[PATCH nf,v2 1/3] rculist: add list_splice_rcu() for private lists

[Pablo Neira Ayuso <pablo@netfilter.org>]

This patch adds a helper function, list_splice_rcu(), to safely splice
a private (non-RCU-protected) list into an RCU-protected list.

The function ensures that only the pointer visible to RCU readers
(prev->next) is updated using rcu_assign_pointer(), while the rest of
the list manipulations are performed with regular assignments, as the
source list is private and not visible to concurrent RCU readers.

This is useful for moving elements from a private list into a global
RCU-protected list, ensuring safe publication for RCU readers.
Subsystems with some sort of batching mechanism from userspace can
benefit from this new function.

The function __list_splice_rcu() has been added for clarity and to
follow the same pattern as in the existing list_splice*() interfaces,
where there is a check to ensure that that the list to splice is not
empty. Note that __list_splice_rcu() has no documentation for this
reason.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
---
v2: including comments by Paul McKenney.

    Except, I have deliberately keep back the suggestion to squash
    __list_splice_rcu() into list_splice_rcu(), I instead removed
    the documentation for __list_splice_rcu(). I am looking
    at other existing list_splice*() function in list.h and rculist.h
    to get this aligned with __list_splice(), which also has no users
    in the tree and no documentation. I find it easier to read with
    __list_splice(), but if this explaination is not sound so...

    @Paul: I can post v3 squashing __list_splice_rcu(), just let me
           know.

    Thanks!

 include/linux/rculist.h | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/include/linux/rculist.h b/include/linux/rculist.h
index 2abba7552605..e3bc44225692 100644
--- a/include/linux/rculist.h
+++ b/include/linux/rculist.h
@@ -261,6 +261,35 @@ static inline void list_replace_rcu(struct list_head *old,
 	old->prev = LIST_POISON2;
 }
 
+static inline void __list_splice_rcu(struct list_head *list,
+				     struct list_head *prev,
+				     struct list_head *next)
+{
+	struct list_head *first = list->next;
+	struct list_head *last = list->prev;
+
+	last->next = next;
+	first->prev = prev;
+	next->prev = last;
+	rcu_assign_pointer(list_next_rcu(prev), first);
+}
+
+/**
+ * list_splice_rcu - splice a non-RCU list into an RCU-protected list,
+ *                   designed for stacks.
+ * @list:	the non RCU-protected list to splice
+ * @head:	the place in the existing RCU-protected list to splice
+ *
+ * The list pointed to by @head can be RCU-read traversed concurrently with
+ * this function.
+ */
+static inline void list_splice_rcu(struct list_head *list,
+				   struct list_head *head)
+{
+	if (!list_empty(list))
+		__list_splice_rcu(list, head, head->next);
+}
+
 /**
  * __list_splice_init_rcu - join an RCU-protected list into an existing list.
  * @list:	the RCU-protected list to splice
-- 
2.47.3