On Sun, Jun 6, 2010 at 20:37, Brad Boyer <f...@allandria.com> wrote:
> On Sun, Jun 06, 2010 at 10:42:40AM +0200, Andreas Schwab wrote:
>> Brad Boyer <f...@allandria.com> writes:
>>
>> > I may be missing something obvious, but why don't we just change the
>> > kernel to allow 2-byte alignment for m68k? The comment in futex.c
>> > just says "natural" alignment but is then hard-coded for sizeof(u32).
>>
>> Natural alignment *is* sizeof.
>
> OK, then I guess my real question is "why does it need alignment"? I
> obviously don't have a good understanding of the futex code, but this
> seems like something that could have pretty significant effects. It
> looks like the need for natural alignment is to avoid alignment
> exceptions in the kernel or something like that, but m68k doesn't
> have such a thing since the hardware always does it. Please let
> me know what I missing here. I even looked at the 68040 UM to see
> if the CAS instruction does anything odd, since I know some other
> chips have some weird limits on atomic instructions.
As Andreas already pointed out:
kernel/futex.c:
| static void get_futex_key_refs(union futex_key *key)
| {
| if (!key->both.ptr)
| return;
|
| switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {
| case FUT_OFF_INODE:
| atomic_inc(&key->shared.inode->i_count);
| break;
| case FUT_OFF_MMSHARED:
| atomic_inc(&key->private.mm->mm_count);
| break;
| }
| }
and include/linux/futex.h:
| /*
| * Futexes are matched on equal values of this key.
| * The key type depends on whether it's a shared or private mapping.
| * Don't rearrange members without looking at hash_futex().
| *
| * offset is aligned to a multiple of sizeof(u32) (== 4) by definition.
| * We use the two low order bits of offset to tell what is the kind of key :
| * 00 : Private process futex (PTHREAD_PROCESS_PRIVATE)
| * (no reference on an inode or mm)
| * 01 : Shared futex (PTHREAD_PROCESS_SHARED)
| * mapped on a file (reference on the underlying inode)
| * 10 : Shared futex (PTHREAD_PROCESS_SHARED)
| * (but private mapping on an mm, and reference taken on it)
| */
|
| #define FUT_OFF_INODE 1 /* We set bit 0 if key has a reference on inode */
| #define FUT_OFF_MMSHARED 2 /* We set bit 1 if key has a reference on mm */
I.e. the futex code uses the two low order bits for its private
purposes (nihil novi sub sole).
Not much we can do about that...
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- ge...@linux-m68k.org
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
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