[tip: locking/core] linux/include: add non-atomic version of xchg

[tip-bot2 for Andrzej Hajda]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     6e58fca8f12015d783df9d155bbb969a09a29e64
Gitweb:        https://git.kernel.org/tip/6e58fca8f12015d783df9d155bbb969a09a29e64
Author:        Andrzej Hajda <andrzej.hajda@xxxxxxxxx>
AuthorDate:    Wed, 18 Jan 2023 16:44:45 +01:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Sat, 11 Mar 2023 14:03:58 +01:00

linux/include: 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.

Signed-off-by: Andrzej Hajda <andrzej.hajda@xxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
Reviewed-by: Andi Shyti <andi.shyti@xxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20230118154450.73842-2-andrzej.hajda@xxxxxxxxx
---
 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 0000000..f7fa5dd
--- /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