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Subject: Re: [PATCH 00/13] [RFC] Rust support
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Wedson Almeida Filho <wedsonaf@google.com>, ojeda@kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        rust-for-linux@vger.kernel.org, linux-kbuild@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org
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From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <7287eac3-f492-bab1-9ea8-b89ceceed560@redhat.com>
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