Re: [PATCH net-next v4 1/3] rculist: Add __hlist_nulls_replace_rcu() and hlist_nulls_replace_init_rcu()
From: Kuniyuki Iwashima <kuniyu@google.com>
Date: 2025-09-23 00:19:55
On Sat, Sep 20, 2025 at 4:00 AM [off-list ref] wrote:
quoted hunk ↗ jump to hunk
From: Xuanqiang Luo <redacted> 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: efd04f8a8b45 ("rcu: Use WRITE_ONCE() for assignments to ->next for rculist_nulls") 860c8802ace1 ("rcu: Use WRITE_ONCE() for assignments to ->pprev for hlist_nulls") Signed-off-by: Xuanqiang Luo <redacted> --- include/linux/rculist_nulls.h | 52 +++++++++++++++++++++++++++++++++++ 1 file changed, 52 insertions(+)diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h index 89186c499dd4..d86331ce22c4 100644 --- a/include/linux/rculist_nulls.h +++ b/include/linux/rculist_nulls.h@@ -152,6 +152,58 @@ 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
nit: '__' is not needed as there is not no-'__' version.
+ * @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(*(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.
+ *
+ * Note: @old should be hashed.nit: s/should/must/
+ *
+ * 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.
--
2.25.1