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

[Andrew Morton]

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.  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?

> 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);