Re: [PATCH 1/5] linux/minmax.h: add non-atomic version of xchg

Arnd Bergmann
2022-12-09T17:17:23Z

On Fri, Dec 9, 2022, at 16:48, Andrzej Hajda wrote:
> The pattern of setting variable with new value and returning old
> one is very common in kernel. Usually atomicity of the operation
> is not required, so xchg seems to be suboptimal and confusing in
> such cases. Since name xchg is already in use and __xchg is used
> in architecture code, proposition is to name the macro exchange.
>
> Signed-off-by: Andrzej Hajda <andrzej.hajda@intel.com>

While I generally don't like type invariant calling conventions
of xchg() and cmpxchg(), having a new function that has a similar
name without being able to tell which one is which from the
name seems more confusing.

Since __xchg() is only used on 11 architectures as an internal
name for the backing of arch_xchg() or arch_xchg_relaxed(),
maybe we can instead rename those to __arch_xchg() and use the
__xchg() name for the new non-atomic version?

> +/**
> + * exchange - set variable pointed by @ptr to @val, return old value
> + * @ptr: pointer to affected variable
> + * @val: value to be written
> + *
> + * This is non-atomic variant of xchg.
> + */
> +#define exchange(ptr, val) ({ \
> + typeof(ptr) __ptr = ptr; \
> + typeof(*__ptr) __t = *__ptr; \

I think you can better express this using __auto_type than typeof(),
it is now provided by all supported compilers now.

Arnd