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-rw-r--r--include/asm-generic/rwonce.h90
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diff --git a/include/asm-generic/rwonce.h b/include/asm-generic/rwonce.h
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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 */