rcu: Make list_splice_init_rcu() account for RCU readers

The list_splice_init_rcu() function allows a list visible to RCU readers
to be spliced into another list visible to RCU readers.  This is OK,
except for the use of INIT_LIST_HEAD(), which does pointer updates
without doing anything to make those updates safe for concurrent readers.

Of course, most of the time INIT_LIST_HEAD() is being used in reader-free
contexts, such as initialization or cleanup, so it is OK for it to update
pointers in an unsafe-for-RCU-readers manner.  This commit therefore
creates an INIT_LIST_HEAD_RCU() that uses ACCESS_ONCE() to make the updates
reader-safe.  The reason that we can use ACCESS_ONCE() instead of the more
typical rcu_assign_pointer() is that list_splice_init_rcu() is updating the
pointers to reference something that is already visible to readers, so
that there is no problem with pre-initialized values.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

include/linux/rculist.h

@@ -18,6 +18,21 @@
  * be used anywhere you would want to use a list_empty_rcu().
  */
 
+/*
+ * INIT_LIST_HEAD_RCU - Initialize a list_head visible to RCU readers
+ * @list: list to be initialized
+ *
+ * You should instead use INIT_LIST_HEAD() for normal initialization and
+ * cleanup tasks, when readers have no access to the list being initialized.
+ * However, if the list being initialized is visible to readers, you
+ * need to keep the compiler from being too mischievous.
+ */
+static inline void INIT_LIST_HEAD_RCU(struct list_head *list)
+{
+	ACCESS_ONCE(list->next) = list;
+	ACCESS_ONCE(list->prev) = list;
+}
+
 /*
  * return the ->next pointer of a list_head in an rcu safe
  * way, we must not access it directly
@@ -191,9 +206,13 @@ static inline void list_splice_init_rcu(struct list_head *list,
 	if (list_empty(list))
 		return;
 
-	/* "first" and "last" tracking list, so initialize it. */
+	/*
+	 * "first" and "last" tracking list, so initialize it.  RCU readers
+	 * have access to this list, so we must use INIT_LIST_HEAD_RCU()
+	 * instead of INIT_LIST_HEAD().
+	 */
 
-	INIT_LIST_HEAD(list);
+	INIT_LIST_HEAD_RCU(list);
 
 	/*
 	 * At this point, the list body still points to the source list.