On Tue, Sep 16, 2025 at 9:27 PM Kuniyuki Iwashima [off-list ref] wrote:

On Tue, Sep 16, 2025 at 8:27 PM luoxuanqiang [off-list ref] wrote:

quoted

在 2025/9/17 02:58, Kuniyuki Iwashima 写道:

quoted

On Tue, Sep 16, 2025 at 3:31 AM [off-list ref] wrote:

quoted

From: Xuanqiang Luo <redacted>

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

Signed-off-by: Xuanqiang Luo <redacted>
---
  include/linux/rculist_nulls.h | 61 +++++++++++++++++++++++++++++++++++
  1 file changed, 61 insertions(+)

diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h
index 89186c499dd4..8ed604f65a3e 100644
--- a/include/linux/rculist_nulls.h
+++ b/include/linux/rculist_nulls.h

@@ -152,6 +152,67 @@ 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;

Do we need to use WRITE_ONCE() here, as mentioned in efd04f8a8b45
("rcu: Use WRITE_ONCE() for assignments to ->next for rculist_nulls")?
I am more inclined to think that it is necessary.

Good point, then WRITE_ONCE() makes sense.

and it would be nice to have such reasoning in the commit
message as we were able to learn from efd04f8a8b45.