linux-kernel - [PATCH v5 1/7] list: Add function list_rotate_to

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Message-Id: <20190402230545.2929-2-tobin@kernel.org>
Date:   Wed,  3 Apr 2019 10:05:39 +1100
From:   "Tobin C. Harding" <tobin@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     "Tobin C. Harding" <tobin@...nel.org>,
        Roman Gushchin <guro@...com>, Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Matthew Wilcox <willy@...radead.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH v5 1/7] list: Add function list_rotate_to_front()

Currently if we wish to rotate a list until a specific item is at the
front of the list we can call list_move_tail(head, list).  Note that the
arguments are the reverse way to the usual use of list_move_tail(list,
head).  This is a hack, it depends on the developer knowing how the
list_head operates internally which violates the layer of abstraction
offered by the list_head.  Also, it is not intuitive so the next
developer to come along must study list.h in order to fully understand
what is meant by the call, while this is 'good for' the developer it
makes reading the code harder.  We should have an function appropriately
named that does this if there are users for it intree.

By grep'ing the tree for list_move_tail() and list_tail() and attempting
to guess the argument order from the names it seems there is only one
place currently in the tree that does this - the slob allocatator.

Add function list_rotate_to_front() to rotate a list until the specified
item is at the front of the list.

Signed-off-by: Tobin C. Harding <tobin@...nel.org>
---
 include/linux/list.h | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/include/linux/list.h b/include/linux/list.h
index 58aa3adf94e6..9e9a6403dbe4 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -270,6 +270,24 @@ static inline void list_rotate_left(struct list_head *head)
 	}
 }

+/**
+ * list_rotate_to_front() - Rotate list to specific item.
+ * @list: The desired new front of the list.
+ * @head: The head of the list.
+ *
+ * Rotates list so that @list becomes the new front of the list.
+ */
+static inline void list_rotate_to_front(struct list_head *list,
+					struct list_head *head)
+{
+	/*
+	 * Deletes the list head from the list denoted by @head and
+	 * places it as the tail of @list, this effectively rotates the
+	 * list so that @list is at the front.
+	 */
+	list_move_tail(head, list);
+}
+
 /**
  * list_is_singular - tests whether a list has just one entry.
  * @head: the list to test.
-- 
2.21.0