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

[Andrzej Hajda]

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 at intel.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