linux-kernel - Re: [PATCH] atomic: Specify natural alignment for atomic

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Message-ID: <CAMuHMdV-AtPm-W-QUC1HixJ8Koy_HdESwCCOhRs3Q26=wjWwog@mail.gmail.com>
Date: Mon, 1 Sep 2025 10:45:46 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Lance Yang <lance.yang@...ux.dev>
Cc: Finn Thain <fthain@...ux-m68k.org>, akpm@...ux-foundation.org, 
	linux-kernel@...r.kernel.org, mhiramat@...nel.org, oak@...sinkinet.fi, 
	peterz@...radead.org, stable@...r.kernel.org, will@...nel.org, 
	Lance Yang <ioworker0@...il.com>, linux-m68k@...ts.linux-m68k.org
Subject: Re: [PATCH] atomic: Specify natural alignment for atomic_t

Hi Lance,

On Thu, 28 Aug 2025 at 04:05, Lance Yang <lance.yang@...ux.dev> wrote:
> On 2025/8/28 07:43, Finn Thain wrote:
> > On Mon, 25 Aug 2025, Lance Yang wrote:
> >> Same here, using a global static variable instead of a local one. The
> >> result is consistently misaligned.
> >>
> >> ```
> >> #include <linux/module.h>
> >> #include <linux/init.h>
> >>
> >> static struct __attribute__((packed)) test_container {
> >>      char padding[49];
> >>      struct mutex io_lock;
> >> } cont;
> >>
> >> static int __init alignment_init(void)
> >> {
> >>      pr_info("Container base address      : %px\n", &cont);
> >>      pr_info("io_lock member address      : %px\n", &cont.io_lock);
> >>      pr_info("io_lock address offset mod 4: %lu\n", (unsigned long)&cont.io_lock % 4);
> >>      return 0;
> >> }
> >>
> >> static void __exit alignment_exit(void)
> >> {
> >>      pr_info("Module unloaded\n");
> >> }
> >>
> >> module_init(alignment_init);
> >> module_exit(alignment_exit);
> >> MODULE_LICENSE("GPL");
> >> MODULE_AUTHOR("x");
> >> MODULE_DESCRIPTION("x");
> >> ```
> >>
> >> Result from dmesg:
> >>
> >> ```
> >> [Mon Aug 25 19:33:28 2025] Container base address      : ffffffffc28f0940
> >> [Mon Aug 25 19:33:28 2025] io_lock member address      : ffffffffc28f0971
> >> [Mon Aug 25 19:33:28 2025] io_lock address offset mod 4: 1
> >> ```
> >>
> >
> > FTR, I was able to reproduce that result (i.e. static storage):
> >
> > [    0.320000] Container base address      : 0055d9d0
> > [    0.320000] io_lock member address      : 0055da01
> > [    0.320000] io_lock address offset mod 4: 1
> >
> > I think the experiments you sent previously would have demonstrated the
> > same result, except for the unpredictable base address that you sensibly
> > logged in this version.
>
> Thanks for taking the time to reproduce it!
>
> This proves the problem can happen in practice (e.g., with packed structs),
> so we need to ignore the unaligned pointers on the architectures that don't
> trap for now.

Putting locks inside a packed struct is definitely a Very Bad Idea
and a No Go.  Packed structs are meant to describe memory data and
MMIO register layouts, and must not contain control data for critical
sections.

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