netdev - [PATCH net-next v1 1/3] rculist: Add __hlist_nulls_replace_rcu() and hlist_nulls_replace_init

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Message-Id: <20250915070308.111816-2-xuanqiang.luo@linux.dev>
Date: Mon, 15 Sep 2025 15:03:06 +0800
From: xuanqiang.luo@...ux.dev
To: edumazet@...gle.com,
	kuniyu@...gle.com
Cc: kerneljasonxing@...il.com,
	davem@...emloft.net,
	kuba@...nel.org,
	netdev@...r.kernel.org,
	Xuanqiang Luo <luoxuanqiang@...inos.cn>
Subject: [PATCH net-next v1 1/3] rculist: Add __hlist_nulls_replace_rcu() and hlist_nulls_replace_init_rcu()

From: Xuanqiang Luo <luoxuanqiang@...inos.cn>

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

Signed-off-by: Xuanqiang Luo <luoxuanqiang@...inos.cn>
---
 include/linux/rculist_nulls.h | 62 +++++++++++++++++++++++++++++++++++
 1 file changed, 62 insertions(+)

diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h
index 89186c499dd4..eaa3a0d2f206 100644
--- a/include/linux/rculist_nulls.h
+++ b/include/linux/rculist_nulls.h
@@ -152,6 +152,68 @@ static inline void hlist_nulls_add_fake(struct hlist_nulls_node *n)
 	n->next = (struct hlist_nulls_node *)NULLS_MARKER(NULL);
 }
 
+/**
+ * __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;
+
+	new->next = next;
+	WRITE_ONCE(new->pprev, old->pprev);
+	rcu_assign_pointer(*(struct hlist_nulls_node __rcu **)new->pprev, new);
+	if (!is_a_nulls(next))
+		WRITE_ONCE(new->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.
+ *
+ * Return: true if the old entry was hashed and was replaced successfully, false
+ * otherwise.
+ *
+ * Note: hlist_nulls_unhashed() on the old node returns true after this.
+ * It is useful for RCU based read lockfree traversal if the writer side must
+ * know if the list entry is still hashed or already unhashed.
+ *
+ * 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 bool hlist_nulls_replace_init_rcu(struct hlist_nulls_node *old,
+						struct hlist_nulls_node *new)
+{
+	if (!hlist_nulls_unhashed(old)) {
+		__hlist_nulls_replace_rcu(old, new);
+		old->pprev = NULL;
+		return true;
+	}
+
+	return false;
+}
+
 /**
  * hlist_nulls_for_each_entry_rcu - iterate over rcu list of given type
  * @tpos:	the type * to use as a loop cursor.
-- 
2.27.0