linux-kernel - Re: [PATCH] locking/atomics: don't alias ___

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Message-ID: <CACT4Y+b+a04gyjcsaBGHnm_JhQW4V+TWjfw7POjTRMB4mA6icQ@mail.gmail.com>
Date:   Wed, 28 Jun 2017 14:27:37 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        "x86@...nel.org" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] locking/atomics: don't alias ____ptr

On Wed, Jun 28, 2017 at 2:24 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Wed, 28 Jun 2017, Dmitry Vyukov wrote:
>> On Wed, Jun 28, 2017 at 1:10 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
>> >  #define cmpxchg_local(ptr, old, new)                   \
>> >  ({                                                     \
>> > -       __typeof__(ptr) ____ptr = (ptr);                \
>> > -       kasan_check_write(____ptr, sizeof(*____ptr));   \
>> > -       arch_cmpxchg_local(____ptr, (old), (new));      \
>> > +       kasan_check_write((ptr), sizeof(*(ptr)));       \
>> > +       arch_cmpxchg_local((ptr), (old), (new));        \
>>
>>
>> /\/\/\/\/\/\/\/\/\/\/\/\
>>
>> These are macros.
>> If ptr is foo(), then we will call foo() twice.
>
> If that's true, the foo() will be evaluated a gazillion more times down the
> way to the end of this macro maze.

No. If we do:

__typeof__(ptr) ____ptr = (ptr);

and then only use ____ptr, then ptr is evaluated only once regardless
of what the rest of macros do.