1/* SPDX-License-Identifier: GPL-2.0 */
2/*
3 * Prevent the compiler from merging or refetching reads or writes. The
4 * compiler is also forbidden from reordering successive instances of
5 * READ_ONCE and WRITE_ONCE, but only when the compiler is aware of some
6 * particular ordering. One way to make the compiler aware of ordering is to
7 * put the two invocations of READ_ONCE or WRITE_ONCE in different C
8 * statements.
9 *
10 * These two macros will also work on aggregate data types like structs or
11 * unions.
12 *
13 * Their two major use cases are: (1) Mediating communication between
14 * process-level code and irq/NMI handlers, all running on the same CPU,
15 * and (2) Ensuring that the compiler does not fold, spindle, or otherwise
16 * mutilate accesses that either do not require ordering or that interact
17 * with an explicit memory barrier or atomic instruction that provides the
18 * required ordering.
19 */
20#ifndef __ASM_GENERIC_RWONCE_H
21#define __ASM_GENERIC_RWONCE_H
22
23#ifndef __ASSEMBLY__
24
25#include <linux/compiler_types.h>
26#include <linux/kasan-checks.h>
27#include <linux/kcsan-checks.h>
28
29/*
30 * Yes, this permits 64-bit accesses on 32-bit architectures. These will
31 * actually be atomic in some cases (namely Armv7 + LPAE), but for others we
32 * rely on the access being split into 2x32-bit accesses for a 32-bit quantity
33 * (e.g. a virtual address) and a strong prevailing wind.
34 */
35#define compiletime_assert_rwonce_type(t)					\
36	compiletime_assert(__native_word(t) || sizeof(t) == sizeof(long long),	\
37		"Unsupported access size for {READ,WRITE}_ONCE().")
38
39/*
40 * Use __READ_ONCE() instead of READ_ONCE() if you do not require any
41 * atomicity. Note that this may result in tears!
42 */
43#ifndef __READ_ONCE
44#define __READ_ONCE(x)	(*(const volatile __unqual_scalar_typeof(x) *)&(x))
45#endif
46
47#define READ_ONCE(x)							\
48({									\
49	compiletime_assert_rwonce_type(x);				\
50	__READ_ONCE(x);							\
51})
52
53#define __WRITE_ONCE(x, val)						\
54do {									\
55	*(volatile typeof(x) *)&(x) = (val);				\
56} while (0)
57
58#define WRITE_ONCE(x, val)						\
59do {									\
60	compiletime_assert_rwonce_type(x);				\
61	__WRITE_ONCE(x, val);						\
62} while (0)
63
64static __no_sanitize_or_inline
65unsigned long __read_once_word_nocheck(const void *addr)
66{
67	return __READ_ONCE(*(unsigned long *)addr);
68}
69
70/*
71 * Use READ_ONCE_NOCHECK() instead of READ_ONCE() if you need to load a
72 * word from memory atomically but without telling KASAN/KCSAN. This is
73 * usually used by unwinding code when walking the stack of a running process.
74 */
75#define READ_ONCE_NOCHECK(x)						\
76({									\
77	compiletime_assert(sizeof(x) == sizeof(unsigned long),		\
78		"Unsupported access size for READ_ONCE_NOCHECK().");	\
79	(typeof(x))__read_once_word_nocheck(&(x));			\
80})
81
82static __no_kasan_or_inline
83unsigned long read_word_at_a_time(const void *addr)
84{
85	kasan_check_read(addr, 1);
86	return *(unsigned long *)addr;
87}
88
89#endif /* __ASSEMBLY__ */
90#endif	/* __ASM_GENERIC_RWONCE_H */
91