linux-kernel - Re: [PATCH 00/19] Introduce _

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Date:   Thu, 22 Dec 2022 15:12:19 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Andrzej Hajda <andrzej.hajda@...el.com>
Cc:     linux-alpha@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-snps-arc@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-hexagon@...r.kernel.org, linux-ia64@...r.kernel.org,
        loongarch@...ts.linux.dev, linux-m68k@...ts.linux-m68k.org,
        linux-mips@...r.kernel.org, openrisc@...ts.librecores.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, intel-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, Arnd Bergmann <arnd@...db.de>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH 00/19] Introduce __xchg, non-atomic xchg

Hi Andrzej,

Thanks for your series!

On Thu, Dec 22, 2022 at 12:49 PM Andrzej Hajda <andrzej.hajda@...el.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@...ux-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