rculist: Add hlist_nulls_replace_rcu() and hlist_nulls_replace_init_rcu()

Add two functions to atomically replace RCU-protected hlist_nulls entries.

Keep using WRITE_ONCE() to assign values to ->next and ->pprev, as
mentioned in the patch below:
commit efd04f8a8b45 ("rcu: Use WRITE_ONCE() for assignments to ->next for
rculist_nulls")
commit 860c8802ace1 ("rcu: Use WRITE_ONCE() for assignments to ->pprev for
hlist_nulls")

Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Xuanqiang Luo <luoxuanqiang@kylinos.cn>
Link: https://patch.msgid.link/20251015020236.431822-2-xuanqiang.luo@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 59 insertions, 0 deletions

diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h
index 89186c499dd4..c26cb83ca071 100644
--- a/include/linux/rculist_nulls.h
+++ b/include/linux/rculist_nulls.h
@@ -53,6 +53,13 @@ static inline void hlist_nulls_del_init_rcu(struct hlist_nulls_node *n)
 	(*((struct hlist_nulls_node __rcu __force **)&(node)->next))
 
 /**
+ * hlist_nulls_pprev_rcu - returns the dereferenced pprev of @node.
+ * @node: element of the list.
+ */
+#define hlist_nulls_pprev_rcu(node) \
+	(*((struct hlist_nulls_node __rcu __force **)(node)->pprev))
+
+/**
  * hlist_nulls_del_rcu - deletes entry from hash list without re-initialization
  * @n: the element to delete from the hash list.
  *
@@ -153,6 +160,58 @@ static inline void hlist_nulls_add_fake(struct hlist_nulls_node *n)
 }
 
 /**
+ * hlist_nulls_replace_rcu - replace an old entry by a new one
+ * @old: the element to be replaced
+ * @new: the new element to insert
+ *
+ * Description:
+ * Replace the old entry with the new one in a RCU-protected hlist_nulls, 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_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().
+ */
+static inline void hlist_nulls_replace_rcu(struct hlist_nulls_node *old,
+					   struct hlist_nulls_node *new)
+{
+	struct hlist_nulls_node *next = old->next;
+
+	WRITE_ONCE(new->next, next);
+	WRITE_ONCE(new->pprev, old->pprev);
+	rcu_assign_pointer(hlist_nulls_pprev_rcu(new), new);
+	if (!is_a_nulls(next))
+		WRITE_ONCE(next->pprev, &new->next);
+}
+
+/**
+ * hlist_nulls_replace_init_rcu - replace an old entry by a new one and
+ * initialize the old
+ * @old: the element to be replaced
+ * @new: the new element to insert
+ *
+ * Description:
+ * Replace the old entry with the new one in a RCU-protected hlist_nulls, while
+ * permitting racing traversals, and reinitialize the old entry.
+ *
+ * Note: @old must be hashed.
+ *
+ * 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().
+ */
+static inline void hlist_nulls_replace_init_rcu(struct hlist_nulls_node *old,
+						struct hlist_nulls_node *new)
+{
+	hlist_nulls_replace_rcu(old, new);
+	WRITE_ONCE(old->pprev, NULL);
+}
+
+/**
  * hlist_nulls_for_each_entry_rcu - iterate over rcu list of given type
  * @tpos:	the type * to use as a loop cursor.
  * @pos:	the &struct hlist_nulls_node to use as a loop cursor.