Re: [PATCH tip/core/rcu 2/2] documentation: Record reason for rcu_head two-byte alignment

[Eric Dumazet]

On Mon, Aug 22, 2016 at 10:34 AM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:

> That does have much to say for itself, though I would prefer sizeof(void
> *) to sizeof(unsigned long).  But would it make sense to define a mask
> on a per-architecture basis, with the default being (sizeof(void *) - 1)?
> Then maybe an IMPROPERLY_ALIGNED_POINTER():
>
>         #ifndef CONFIG_ARCH_POINTER_ALIGNMENT
>         #define CONFIG_ARCH_POINTER_ALIGNMENT (sizeof(void *) - 1)
>         #endif
>
>         #define IMPROPERLY_ALIGNED_POINTER(p) \
>                 ((p) & CONFIG_ARCH_POINTER_ALIGNMENT)
>
> m68k would define ARCH_POINTER_ALIGNMENT to 1, and all other arches
> would leave it undefined.
>
> Then __call_rcu() could to this:
>
>         WARN_ON_ONCE(IMPROPERLY_ALIGNED_POINTER(head));

Don't we have __alignof__(void *) to avoid #ifdef CONFIG_M68K and
other new macros ?