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
19 union u8_32 {
20 u8 b[4];
21 u32 w;
22 };
23
24 /* Emulate one-byte cmpxchg() in terms of 4-byte cmpxchg. */
25 uintptr_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 }
45 EXPORT_SYMBOL_GPL(cmpxchg_emu_u8);
46
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