Is core.internal.atomic.atomicFetchAdd implementation really lock free?

[Vladimir Panteleev]

(Disclaimer: I have not studied lock-free programming so I might 
be wrong.)

On Wednesday, 30 November 2022 at 00:04:22 UTC, mw wrote:
> ```
> lock; xadd[%0], %1;
> ```
>
> Is this really lock free?

`lock` here refers not to some mutex (which is what "lock free" 
generally means), but to the processor cache line:

https://stackoverflow.com/questions/8891067/what-does-the-lock-instruction-mean-in-x86-assembly

> If not, can we use `cas` to implement it, e.g pseudo code:
>
>
> ```
> int atomicFetchAdd(int * pAddr, int nIncr ) {
>
>  while (true) {
>   int ncur = atomicLoad(pAddr);
>   if (cas( pAddr, ncur, ncur + nIncr )
>     return ncur ;
>  }
>
> }
> ```

`CMPXCHG`, used to implement CAS on x86, needs LOCK as well:

https://stackoverflow.com/questions/27837731/is-x86-cmpxchg-atomic-if-so-why-does-it-need-lock