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

[mw]

According to the doc:

https://dlang.org/library/core/atomic/atomic_fetch_add.html

Atomically adds mod to the value referenced by val and returns 
the value val held previously. This operation is both lock-free 
and atomic.


However, when I check its final implementation, I saw


https://github.com/dlang/dmd/blob/master/druntime/src/core/internal/atomic.d#L275

```
lock; xadd[%0], %1;
```

Is this really lock free?

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 ;
  }

}
```