Re: [PATCH] atomic: Specify natural alignment for atomic_t

[David Laight <david.laight.linux@gmail.com>]

On Mon, 1 Sep 2025 10:45:46 +0200
Geert Uytterhoeven <geert@linux-m68k.org> wrote:

> Hi Lance,
> 
> On Thu, 28 Aug 2025 at 04:05, Lance Yang <lance.yang@linux.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.

Even for MMIO register layouts you don't (usually) want 'packed'.
You may need to add explicit padding, and an 'error if padded' attribute
you be useful.
Sometimes you have (eg) a 64bit item on a 32bit boundary, marking the
member 'packed' will remove the gap before it - usually what is wanted.

In reality pretty much nothing should be 'packed'.

	David.

> 
> Gr{oetje,eeting}s,
> 
>                         Geert
>