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

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Date:   Thu, 2 Mar 2017 18:55:47 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Christian Borntraeger <borntraeger@...ibm.com>
Cc:     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 Thu, Mar 2, 2017 at 5:51 PM, Christian Borntraeger
<borntraeger@...ibm.com> wrote:
> On 03/02/2017 05:38 PM, Arnd Bergmann wrote:
>>
>> This attempts a rewrite of the two macros, using a simpler implementation
>> for the most common case of having a naturally aligned 1, 2, 4, or (on
>> 64-bit architectures) 8  byte object that can be accessed with a single
>> instruction.  For these, we go back to a volatile pointer dereference
>> that we had with the ACCESS_ONCE macro.
>
> We had changed that back then because gcc 4.6 and 4.7 had a bug that could
> removed the volatile statement on aggregate types like the following one
>
> union ipte_control {
>         unsigned long val;
>         struct {
>                 unsigned long k  : 1;
>                 unsigned long kh : 31;
>                 unsigned long kg : 32;
>         };
> };
>
> See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145
>
> If I see that right, your __ALIGNED_WORD(x)
> macro would say that for above structure  sizeof(x) == sizeof(long)) is true,
> so it would fall back to the old volatile cast and might reintroduce the
> old compiler bug?

Ah, right, that's the missing piece. For some reason I didn't find
the reference in the source or the git log.

> Could you maybe you fence your simple macro for anything older than 4.9? After
> all there was no kasan support anyway on these older gcc version.

Yes, that should work, thanks!

     Arnd