Re: [PATCH v4 1/5] getcpu_cache system call: cache CPU number of running thread

[Geert Uytterhoeven <geert-Td1EMuHUCqxL1ZNQvxDV9g@public.gmane.org>]

On Mon, Feb 29, 2016 at 1:41 PM, Mathieu Desnoyers
<mathieu.desnoyers-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org> wrote:
> ----- On Feb 29, 2016, at 5:39 AM, Arnd Bergmann arnd-r2nGTMty4D4@public.gmane.org wrote:
>
>> On Monday 29 February 2016 11:32:21 Peter Zijlstra wrote:
>>> On Sun, Feb 28, 2016 at 12:39:54AM +0000, Mathieu Desnoyers wrote:
>>>
>>> > /* This structure needs to be aligned cache line size. */
>>> > struct thread_local_abi {
>>> >   int32_t  cpu_id;
>>> >   uint32_t rseq_seqnum;
>>> >   uint64_t rseq_post_commit_ip;
>>> >   /* Add new fields at the end. */
>>> > } __attribute__((packed));
>>>
>>> I would really not use packed; that can lead to horrible layout.
>>>
>>> Suppose someone would add:
>>>
>>>      uint32_t foo;
>>>      uint64_t bar;
>>>
>>> With packed, you get an unaligned uint64_t in there, which is horrible.
>>> Without packed, you get a hole, which you can later fill.
>>
>
> Actually, Peter is wrong about the hole there. On some 32-bit architectures,
> 64-bit integers are aligned on 32-bit, not 64-bit. So there may or may not

... or even on 16-bit.

> be a hole there, and that would lead to a mess.

indeed.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert-Td1EMuHUCqxL1ZNQvxDV9g@public.gmane.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