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 11:16 PM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> On Mon, Aug 22, 2016 at 10:48:57PM +0200, Geert Uytterhoeven wrote:
>> 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 *).
>
> Right you are.  Commit 720abae3d68ae from Kirill A. Shutemov in November
> 2015.
>
> Given that you haven't complained, I am guessing that this works for you.
> If so, I can make the __call_rcu() WARN_ON() more strict.
>
>> > #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!
>
> Gah...  ((__alignof__(void *) + 1) & ~0x1), eh?
>
>> 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
>
> Thank you for checking!
>
> Again, does the current state work for you?

Yes it does. See also your commit 1146edcbef378922 ("rcu: Loosen __call_rcu()'s
rcu_head alignment constraint").

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