[cpp-threads] Failed compare-and-swap

[Peter Dimov]

Boehm, Hans wrote:

>> Why do you people keep claiming that acquire semantics
>> provide sequential consistency? I must be missing something.

> In my view, acquire semantics in the failure case are necessary to
> provide sequential consistency, if a failed CAS returns the value that
> was read.  Such a CAS can clearly be used as an (expensive)
> replacement for a load.  This a failed SC CAS has to provide at least
> the samantics of an SC load.  Acquire semantics are clearly not
> sufficient, but they are necessary.

Yes, I was talking about the "not sufficient" part. A failed seq_cst CAS 
must have seq_cst semantics and not merely acquire semantics if the program 
is to remain SC. Conversely, a non-seq_cst CAS does not guarantee an SC 
program under any semantics for the failure case.

That is why I think that using sequential consistency as an argument for 
acquire semantics in the failed acquire case is not justified.