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

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Date:   Fri,  9 Dec 2022 16:48:39 +0100
From:   Andrzej Hajda <andrzej.hajda@...el.com>
To:     linux-kernel@...r.kernel.org, intel-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org
Cc:     Andrzej Hajda <andrzej.hajda@...el.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        Andi Shyti <andi.shyti@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>
Subject: [PATCH 1/5] linux/minmax.h: add non-atomic version of xchg

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@...el.com>
---
Hi,

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 used, of course I do not intend to do it :).

I was not sure where to put this macro, I hope near swap definition
is the most suitable place.

Moreover sorry if to/cc is not correct - get_maintainers.pl was
more confused than me, to who address this patch.

Regards
Andrzej
---
 include/linux/minmax.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/include/linux/minmax.h b/include/linux/minmax.h
index 5433c08fcc6858..17d48769203bd5 100644
--- a/include/linux/minmax.h
+++ b/include/linux/minmax.h
@@ -144,4 +144,18 @@
 #define swap(a, b) \
 	do { typeof(a) __tmp = (a); (a) = (b); (b) = __tmp; } while (0)

+/**
+ * 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;	\
+	*(__ptr) = (val);		\
+	__t;				\
+})
+
 #endif	/* _LINUX_MINMAX_H */
-- 
2.34.1