On Mon, Feb 29, 2016 at 1:41 PM, Mathieu Desnoyers
[off-list ref] wrote:

----- On Feb 29, 2016, at 5:39 AM, Arnd Bergmann arnd-r2nGTMty4D4@public.gmane.org wrote:

quoted

On Monday 29 February 2016 11:32:21 Peter Zijlstra wrote:

quoted

On Sun, Feb 28, 2016 at 12:39:54AM +0000, Mathieu Desnoyers wrote:

quoted

/* 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