Re: [PATCH net-next v6 1/3] rculist: Add hlist_nulls_replace_rcu() and hlist_nulls_replace_init_rcu()
From: Kuniyuki Iwashima <kuniyu@google.com>
Date: 2025-09-25 17:57:08
On Wed, Sep 24, 2025 at 7:18 PM [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: 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") 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..c3ba74b1890d 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 + * @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)
nit: checkpatch complains here.. https://patchwork.kernel.org/project/netdevbpf/patch/20250925021628.886203-2-xuanqiang.luo@linux.dev/ CHECK: Alignment should match open parenthesis #46: FILE: include/linux/rculist_nulls.h:171: +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);nit: define hlist_nulls_prev_rcu() like hlist_nulls_next_rcu().
+ if (!is_a_nulls(next)) + WRITE_ONCE(new->next->pprev, &new->next);
nit: s/new->next->pprev/next->pprev/
+}
+
+/**
+ * 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.
--
2.25.1