1// SPDX-License-Identifier: GPL-2.0+
2/*
3 * Emulated 1-byte cmpxchg operation for architectures lacking direct
4 * support for this size.  This is implemented in terms of 4-byte cmpxchg
5 * operations.
6 *
7 * Copyright (C) 2024 Paul E. McKenney.
8 */
9
10#include <linux/types.h>
11#include <linux/export.h>
12#include <linux/instrumented.h>
13#include <linux/atomic.h>
14#include <linux/panic.h>
15#include <linux/bug.h>
16#include <asm-generic/rwonce.h>
17#include <linux/cmpxchg-emu.h>
18
19union u8_32 {
20	u8 b[4];
21	u32 w;
22};
23
24/* Emulate one-byte cmpxchg() in terms of 4-byte cmpxchg. */
25uintptr_t cmpxchg_emu_u8(volatile u8 *p, uintptr_t old, uintptr_t new)
26{
27	u32 *p32 = (u32 *)(((uintptr_t)p) & ~0x3);
28	int i = ((uintptr_t)p) & 0x3;
29	union u8_32 old32;
30	union u8_32 new32;
31	u32 ret;
32
33	ret = READ_ONCE(*p32);
34	do {
35		old32.w = ret;
36		if (old32.b[i] != old)
37			return old32.b[i];
38		new32.w = old32.w;
39		new32.b[i] = new;
40		instrument_atomic_read_write(p, 1);
41		ret = data_race(cmpxchg(p32, old32.w, new32.w)); // Overridden above.
42	} while (ret != old32.w);
43	return old;
44}
45EXPORT_SYMBOL_GPL(cmpxchg_emu_u8);
46