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
__read_once_word_nocheck(const void * addr)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
read_word_at_a_time(const void * addr)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