Messages in this thread | | | Subject | Re: [PATCH 00/13] [RFC] Rust support | From | Paolo Bonzini <> | Date | Mon, 19 Apr 2021 09:53:06 +0200 |
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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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