Add hlist_add_tail_rcu() (Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net)

BugLink: https://bugs.launchpad.net/bugs/1826212

commit 1602f49b upstream.

[This commit was a merge, but it added hlist_add_tail_rcu(), which is what we
 need in this stable tree, so I've changed the subject to be more descriptive
 - gregkh]
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

include/linux/rculist.h

@@ -401,6 +401,42 @@ static inline void hlist_add_head_rcu(struct hlist_node *n,
 		first->pprev = &n->next;
 }
 
+/**
+ * hlist_add_tail_rcu
+ * @n: the element to add to the hash list.
+ * @h: the list to add to.
+ *
+ * Description:
+ * Adds the specified element to the specified hlist,
+ * while permitting racing traversals.
+ *
+ * The caller must take whatever precautions are necessary
+ * (such as holding appropriate locks) to avoid racing
+ * with another list-mutation primitive, such as hlist_add_head_rcu()
+ * or hlist_del_rcu(), running on this same list.
+ * However, it is perfectly legal to run concurrently with
+ * the _rcu list-traversal primitives, such as
+ * hlist_for_each_entry_rcu(), used to prevent memory-consistency
+ * problems on Alpha CPUs.  Regardless of the type of CPU, the
+ * list-traversal primitive must be guarded by rcu_read_lock().
+ */
+static inline void hlist_add_tail_rcu(struct hlist_node *n,
+				      struct hlist_head *h)
+{
+	struct hlist_node *i, *last = NULL;
+
+	for (i = hlist_first_rcu(h); i; i = hlist_next_rcu(i))
+		last = i;
+
+	if (last) {
+		n->next = last->next;
+		n->pprev = &last->next;
+		rcu_assign_pointer(hlist_next_rcu(last), n);
+	} else {
+		hlist_add_head_rcu(n, h);
+	}
+}
+
 /**
  * hlist_add_before_rcu
  * @n: the new element to add to the hash list.