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

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Date:   Thu, 22 Dec 2022 15:17:18 +0100
From:   Andrzej Hajda <andrzej.hajda@...el.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
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



On 22.12.2022 15:12, Geert Uytterhoeven wrote:
> 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);
>
> ?

Yes, you are right, the first argument is a pointer.

Regards
Andrzej

>
>> 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