1 // SPDX-License-Identifier: GPL-2.0+
2 /*
3 * Emulated 1-byte and 2-byte cmpxchg operations for architectures lacking
4 * direct support for these sizes. These are implemented in terms of
5 * 4-byte cmpxchg operations.
6 *
7 * Copyright (C) 2024 Paul E. McKenney <paulmck@kernel.org>
8 * Copyright (C) 2026 Bradley Morgan <brads@mainlining.org>
9 */
10
11 #include <linux/types.h>
12 #include <linux/export.h>
13 #include <linux/instrumented.h>
14 #include <linux/atomic.h>
15 #include <linux/panic.h>
16 #include <linux/bug.h>
17 #include <asm-generic/rwonce.h>
18 #include <linux/cmpxchg-emu.h>
19
20 union u8_32 {
21 u8 b[4];
22 u32 w;
23 };
24
25 /* Emulate one-byte cmpxchg() in terms of 4-byte cmpxchg. */
cmpxchg_emu_u8(volatile u8 * p,uintptr_t old,uintptr_t new)26 uintptr_t cmpxchg_emu_u8(volatile u8 *p, uintptr_t old, uintptr_t new)
27 {
28 u32 *p32 = (u32 *)(((uintptr_t)p) & ~0x3);
29 int i = ((uintptr_t)p) & 0x3;
30 union u8_32 old32;
31 union u8_32 new32;
32 u32 ret;
33
34 ret = READ_ONCE(*p32);
35 do {
36 old32.w = ret;
37 if (old32.b[i] != old)
38 return old32.b[i];
39 new32.w = old32.w;
40 new32.b[i] = new;
41 instrument_atomic_read_write(p, 1);
42 ret = data_race(cmpxchg(p32, old32.w, new32.w)); // Overridden above.
43 } while (ret != old32.w);
44 return old;
45 }
46 EXPORT_SYMBOL_GPL(cmpxchg_emu_u8);
47
48 union u16_32 {
49 u16 h[2];
50 u32 w;
51 };
52
53 /* Emulate two-byte cmpxchg() in terms of 4-byte cmpxchg. */
cmpxchg_emu_u16(volatile u16 * p,unsigned long old,unsigned long new)54 unsigned long cmpxchg_emu_u16(volatile u16 *p, unsigned long old, unsigned long new)
55 {
56 u32 *p32 = (u32 *)(((uintptr_t)p) & ~0x3);
57 int i = (((uintptr_t)p) & 0x2) / 2;
58 union u16_32 old32;
59 union u16_32 new32;
60 u32 ret;
61
62 WARN_ON_ONCE(((uintptr_t)p) & 0x1);
63 ret = READ_ONCE(*p32);
64 do {
65 old32.w = ret;
66 if (old32.h[i] != (u16)old)
67 return (unsigned long)old32.h[i];
68 new32.w = old32.w;
69 new32.h[i] = (u16)new;
70 instrument_atomic_read_write(p, 2);
71 ret = data_race(cmpxchg(p32, old32.w, new32.w)); // Overridden above.
72 } while (ret != old32.w);
73 return (u16)old;
74 }
75 EXPORT_SYMBOL_GPL(cmpxchg_emu_u16);
76