rcu: Add ACCESS_ONCE() for RCU_INIT_POINTER()

It is permissible to use RCU_INIT_POINTER() instead of rcu_assign_pointer()
in a couple of cases where readers might be concurrently accessing the
pointer being assigned to: (1) When assigning NULL and (2) When assigning
a pointer that is already visible to readers.  In these two cases, we
should really be using ACCESS_ONCE() to prevent compiler mischief.  This
commit therefore adds the ACCESS_ONCE().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

include/linux/rcupdate.h

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 334ff89..315772c 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -997,7 +997,7 @@
  */
 #define RCU_INIT_POINTER(p, v) \
 	do { \
-		p = RCU_INITIALIZER(v); \
+		ACCESS_ONCE(p) = (v); \
 	} while (0)
 
 /**