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

[Alexander Lobakin]

From: Andrew Morton <akpm at linux-foundation.org>
Date: Thu, 22 Dec 2022 09:21:47 -0800

> 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);

Unforunately, swap()'s arguments get passed by names, not as
pointers, so you can't do

	swap(some_ptr, NULL);

 -- pretty common pattern for xchg.

Thanks,
Olek