Re: [PATCH 00/13] [RFC] Rust support

[Paolo Bonzini]

On 19/04/21 11:36, Peter Zijlstra wrote:
> On Mon, Apr 19, 2021 at 11:02:12AM +0200, Paolo Bonzini wrote:
> > > void writer(void)
> > > {
> > >       atomic_store_explicit(&seq, seq+1, memory_order_relaxed);
> > >       atomic_thread_fence(memory_order_acquire);
> >
> > This needs to be memory_order_release.  The only change in the resulting
> > assembly is that "dmb ishld" becomes "dmb ish", which is not as good as the
> > "dmb ishst" you get from smp_wmb() but not buggy either.
>
> Yuck! And that is what requires the insides to be
> atomic_store_explicit(), otherwise this fence doesn't have to affect
> them.

Not just that, even the write needs to be atomic_store_explicit in order 
to avoid a data race.atomic_store_explicit

> I also don't see how this is better than seq_cst.

It is better than seq_cst on TSO architectures.  Another possibility is 
to use release stores for everything (both increments and the stores 
between them).

> But yes, not broken, but also very much not optimal.

Agreed on that, just like RCU/memory_order_consume.

Paolo