Move ACCESS_ONCE() to <linux/compiler.h>

It actually makes much more sense there, and we do tend to need it for
non-RCU usage too.  Moving it to <linux/compiler.h> will allow some
other cases that have open-coded the same logic to use the same helper
function that RCU has used.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/compiler.h

@@ -182,4 +182,16 @@ extern void __chk_io_ptr(const volatile void __iomem *);
 # define __section(S) __attribute__ ((__section__(#S)))
 #endif
 
+/*
+ * Prevent the compiler from merging or refetching accesses.  The compiler
+ * is also forbidden from reordering successive instances of ACCESS_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
+ * ACCESS_ONCE() in different C statements.
+ *
+ * This macro does absolutely -nothing- to prevent the CPU from reordering,
+ * merging, or refetching absolutely anything at any time.
+ */
+#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
+
 #endif /* __LINUX_COMPILER_H */

include/linux/rcupdate.h

@@ -131,18 +131,6 @@ struct rcu_head {
  */
 #define rcu_read_unlock_bh() __rcu_read_unlock_bh()
 
-/*
- * Prevent the compiler from merging or refetching accesses.  The compiler
- * is also forbidden from reordering successive instances of ACCESS_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
- * ACCESS_ONCE() in different C statements.
- *
- * This macro does absolutely -nothing- to prevent the CPU from reordering,
- * merging, or refetching absolutely anything at any time.
- */
-#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
-
 /**
  * rcu_dereference - fetch an RCU-protected pointer in an
  * RCU read-side critical section.  This pointer may later