[PATCH 1/2] list: introduce a new cutting helper

[Keith Busch <kbusch@meta.com>]

From: Keith Busch <kbusch@kernel.org>

Provide a helper to remove elements from a list to the end, and place
those elements in a new list.

Signed-off-by: Keith Busch <kbusch@kernel.org>
---
 include/linux/list.h | 20 ++++++++++++++++++++
 lib/list-test.c      | 29 +++++++++++++++++++++++++++++
 2 files changed, 49 insertions(+)

diff --git a/include/linux/list.h b/include/linux/list.h
index 5f4b0a39cf46a..f22850e854820 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -520,6 +520,26 @@ static inline void list_cut_before(struct list_head *list,
 	entry->prev = head;
 }
 
+/**
+ * list_cut - cut a list into two from the entry
+ * @list: a new list to add all removed entries
+ * @head: a list with entries
+ * @entry: an entry within head, could be the head itself
+ *
+ * This helper removes elements from @head starting at @entry until the end,
+ * and appends them to @lists.
+ */
+static inline void list_cut(struct list_head *list,
+		struct list_head *head, struct list_head *entry)
+{
+	list->next = entry;
+	list->prev = head->prev;
+	head->prev = entry->prev;
+	entry->prev->next = head;
+	entry->prev = list;
+	list->prev->next = list;
+}
+
 static inline void __list_splice(const struct list_head *list,
 				 struct list_head *prev,
 				 struct list_head *next)
diff --git a/lib/list-test.c b/lib/list-test.c
index 0cc27de9cec88..1507f46cf1ade 100644
--- a/lib/list-test.c
+++ b/lib/list-test.c
@@ -382,6 +382,34 @@ static void list_test_list_is_singular(struct kunit *test)
 	KUNIT_EXPECT_FALSE(test, list_is_singular(&list));
 }
 
+static void list_test_list_cut(struct kunit *test)
+{
+	struct list_head entries[3], *cur;
+	LIST_HEAD(list1);
+	LIST_HEAD(list2);
+	int i = 0;
+
+	list_add_tail(&entries[0], &list1);
+	list_add_tail(&entries[1], &list1);
+	list_add_tail(&entries[2], &list1);
+
+	/* before: [list1] -> entries[0] -> entries[1] -> entries[2] */
+	list_cut(&list2, &list1, &entries[1]);
+	/* after: [list1] -> entries[0], [list2] -> entries[1] -> entries[2] */
+
+	list_for_each(cur, &list1) {
+		KUNIT_EXPECT_PTR_EQ(test, cur, &entries[i]);
+		i++;
+	}
+
+	KUNIT_EXPECT_EQ(test, i, 1);
+
+	list_for_each(cur, &list2) {
+		KUNIT_EXPECT_PTR_EQ(test, cur, &entries[i]);
+		i++;
+	}
+}
+
 static void list_test_list_cut_position(struct kunit *test)
 {
 	struct list_head entries[3], *cur;
@@ -780,6 +808,7 @@ static struct kunit_case list_test_cases[] = {
 	KUNIT_CASE(list_test_list_is_singular),
 	KUNIT_CASE(list_test_list_cut_position),
 	KUNIT_CASE(list_test_list_cut_before),
+	KUNIT_CASE(list_test_list_cut),
 	KUNIT_CASE(list_test_list_splice),
 	KUNIT_CASE(list_test_list_splice_tail),
 	KUNIT_CASE(list_test_list_splice_init),
-- 
2.43.0