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

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Message-ID: <CAMuHMdVwYvJSoAWDTJJYOTt8mgtfa+sB_uevYR3NXFUK2fVS5A@mail.gmail.com>
Date: Wed, 1 Oct 2025 09:08:17 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Finn Thain <fthain@...ux-m68k.org>
Cc: Arnd Bergmann <arnd@...db.de>, Peter Zijlstra <peterz@...radead.org>, Will Deacon <will@...nel.org>, 
	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>, 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

Hi Finn,

On Wed, 1 Oct 2025 at 03:46, Finn Thain <fthain@...ux-m68k.org> wrote:
> On Tue, 30 Sep 2025, Geert Uytterhoeven wrote:
> > > To silence the misalignment WARN from CONFIG_DEBUG_ATOMIC, for 64-bit
> > > atomic operations, for my small m68k .config, it was also necesary to
> > > increase ARCH_SLAB_MINALIGN to 8. However, I'm not advocating a
> >
> > Probably ARCH_SLAB_MINALIGN should be 4 on m68k.  Somehow I thought that
> > was already the case, but it is __alignof__(unsigned long long) = 2.
>
> I agree -- setting ARCH_SLAB_MINALIGN to 4 would be better style, and may
> avoid surprises in future. Right now that won't have any effect because
> that value gets increased to sizeof(void *) by calculate_alignment() and

Ah, so there it happens...

> gets increased to ARCH_KMALLOC_MINALIGN or ARCH_DMA_MINALIGN by
> __kmalloc_minalign().

Thanks for checking!

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds