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

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Date:   Thu, 29 Dec 2022 10:54:50 +0100
From:   Andrzej Hajda <andrzej.hajda@...el.com>
To:     Andrew Morton <akpm@...ux-foundation.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@...r.kernel.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>,
        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

Forgive me late response - Holidays,

On 22.12.2022 18:21, Andrew Morton wrote:
> On Thu, 22 Dec 2022 12:46:16 +0100 Andrzej Hajda <andrzej.hajda@...el.com> wrote:
>
>> Hi all,
>>
>> 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.
> So to clarify, the intent here is a simple readability cleanup for
> existing open-coded exchange operations.

And replace private helpers with common one, see the last patch - the 
ultimate goal
would be to replace all occurrences of fetch_and_zero with __xchg.

> The intent is *not* to
> identify existing xchg() sites which are unnecessarily atomic and to
> optimize them by using the non-atomic version.
>
> Have you considered the latter?

If you mean some way of (semi-)automatic detection of such cases, then 
no. Anyway this could be quite interesting challenge.

>
>> I am not sure who is good person to review/ack such patches,
> I can take 'em.
>
>> 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);
>>
>> 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.
> I like replace(), or, shockingly, exchange().
>
> But...   Can we simply make swap() return the previous value?
>
> 	previous_min_rate = swap(&evport->qos.min_rate, min_rate);

As Alexander already pointed out, swap requires 'references' to two 
variables,
in contrast to xchg which requires reference to variable and value.
So we cannot use swap for cases:
     old_value = __xchg(&x, new_value);

Regards
Andrzej