linux-kernel - Re: [RFC v4 3/5] atomic: Specify alignment for atomic_t and atomic64

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Message-ID: <CAFr9PX=MYUDGJS2kAvPMkkfvH+0-SwQB_kxE4ea0J_wZ_pk=7w@mail.gmail.com>
Date: Mon, 24 Nov 2025 09:23:35 +0900
From: Daniel Palmer <daniel@...f.com>
To: Finn Thain <fthain@...ux-m68k.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Will Deacon <will@...nel.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Arnd Bergmann <arnd@...db.de>, 
	Boqun Feng <boqun.feng@...il.com>, Geert Uytterhoeven <geert@...ux-m68k.org>, linux-arch@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-m68k@...r.kernel.org, 
	Mark Rutland <mark.rutland@....com>
Subject: Re: [RFC v4 3/5] atomic: Specify alignment for atomic_t and atomic64_t

Hi Finn,

On Tue, 21 Oct 2025 at 07:39, Finn Thain <fthain@...ux-m68k.org> wrote:
>
> Some recent commits incorrectly assumed 4-byte alignment of locks.
> That assumption fails on Linux/m68k (and, interestingly, would have
> failed on Linux/cris also). Specify the minimum alignment of atomic
> variables for fewer surprises and (hopefully) better performance.

FWIW I implemented jump labels for m68k and I think there is a problem
with this in there too.
jump_label_init() calls static_key_set_entries() and setting
key->entries in there is corrupting 'atomic_t enabled' at the start of
key.

With this patch the problem goes away.

Cheers,

Daniel