add READ_ONCE() and WRITE_ONCE()

c/rwonce.h

@@ -24,6 +24,9 @@
 #ifndef __BR_RWONCE_H
 #define __BR_RWONCE_H
 
+/************ originally in <include/linux/compiler_types.h> */
+# define __compiletime_error(message) __attribute__((error(message)))
+
 /************ originally in <include/linux/compiler_types.h> */
 /*
  * __unqual_scalar_typeof(x) - Declare an unqualified scalar type, leaving
@@ -52,6 +55,36 @@
 	(sizeof(t) == sizeof(char) || sizeof(t) == sizeof(short) || \
 	 sizeof(t) == sizeof(int) || sizeof(t) == sizeof(long))
 
+#ifdef __OPTIMIZE__
+# define __compiletime_assert(condition, msg, prefix, suffix)		\
+	do {								\
+		extern void prefix ## suffix(void) __compiletime_error(msg); \
+		if (!(condition))					\
+			prefix ## suffix();				\
+	} while (0)
+#else
+# define __compiletime_assert(condition, msg, prefix, suffix) do { } while (0)
+#endif
+
+#define _compiletime_assert(condition, msg, prefix, suffix) \
+	__compiletime_assert(condition, msg, prefix, suffix)
+
+/**
+ * compiletime_assert - break build and emit msg if condition is false
+ * @condition: a compile-time constant condition to check
+ * @msg:       a message to emit if condition is false
+ *
+ * In tradition of POSIX assert, this macro will break the build if the
+ * supplied condition is *false*, emitting the supplied error message if the
+ * compiler has support to do so.
+ */
+#define compiletime_assert(condition, msg) \
+	_compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
+
+#define compiletime_assert_atomic_type(t)				\
+	compiletime_assert(__native_word(t),				\
+		"Need native word sized stores/loads for atomicity.")
+
 /************ originally in <asm-generic/rwonce.h> */
 /*
  * Yes, this permits 64-bit accesses on 32-bit architectures. These will