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Subject: Re: [PATCH 00/13] [RFC] Rust support
To:     Peter Zijlstra <peterz@infradead.org>,
        Wedson Almeida Filho <wedsonaf@google.com>
Cc:     ojeda@kernel.org, Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
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        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org
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From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <aa6e44ab-e223-73aa-279e-8103732460ac@redhat.com>
Date:   Sat, 17 Apr 2021 16:51:58 +0200
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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.

> Simlar thing for RCU; C11 can't optimally do that

Technically if you know what you're doing (i.e. that you're not on 
Alpha) you can do RCU using a relaxed load followed by an 
atomic_signal_fence(memory_order_consume).  Which I agree is horrible 
and not entirely within the standard, but it works in practice.  The 
Linux implementation of memory barriers, atomic RMW primitives, 
load-acquire/store-release etc. is also completely outside the standard, 
so it's not much different and more portable.

The only thing that I really, really miss when programming with C11 
atomics is smp_mb__{before,after}_atomic().

Paolo