Re: [RFC v2 2/3] atomic: Specify alignment for atomic_t and atomic64_t

[Geert Uytterhoeven]

Hi Finn,

On Sun, 14 Sept 2025 at 02:59, Finn Thain <fthain@xxxxxxxxxxxxxx> 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.
>
> Consistent with i386, atomic64_t is not given natural alignment here.
>
> Cc: Lance Yang <lance.yang@xxxxxxxxx>
> Link: https://lore.kernel.org/lkml/CAMuHMdW7Ab13DdGs2acMQcix5ObJK0O2dG_Fxzr8_g58Rc1_0g@xxxxxxxxxxxxxx/

Thanks for your patch!

> --- a/include/asm-generic/atomic64.h
> +++ b/include/asm-generic/atomic64.h
> @@ -10,7 +10,7 @@
>  #include <linux/types.h>
>
>  typedef struct {
> -       s64 counter;
> +       s64 counter __aligned(sizeof(long));
>  } atomic64_t;
>
>  #define ATOMIC64_INIT(i)       { (i) }
> diff --git a/include/linux/types.h b/include/linux/types.h
> index 6dfdb8e8e4c3..cd5b2b0f4b02 100644
> --- a/include/linux/types.h
> +++ b/include/linux/types.h
> @@ -179,7 +179,7 @@ typedef phys_addr_t resource_size_t;
>  typedef unsigned long irq_hw_number_t;
>
>  typedef struct {
> -       int counter;
> +       int counter __aligned(sizeof(int));
>  } atomic_t;
>
>  #define ATOMIC_INIT(i) { (i) }

This triggers a failure in kernel/bpf/rqspinlock.c:

kernel/bpf/rqspinlock.c: In function ‘bpf_res_spin_lock’:
include/linux/compiler_types.h:572:45: error: call to
‘__compiletime_assert_397’ declared with attribute error: BUILD_BUG_ON
failed: __alignof__(rqspinlock_t) != __alignof__(struct
bpf_res_spin_lock)
  572 |         _compiletime_assert(condition, msg,
__compiletime_assert_, __COUNTER__)
      |                                             ^
include/linux/compiler_types.h:553:25: note: in definition of macro
‘__compiletime_assert’
  553 |                         prefix ## suffix();
         \
      |                         ^~~~~~
include/linux/compiler_types.h:572:9: note: in expansion of macro
‘_compiletime_assert’
  572 |         _compiletime_assert(condition, msg,
__compiletime_assert_, __COUNTER__)
      |         ^~~~~~~~~~~~~~~~~~~
include/linux/build_bug.h:39:37: note: in expansion of macro
‘compiletime_assert’
   39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
      |                                     ^~~~~~~~~~~~~~~~~~
include/linux/build_bug.h:50:9: note: in expansion of macro ‘BUILD_BUG_ON_MSG’
   50 |         BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
      |         ^~~~~~~~~~~~~~~~
kernel/bpf/rqspinlock.c:695:9: note: in expansion of macro ‘BUILD_BUG_ON’
  695 |         BUILD_BUG_ON(__alignof__(rqspinlock_t) !=
__alignof__(struct bpf_res_spin_lock));
      |         ^~~~~~~~~~~~

I haven't investigated it yet.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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