/* 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_inlineunsigned 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_sanitize_or_inlineunsigned long read_word_at_a_time(const void *addr){	/* open-coded instrument_read(addr, 1) */	kasan_check_read(addr, 1);	kcsan_check_read(addr, 1); 	/*	 * This load can race with concurrent stores to out-of-bounds memory,	 * but READ_ONCE() can't be used because it requires higher alignment	 * than plain loads in arm64 builds with LTO.	 */	return *(unsigned long *)addr;} #endif /* __ASSEMBLY__ */#endif	/* __ASM_GENERIC_RWONCE_H */