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

[Geert Uytterhoeven]

Hi Paul,

On Mon, Aug 22, 2016 at 9:54 PM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> On Mon, Aug 22, 2016 at 03:18:54PM -0400, Steven Rostedt wrote:
>> On Mon, 22 Aug 2016 20:56:09 +0200
>> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>>
>> > > Don't we have __alignof__(void *) to avoid #ifdef CONFIG_M68K and
>> > > other new macros ?
>
> Hmmm...  Does __alignof__(void *) give two-byte alignment on m68k,
> allowing something like this?  Heh!!!  It is already there.  ;-)
>
> struct callback_head {
>         struct callback_head *next;
>         void (*func)(struct callback_head *head);
> } __attribute__((aligned(sizeof(void *))));

No, it's aligning to sizeof(void *) (4 on m68k), not __alignof__(void *).

> #define rcu_head callback_head
>
> If so, that does sound quite attractive!  Might need the WARN_ON()
> anyway, to flag wild pointers if nothing else.
>
> Adding Geert on CC for his thoughts.

__alignof__(void *)  is indeed 2 on m68k, and h8300.

Note that it is 1 on crisv32!

It's 4 or 8 on anything else I have a cross-compiler for.

$ cat a.c
unsigned x = __alignof__(void *);
$ for i in /opt/cross/*/*/bin/*gcc; do echo +++ $i +++; $i -c -S a.c;
cat a.s; done | less

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