Lock is NOT the superset of mfence

From a recent post of Dmitriy at com.programming.threads. I keep this here as a memo.

I was always thinking that LOCK is a kind of superset of MFENCE. It turns out that this is architecture dependent:

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For the P6 family processors, locked operations serialize all outstanding load and store operations (that is, wait for them to complete). This rule is also true for the Pentium 4 and Intel Xeon processors, with one exception. Load operations that reference weakly ordered memory types (such as the WC memory type) may not be
serialized.
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Dmitriy V'jukov

Thanks, Dmitriy!