btrfs-progs: kernel-lib: add simplified READ_ONCE and WRITE_ONCE

For easier source synchronization with kernel, add the _ONCE wrappers,
but only the simplified version as we don't do any lock-less
algorithms or use the semantics in userspace.

Signed-off-by: David Sterba <dsterba@suse.com>

kerncompat.h

@@ -531,4 +531,36 @@ struct __una_u64 { __le64 x; } __attribute__((__packed__));
 #define noinline
 #endif
 
+/*
+ * Note: simplified versions of READ_ONCE and WRITE_ONCE for source
+ * compatibility only, not usable for lock-less implementation like in kernel.
+ *
+ * Changed:
+ * - __unqual_scalar_typeof: volatile cast to typeof()
+ * - compiletime_assert_rwonce_type: no word size compatibility checks
+ */
+
+/*
+ * 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 typeof(x) *)&(x))
+#endif
+
+#define READ_ONCE(x)							\
+({									\
+	__READ_ONCE(x);							\
+})
+
+#define __WRITE_ONCE(x, val)						\
+do {									\
+	*(volatile typeof(x) *)&(x) = (val);				\
+} while (0)
+
+#define WRITE_ONCE(x, val)						\
+do {									\
+	__WRITE_ONCE(x, val);						\
+} while (0)
+
 #endif