Re: [PATCH -rcu/kcsan 16/23] locking/atomics, kcsan: Add instrumentation for barriers

[Marco Elver]

On Tue, 5 Oct 2021 at 14:03, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Tue, Oct 05, 2021 at 12:58:58PM +0200, Marco Elver wrote:
> > @@ -59,6 +60,7 @@ atomic_add(int i, atomic_t *v)
> >  static __always_inline int
> >  atomic_add_return(int i, atomic_t *v)
> >  {
> > +     kcsan_mb();
> >       instrument_atomic_read_write(v, sizeof(*v));
> >       return arch_atomic_add_return(i, v);
> >  }
>
> This and others,.. is this actually correct? Should that not be
> something like:
>
>         kscan_mb();
>         instrument_atomic_read_write(...);
>         ret = arch_atomic_add_return(i, v);
>         kcsan_mb();
>         return ret;
>
> ?

In theory, yes, but right now it's redundant.

Because right now KCSAN only models "buffering", and no "prefetching".
So there's no way that a later instruction would be reordered before
this point. And atomic accesses are never considered for reordering,
so it's also impossible that it would  be reordered later.

Each kcsan_mb() is a call, so right now it makes sense to just have 1
call to be a bit more efficient.