linux-kernel - Re: [PATCH v2 1/1] hung_task: fix warnings caused by unaligned lock pointers

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Message-ID: <CAMuHMdVMw3nUMtXhfhB5mgmsEZNuagna=6ywOuRsRRMFXHYwbA@mail.gmail.com>
Date: Wed, 10 Sep 2025 09:34:02 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
Cc: Kent Overstreet <kent.overstreet@...ux.dev>, Lance Yang <lance.yang@...ux.dev>, 
	akpm@...ux-foundation.org, amaindex@...look.com, anna.schumaker@...cle.com, 
	boqun.feng@...il.com, fthain@...ux-m68k.org, ioworker0@...il.com, 
	joel.granados@...nel.org, jstultz@...gle.com, leonylgao@...cent.com, 
	linux-kernel@...r.kernel.org, linux-m68k@...ts.linux-m68k.org, 
	longman@...hat.com, mhiramat@...nel.org, mingo@...hat.com, 
	mingzhe.yang@...com, oak@...sinkinet.fi, peterz@...radead.org, 
	rostedt@...dmis.org, senozhatsky@...omium.org, tfiga@...omium.org, 
	will@...nel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2 1/1] hung_task: fix warnings caused by unaligned lock pointers

On Tue, 9 Sept 2025 at 18:55, John Paul Adrian Glaubitz
<glaubitz@...sik.fu-berlin.de> wrote:
> On Tue, 2025-09-09 at 12:46 -0400, Kent Overstreet wrote:
> > On Tue, Sep 09, 2025 at 10:52:43PM +0800, Lance Yang wrote:
> > > From: Lance Yang <lance.yang@...ux.dev>
> > >
> > > The blocker tracking mechanism assumes that lock pointers are at least
> > > 4-byte aligned to use their lower bits for type encoding.
> > >
> > > However, as reported by Eero Tamminen, some architectures like m68k
> > > only guarantee 2-byte alignment of 32-bit values. This breaks the
> > > assumption and causes two related WARN_ON_ONCE checks to trigger.
> >
> > Isn't m68k the only architecture that's weird like this?
>
> Yes, and it does this on Linux only. I have been trying to change it upstream
> though as the official SysV ELF ABI for m68k requires a 4-byte natural alignment [1].

M68k does this on various OSes and ABIs that predate or are not
explicitly compatible with the SysV ELF ABI.

Other architectures like CRIS (1-byte alignment!) are no longer supported
by Linux.

FWIW, doubles (and doublewords) are not naturally aligned in the
SysV ELF ABI for i386, while doubles (no mention of doublewords)
are naturally aligned in the SysV ELF ABI for m68k.

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