
mov ebx, eax
add ebx, 1
adc ecx, 0
mfence
lock cmpxchg8b [edi]
mfence
jnz again
mov eax, temp
mov [eax], ebx
mov [eax+4], ecx
pop ebx
pop edi
}
}

Requirement is that it should work on single and multi-core hardwares.

1. Use of mfence is not clear to me. So I temporarily put mfence before and after cmpxchg8b. I am looking for correct use of mfence opcode in this context. I would like to know how many lines mfence serializes before its use.
MFENCE:
This serializing operation guarantees that every load and store instruction that precedes in program order the MFENCE instruction is globally visible before any load or store instruction that follows the MFENCE instruction is globally visible

2. If i use mfence then also i need to use lock opcode before cmpxchg8b?

What is the correct use of mfence in above assembly code.

Thanks,
Ram

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