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

[Geert Uytterhoeven]

Hi Paul,

On Tue, Aug 23, 2016 at 3:43 PM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> On Tue, Aug 23, 2016 at 08:39:18AM +0200, Geert Uytterhoeven wrote:
>> 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.
>> > 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").
>
> Understood!
>
> But given that all architectures now provide at least four-byte alignment
> for the rcu_head structure, isn't it now OK for me to tighten up __call_rcu()'s
> check, for example, to this?
>
>         WARN_ON_ONCE((unsigned long)head & (sizeof(void *) - 1));

Yes, I agree with that.

Thanks!

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