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

Andrew Morton
2022-12-22T17:21:56Z

On Thu, 22 Dec 2022 12:46:16 +0100 Andrzej Hajda <andrzej.hajda@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);