linux-kernel - Re: [PATCH] headers/deps: dcache: Move the ____cacheline_aligned attribute to the head of the definition

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Message-ID: <YdTkUBSLCQv7++FN@gmail.com>
Date:   Wed, 5 Jan 2022 01:20:32 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "David S. Miller" <davem@...emloft.net>,
        Ard Biesheuvel <ardb@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jonathan Corbet <corbet@....net>, llvm@...ts.linux.dev
Subject: Re: [PATCH] headers/deps: dcache: Move the ____cacheline_aligned
 attribute to the head of the definition


* Nathan Chancellor <nathan@...nel.org> wrote:

> Nowhere does it mention that it accepts the attribute before the type 
> keyword and neither compiler respects the attribute if it comes before 
> the keyword but at least clang warns: https://godbolt.org/z/E9fTecKPv
>
> $ cat test.c
> #include <stdio.h>
> 
> struct foo {
>     int a;
>     int b;
> };
> 
> struct __attribute__ ((aligned (64))) bar {
>     int a;
>     int b;
> };
> 
> __attribute__ ((aligned (64))) struct baz {
>     int a;
>     int b;
> };
> 
> int main(void)
> {
>     printf("struct foo alignment: %zd\n", _Alignof(struct foo));
>     printf("struct bar alignment: %zd\n", _Alignof(struct bar));
>     printf("struct baz alignment: %zd\n", _Alignof(struct baz));
>     return 0;
> }
> 
> $ gcc --version | head -1
> gcc (GCC) 11.2.1 20211231
> 
> $ gcc -std=gnu89 -Wall -Wextra test.c; and ./a.out
> struct foo alignment: 4
> struct bar alignment: 64
> struct baz alignment: 4

Ugh - so my changes there are outright buggy.

I'm reverting all those attribute position changes as we speak ...

I'm actually happy about this in a way, as it settles the issue nicely. :-)

Thanks,

	Ingo