linux-kernel - Re: [RFC v2 2/3] atomic: Specify alignment for atomic_t and atomic64

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Message-Id: <f1f95870-9ef1-42e8-bb74-b7120820028e@app.fastmail.com>
Date: Mon, 15 Sep 2025 09:35:08 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Finn Thain" <fthain@...ux-m68k.org>,
 "Peter Zijlstra" <peterz@...radead.org>, "Will Deacon" <will@...nel.org>
Cc: "Andrew Morton" <akpm@...ux-foundation.org>,
 "Boqun Feng" <boqun.feng@...il.com>, "Jonathan Corbet" <corbet@....net>,
 "Mark Rutland" <mark.rutland@....com>, linux-kernel@...r.kernel.org,
 Linux-Arch <linux-arch@...r.kernel.org>,
 "Geert Uytterhoeven" <geert@...ux-m68k.org>, linux-m68k@...r.kernel.org,
 "Lance Yang" <lance.yang@...ux.dev>
Subject: Re: [RFC v2 2/3] atomic: Specify alignment for atomic_t and atomic64_t

On Sun, Sep 14, 2025, at 02:45, Finn Thain wrote:
> index 100d24b02e52..7ae82ac17645 100644
> --- a/include/asm-generic/atomic64.h
> +++ b/include/asm-generic/atomic64.h
> @@ -10,7 +10,7 @@
>  #include <linux/types.h>
> 
>  typedef struct {
> -	s64 counter;
> +	s64 counter __aligned(sizeof(long));
>  } atomic64_t;

Why is this not aligned to 8 bytes? I checked all supported architectures
and found that arc, csky, m68k, microblaze, openrisc, sh and x86-32
use a smaller alignment by default, but arc and x86-32 override it
to 8 bytes already. x86 changed it back in 2009 with commit
bbf2a330d92c ("x86: atomic64: The atomic64_t data type should be 8
bytes aligned on 32-bit too"), and arc uses the same one.

Changing csky, m68k, microblaze, openrisc and sh to use the same
alignment as all others is probably less risky in the long run in
case anything relies on that the same way that code expects native
alignment on atomic_t.

     Arnd