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Date:   Fri, 3 Mar 2017 15:49:38 +0100
From:   Peter Zijlstra <>
To:     Christian Borntraeger <>
Cc:     Arnd Bergmann <>,
        kasan-dev <>,
        Andrey Ryabinin <>,
        Alexander Potapenko <>,
        Dmitry Vyukov <>,
        Networking <>,
        Linux Kernel Mailing List <>,,
        linux-wireless <>,,
        "David S . Miller" <>,
        Paul McKenney <>
Subject: Re: [PATCH 02/26] rewrite READ_ONCE/WRITE_ONCE

On Fri, Mar 03, 2017 at 09:26:50AM +0100, Christian Borntraeger wrote:
> Right. The main purpose is to read/write _ONCE_. You can assume a somewhat
> atomic access for sizes <= word size. And there are certainly places that
> rely on that. But the *ONCE thing is mostly used for things where we used
> barrier() 10 years ago.

A lot of code relies on READ/WRITE_ONCE() to generate single
instructions for naturally aligned machined word sized loads/stores
(something GCC used to guarantee, but does no longer IIRC).

So much so that I would say its a bug if READ/WRITE_ONCE() doesn't
generate a single instruction under those conditions.

However, every time I've tried to introduce stricter
semantics/primitives to verify things Linus hated it.

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