On 10/12/12 12:55, Geert Uytterhoeven wrote:
> On Mon, Dec 10, 2012 at 11:22 AM, James Hogan <james.hogan@xxxxxxxxxx> wrote:
>> On 08/12/12 03:43, H. Peter Anvin wrote:
>>> On 12/05/2012 08:08 AM, James Hogan wrote:
>>>> On 64 bit architectures with no efficient unaligned access, taskstats
>>>> has to add some padding to a reply to prevent unaligned access warnings.
>>>> However this also needs to apply to 32 bit architectures with 64 bit
>>>> struct alignment such as metag (which has 64 bit memory accesses).
>>>
>>> Wait... 64-bit struct alignment on structures with only 32-bit members?
>>> That might be... interesting... in a number of places...
>>
>> I'll rewrite the description as it's a bit misleading. On metag 64bit
>> struct alignment is required when it contains 64bit members, not if it
>> only contains 32bit members. Although metag is a 32bit arch, it can do
>> 64bit memory accesses which must be aligned.
>
> The C alignment rules should take care of this automatically (struct alignment
> is the maximum alignment of its members).
Hi Geert,
Please see the comment in mk_reply in kernel/taskstats.c. The structure
is being serialised after 2 NLA headers and a pid which is why the extra
padding needs to be added manually.
Cheers
James
>
> You only have to override this manually in cases like this, where
> 64-bit quantities
> are stored in char arrays:
>
> struct buffer_data_page {
> u64 time_stamp; /* page time stamp */
> local_t commit; /* write committed index */
> - unsigned char data[]; /* data of buffer page */
> + unsigned char data[] RB_ALIGN_DATA; /* data of buffer page */
> };
>
> (cfr. your other patch).
>
> 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
>
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