Re: [PATCH v2] smp: align struct __call_single_node to 8 bytes

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

On Wed, 23 Sep 2026 08:43:07 -0700
Brian Cain <brian.cain@oss.qualcomm.com> wrote:

> Consumers walking call_single_queue cast each node to call_single_data_t
> and, given that type's declared over-alignment, the compiler may load
> llist.next and u_flags as one naturally aligned double word.  Fields using
> this type, such as task_struct::wake_entry only guarantee pointer alignment,
> which faults on strict-alignment 32-bit architectures like hexagon.

I think the same could happen on arm64.
The compiler is being told the alignment is 4*sizeof(long)), but I doubt it
can ever matter that it is more than 2*sizeof(long).

The other option is to only ever use call_single_data_t to get an aligned
version of the structure and use the struct everywhere else.
The compiler won't be able to use the 'load two register' instruction, but
I doubt that will be measurable.

David

> 
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Paul E. McKenney <paulmck@kernel.org>
> Cc: Frederic Weisbecker <frederic@kernel.org>
> Cc: linux-hexagon@vger.kernel.org
> Reviewed-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
> Signed-off-by: Brian Cain <brian.cain@oss.qualcomm.com>
> ---
>  include/linux/smp_types.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/smp_types.h b/include/linux/smp_types.h
> index 2e8461af8df6..5773e1fb2660 100644
> --- a/include/linux/smp_types.h
> +++ b/include/linux/smp_types.h
> @@ -64,6 +64,6 @@ struct __call_single_node {
>  #ifdef CONFIG_64BIT
>  	u16 src, dst;
>  #endif
> -};
> +} __aligned(8);
>  
>  #endif /* __LINUX_SMP_TYPES_H */