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

Andrzej Hajda
2022-12-22T14:17:18Z

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@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);
>
> ?

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@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