linux-kernel - Re: [PATCH 18/19] linux/include: add non-atomic version of xchg

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Date:   Thu, 22 Dec 2022 14:47:42 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Andrzej Hajda <andrzej.hajda@...el.com>
Cc:     linux-alpha@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-snps-arc@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-hexagon@...r.kernel.org, linux-ia64@...r.kernel.org,
        loongarch@...ts.linux.dev, linux-m68k@...ts.linux-m68k.org,
        linux-mips@...r.kernel.org, openrisc@...ts.librecores.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, intel-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, Arnd Bergmann <arnd@...db.de>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH 18/19] linux/include: add non-atomic version of xchg

On Thu, Dec 22, 2022 at 12:46:34PM +0100, 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.

FWIW,
Reviewed-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>

> Signed-off-by: Andrzej Hajda <andrzej.hajda@...el.com>
> ---
>  include/linux/non-atomic/xchg.h | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
>  create mode 100644 include/linux/non-atomic/xchg.h
> 
> diff --git a/include/linux/non-atomic/xchg.h b/include/linux/non-atomic/xchg.h
> new file mode 100644
> index 00000000000000..f7fa5dd746f37d
> --- /dev/null
> +++ b/include/linux/non-atomic/xchg.h
> @@ -0,0 +1,19 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _LINUX_NON_ATOMIC_XCHG_H
> +#define _LINUX_NON_ATOMIC_XCHG_H
> +
> +/**
> + * __xchg - 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 __xchg(ptr, val) ({		\
> +	__auto_type __ptr = ptr;	\
> +	__auto_type __t = *__ptr;	\
> +	*__ptr = (val);			\
> +	__t;				\
> +})
> +
> +#endif
> -- 
> 2.34.1
> 

-- 
With Best Regards,
Andy Shevchenko