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

[Paolo Bonzini]

On 19/04/21 09:32, Peter Zijlstra wrote:
> On Sat, Apr 17, 2021 at 04:51:58PM +0200, Paolo Bonzini wrote:
> > On 16/04/21 09:09, Peter Zijlstra wrote:
> > > Well, the obvious example would be seqlocks. C11 can't do them
> >
> > Sure it can.  C11 requires annotating with (the equivalent of) READ_ONCE all
> > reads of seqlock-protected fields, but the memory model supports seqlocks
> > just fine.
>
> How does that help?
>
> IIRC there's two problems, one on each side the lock. On the write side
> we have:
>
> 	seq++;
> 	smp_wmb();
> 	X = r;
> 	Y = r;
> 	smp_wmb();
> 	seq++;
>
> Which C11 simply cannot do right because it does't have wmb.

It has atomic_thread_fence(memory_order_release), and 
atomic_thread_fence(memory_order_acquire) on the read side.

> You end up
> having to use seq_cst for the first wmb or make both X and Y (on top of
> the last seq) a store-release, both options are sub-optimal.

seq_cst (except for the fence which is just smp_mb) is a pile of manure, 
no doubt about that. :)

Paolo