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- /* SPDX-License-Identifier: GPL-2.0 */
- /*
- * Prevent the compiler from merging or refetching reads or writes. The
- * compiler is also forbidden from reordering successive instances of
- * READ_ONCE and WRITE_ONCE, but only when the compiler is aware of some
- * particular ordering. One way to make the compiler aware of ordering is to
- * put the two invocations of READ_ONCE or WRITE_ONCE in different C
- * statements.
- *
- * These two macros will also work on aggregate data types like structs or
- * unions.
- *
- * Their two major use cases are: (1) Mediating communication between
- * process-level code and irq/NMI handlers, all running on the same CPU,
- * and (2) Ensuring that the compiler does not fold, spindle, or otherwise
- * mutilate accesses that either do not require ordering or that interact
- * with an explicit memory barrier or atomic instruction that provides the
- * required ordering.
- */
- #ifndef __ASM_GENERIC_RWONCE_H
- #define __ASM_GENERIC_RWONCE_H
- #ifndef __ASSEMBLY__
- #include <linux/compiler_types.h>
- #include <linux/kasan-checks.h>
- #include <linux/kcsan-checks.h>
- /*
- * Yes, this permits 64-bit accesses on 32-bit architectures. These will
- * actually be atomic in some cases (namely Armv7 + LPAE), but for others we
- * rely on the access being split into 2x32-bit accesses for a 32-bit quantity
- * (e.g. a virtual address) and a strong prevailing wind.
- */
- #define compiletime_assert_rwonce_type(t) \
- compiletime_assert(__native_word(t) || sizeof(t) == sizeof(long long), \
- "Unsupported access size for {READ,WRITE}_ONCE().")
- /*
- * Use __READ_ONCE() instead of READ_ONCE() if you do not require any
- * atomicity. Note that this may result in tears!
- */
- #ifndef __READ_ONCE
- #define __READ_ONCE(x) (*(const volatile __unqual_scalar_typeof(x) *)&(x))
- #endif
- #define READ_ONCE(x) \
- ({ \
- compiletime_assert_rwonce_type(x); \
- __READ_ONCE(x); \
- })
- #define __WRITE_ONCE(x, val) \
- do { \
- *(volatile typeof(x) *)&(x) = (val); \
- } while (0)
- #define WRITE_ONCE(x, val) \
- do { \
- compiletime_assert_rwonce_type(x); \
- __WRITE_ONCE(x, val); \
- } while (0)
- static __no_sanitize_or_inline
- unsigned long __read_once_word_nocheck(const void *addr)
- {
- return __READ_ONCE(*(unsigned long *)addr);
- }
- /*
- * Use READ_ONCE_NOCHECK() instead of READ_ONCE() if you need to load a
- * word from memory atomically but without telling KASAN/KCSAN. This is
- * usually used by unwinding code when walking the stack of a running process.
- */
- #define READ_ONCE_NOCHECK(x) \
- ({ \
- compiletime_assert(sizeof(x) == sizeof(unsigned long), \
- "Unsupported access size for READ_ONCE_NOCHECK()."); \
- (typeof(x))__read_once_word_nocheck(&(x)); \
- })
- static __no_kasan_or_inline
- unsigned long read_word_at_a_time(const void *addr)
- {
- kasan_check_read(addr, 1);
- return *(unsigned long *)addr;
- }
- #endif /* __ASSEMBLY__ */
- #endif /* __ASM_GENERIC_RWONCE_H */
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