netdev - Re: [PATCH 02/26] rewrite READ_ONCE/WRITE

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Message-ID: <20170303144938.GF6557@twins.programming.kicks-ass.net>
Date:   Fri, 3 Mar 2017 15:49:38 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christian Borntraeger <borntraeger@...ibm.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Networking <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-media@...r.kernel.org,
        linux-wireless <linux-wireless@...r.kernel.org>,
        kernel-build-reports@...ts.linaro.org,
        "David S . Miller" <davem@...emloft.net>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>
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.