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) \
54 do { \
55 *(volatile typeof(x) *)&(x) = (val); \
56 } while (0)
57
58 #define WRITE_ONCE(x, val) \
59 do { \
60 compiletime_assert_rwonce_type(x); \
61 __WRITE_ONCE(x, val); \
62 } while (0)
63
64 static __no_sanitize_or_inline
65 unsigned 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
82 static __no_kasan_or_inline
83 unsigned 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
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