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

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