| 1 |
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| 2 | |
| 3 | * Prevent the compiler from merging or refetching reads or writes. The
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| 4 | * compiler is also forbidden from reordering successive instances of
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| 5 | * READ_ONCE and WRITE_ONCE, but only when the compiler is aware of some
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| 6 | * particular ordering. One way to make the compiler aware of ordering is to
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| 7 | * put the two invocations of READ_ONCE or WRITE_ONCE in different C
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| 8 | * statements.
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| 9 | *
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| 10 | * These two macros will also work on aggregate data types like structs or
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| 11 | * unions.
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| 12 | *
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| 13 | * Their two major use cases are: (1) Mediating communication between
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| 14 | * process-level code and irq/NMI handlers, all running on the same CPU,
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| 15 | * and (2) Ensuring that the compiler does not fold, spindle, or otherwise
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| 16 | * mutilate accesses that either do not require ordering or that interact
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| 17 | * with an explicit memory barrier or atomic instruction that provides the
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| 18 | * required ordering.
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| 19 |
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| 20 | #ifndef __ASM_GENERIC_RWONCE_H
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| 21 | #define __ASM_GENERIC_RWONCE_H
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| 22 |
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| 23 | #ifndef __ASSEMBLY__
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| 24 |
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| 25 | #include <linux/compiler_types.h>
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| 26 | #include <linux/kasan-checks.h>
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| 27 | #include <linux/kcsan-checks.h>
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| 28 |
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| 29 | |
| 30 | * Yes, this permits 64-bit accesses on 32-bit architectures. These will
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| 31 | * actually be atomic in some cases (namely Armv7 + LPAE), but for others we
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| 32 | * rely on the access being split into 2x32-bit accesses for a 32-bit quantity
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| 33 | * (e.g. a virtual address) and a strong prevailing wind.
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| 34 |
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| 35 | #define compiletime_assert_rwonce_type(t) \
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| 36 | compiletime_assert(__native_word(t) || sizeof(t) == sizeof(long long), \
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| 37 | "Unsupported access size for {READ,WRITE}_ONCE().")
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| 38 |
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| 39 | |
| 40 | * Use __READ_ONCE() instead of READ_ONCE() if you do not require any
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| 41 | * atomicity. Note that this may result in tears!
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| 42 |
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| 43 | #ifndef __READ_ONCE
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| 44 | #define __READ_ONCE(x) (*(const volatile __unqual_scalar_typeof(x) *)&(x))
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| 45 | #endif
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| 46 |
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| 47 | #define READ_ONCE(x) \
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| 48 | ({ \
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| 49 | compiletime_assert_rwonce_type(x); \
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| 50 | __READ_ONCE(x); \
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| 51 | })
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| 52 |
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| 53 | #define __WRITE_ONCE(x, val) \
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| 54 | do { \
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| 55 | *(volatile typeof(x) *)&(x) = (val); \
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| 56 | } while (0)
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| 57 |
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| 58 | #define WRITE_ONCE(x, val) \
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| 59 | do { \
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| 60 | compiletime_assert_rwonce_type(x); \
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| 61 | __WRITE_ONCE(x, val); \
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| 62 | } while (0)
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| 63 |
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| 64 | static __no_sanitize_or_inline
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| 65 | unsigned long __read_once_word_nocheck(const void *addr)
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| 66 | {
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| 67 | return __READ_ONCE(*(unsigned long *)addr);
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| 68 | }
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| 69 |
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| 70 | |
| 71 | * Use READ_ONCE_NOCHECK() instead of READ_ONCE() if you need to load a
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| 72 | * word from memory atomically but without telling KASAN/KCSAN. This is
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| 73 | * usually used by unwinding code when walking the stack of a running process.
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| 74 |
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| 75 | #define READ_ONCE_NOCHECK(x) \
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| 76 | ({ \
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| 77 | compiletime_assert(sizeof(x) == sizeof(unsigned long), \
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| 78 | "Unsupported access size for READ_ONCE_NOCHECK()."); \
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| 79 | (typeof(x))__read_once_word_nocheck(&(x)); \
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| 80 | })
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| 81 |
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| 82 | static __no_sanitize_or_inline
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| 83 | unsigned long read_word_at_a_time(const void *addr)
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| 84 | {
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| 85 |
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| 86 | kasan_check_read(addr, 1);
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| 87 | kcsan_check_read(addr, 1);
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| 88 |
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| 89 | |
| 90 | * This load can race with concurrent stores to out-of-bounds memory,
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| 91 | * but READ_ONCE() can't be used because it requires higher alignment
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| 92 | * than plain loads in arm64 builds with LTO.
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| 93 |
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| 94 | return *(unsigned long *)addr;
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| 95 | }
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| 96 |
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| 97 | #endif
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| 98 | #endif
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