[PATCH 1/5] include/linux/rculist_nulls: add hlist_nulls_add_behind_rcu()

[Max Kellermann <max.kellermann@ionos.com>]

Move some code from hlist_nulls_add_tail_rcu() to the new function
hlist_nulls_add_behind_rcu() for callers that already know the last
node of an `hlist_nulls` list, to allow constant-time appends.

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
---
 include/linux/rculist_nulls.h | 36 +++++++++++++++++++++++++++++------
 1 file changed, 30 insertions(+), 6 deletions(-)

diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h
index a97c3bcb1656..054e477dd093 100644
--- a/include/linux/rculist_nulls.h
+++ b/include/linux/rculist_nulls.h
@@ -115,6 +115,33 @@ static inline void hlist_nulls_add_head_rcu(struct hlist_nulls_node *n,
 		WRITE_ONCE(first->pprev, &n->next);
 }
 
+/**
+ * hlist_nulls_add_behind_rcu - append after the last list entry
+ * @n: the new element to add to the hash list.
+ * @last: the last element in the list.
+ *
+ * Adds the specified element after @last while permitting racing RCU
+ * traversals.  @last must be the last list entry, with its next
+ * pointer holding the terminating nulls marker.
+ *
+ * The caller must take whatever precautions are necessary
+ * (such as holding appropriate locks) to avoid racing
+ * with another list-mutation primitive, such as hlist_nulls_add_head_rcu()
+ * or hlist_nulls_del_rcu(), running on this same list.
+ * However, it is perfectly legal to run concurrently with
+ * the _rcu list-traversal primitives, such as
+ * hlist_nulls_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_nulls_add_behind_rcu(struct hlist_nulls_node *n,
+					      struct hlist_nulls_node *last)
+{
+	WRITE_ONCE(n->next, last->next);
+	WRITE_ONCE(n->pprev, &last->next);
+	rcu_assign_pointer(hlist_nulls_next_rcu(last), n);
+}
+
 /**
  * hlist_nulls_add_tail_rcu
  * @n: the element to add to the hash list.
@@ -143,13 +170,10 @@ static inline void hlist_nulls_add_tail_rcu(struct hlist_nulls_node *n,
 	for (i = h->first; !is_a_nulls(i); i = i->next)
 		last = i;
 
-	if (last) {
-		WRITE_ONCE(n->next, last->next);
-		WRITE_ONCE(n->pprev, &last->next);
-		rcu_assign_pointer(hlist_nulls_next_rcu(last), n);
-	} else {
+	if (last)
+		hlist_nulls_add_behind_rcu(n, last);
+	else
 		hlist_nulls_add_head_rcu(n, h);
-	}
 }
 
 /* after that hlist_nulls_del will work */
-- 
2.47.3