Re: [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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