Re: [PATCH 00/19] Introduce __xchg, non-atomic xchg

Geert Uytterhoeven
2022-12-22T14:12:19Z

Hi Andrzej,

Thanks for your series!

On Thu, Dec 22, 2022 at 12:49 PM Andrzej Hajda <andrzej.hajda@intel.com> wrote:
> I hope there will be place for such tiny helper in kernel.
> Quick cocci analyze shows there is probably few thousands places
> where it could be useful.
> I am not sure who is good person to review/ack such patches,
> so I've used my intuition to construct to/cc lists, sorry for mistakes.
> This is the 2nd approach of the same idea, with comments addressed[0].
>
> The helper is tiny and there are advices we can leave without it, so
> I want to present few arguments why it would be good to have it:
>
> 1. Code readability/simplification/number of lines:
>
> Real example from drivers/net/ethernet/mellanox/mlx5/core/esw/qos.c:
> - previous_min_rate = evport->qos.min_rate;
> - evport->qos.min_rate = min_rate;
> + previous_min_rate = __xchg(evport->qos.min_rate, min_rate);

Upon closer look, shouldn't that be

previous_min_rate = __xchg(&evport->qos.min_rate, min_rate);

?

> For sure the code is more compact, and IMHO more readable.
>
> 2. Presence of similar helpers in other somehow related languages/libs:
>
> a) Rust[1]: 'replace' from std::mem module, there is also 'take'
> helper (__xchg(&x, 0)), which is the same as private helper in
> i915 - fetch_and_zero, see latest patch.
> b) C++ [2]: 'exchange' from utility header.
>
> If the idea is OK there are still 2 qestions to answer:
>
> 1. Name of the helper, __xchg follows kernel conventions,
> but for me Rust names are also OK.

Before I realized the missing "&", I wondered how this is different
from swap(), so naming is important.
https://elixir.bootlin.com/linux/latest/source/include/linux/minmax.h#L139

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@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