Re: [PATCH net-next v3 1/3] rculist: Add __hlist_nulls_replace_rcu() and hlist_nulls_replace_init_rcu()
From: Kuniyuki Iwashima <kuniyu@google.com>
Date: 2025-09-16 18:58:19
On Tue, Sep 16, 2025 at 3:31 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. 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; + WRITE_ONCE(new->pprev, old->pprev);
As you don't use WRITE_ONCE() for ->next, the new node must not be published yet, so WRITE_ONCE() is unnecessary for ->pprev too.
+ 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)) {As mentioned in v1, this check is redundant.
+ __hlist_nulls_replace_rcu(old, new); + WRITE_ONCE(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. -- 2.25.1