linux-kernel - Re: [Intel-gfx] [PATCH 1/5] linux/minmax.h: add non-atomic version of xchg

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Date:   Tue, 13 Dec 2022 11:09:12 +0100
From:   Andrzej Hajda <andrzej.hajda@...el.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Arnd Bergmann <arnd@...db.de>, intel-gfx@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [Intel-gfx] [PATCH 1/5] linux/minmax.h: add non-atomic version of
 xchg

On 09.12.2022 19:56, Andy Shevchenko wrote:
> On Fri, Dec 09, 2022 at 04:48:39PM +0100, Andrzej Hajda wrote:
>> The pattern of setting variable with new value and returning old
>> one is very common in kernel. Usually atomicity of the operation
>> is not required, so xchg seems to be suboptimal and confusing in
>> such cases. Since name xchg is already in use and __xchg is used
>> in architecture code, proposition is to name the macro exchange.
>>
>> Signed-off-by: Andrzej Hajda <andrzej.hajda@...el.com>
>> ---
>> Hi,
>>
>> 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 used, of course I do not intend to do it :).
>>
>> I was not sure where to put this macro, I hope near swap definition
>> is the most suitable place.
> 
> Ah, swap() in this context is not the same. minmax.h hosts it because
> it's often related to the swap function in the sort-type algorithms. > >> Moreover sorry if to/cc is not correct - get_maintainers.pl was
>> more confused than me, to who address this patch.
> 
> ...
> 
>>   include/linux/minmax.h | 14 ++++++++++++++
> 
> Does it really suit this header? I would expect something else.
> Maybe include/linux/non-atomic/xchg.h, dunno.

non-atomic seems quite strange for me, I would assume everything not in 
atomic is non-atomic, unless explicitly specified.

> 
> Btw, have you looked if Ingo's gigantic series have done anything to cmpxchg.h
> and related headers? Maybe some ideas can be taken from there?
> 

Grepping it didn't give any clue.

Looking at 'near' languages just to get an idea (they name the function 
differently):

C++ [1]: exchange and swap are in utility header
Rust[2]: replace and swap are in std::mem module

This is some argument to put them together.

[1]: https://en.cppreference.com/w/cpp/header/utility
[2]: https://doc.rust-lang.org/std/mem/index.html

Regards
Andrzej