1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3 * Lock-less NULL terminated single linked list
4 *
5 * The basic atomic operation of this list is cmpxchg on long. On
6 * architectures that don't have NMI-safe cmpxchg implementation, the
7 * list can NOT be used in NMI handlers. So code that uses the list in
8 * an NMI handler should depend on CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG.
9 *
10 * Copyright 2010,2011 Intel Corp.
11 * Author: Huang Ying <ying.huang@intel.com>
12 */
13 #include <linux/kernel.h>
14 #include <linux/export.h>
15 #include <linux/llist.h>
16
17 /**
18 * llist_del_first - delete the first entry of lock-less list
19 * @head: the head for your lock-less list
20 *
21 * If list is empty, return NULL, otherwise, return the first entry
22 * deleted, this is the newest added one.
23 *
24 * Only one llist_del_first user can be used simultaneously with
25 * multiple llist_add users without lock. Because otherwise
26 * llist_del_first, llist_add, llist_add (or llist_del_all, llist_add,
27 * llist_add) sequence in another user may change @head->first->next,
28 * but keep @head->first. If multiple consumers are needed, please
29 * use llist_del_all or use lock between consumers.
30 */
llist_del_first(struct llist_head * head)31 struct llist_node *llist_del_first(struct llist_head *head)
32 {
33 struct llist_node *entry, *next;
34
35 entry = smp_load_acquire(&head->first);
36 do {
37 if (entry == NULL)
38 return NULL;
39 next = READ_ONCE(entry->next);
40 } while (!try_cmpxchg(&head->first, &entry, next));
41
42 return entry;
43 }
44 EXPORT_SYMBOL_GPL(llist_del_first);
45
46 /**
47 * llist_del_first_this - delete given entry of lock-less list if it is first
48 * @head: the head for your lock-less list
49 * @this: a list entry.
50 *
51 * If head of the list is given entry, delete and return %true else
52 * return %false.
53 *
54 * Multiple callers can safely call this concurrently with multiple
55 * llist_add() callers, providing all the callers offer a different @this.
56 */
llist_del_first_this(struct llist_head * head,struct llist_node * this)57 bool llist_del_first_this(struct llist_head *head,
58 struct llist_node *this)
59 {
60 struct llist_node *entry, *next;
61
62 /* acquire ensures orderig wrt try_cmpxchg() is llist_del_first() */
63 entry = smp_load_acquire(&head->first);
64 do {
65 if (entry != this)
66 return false;
67 next = READ_ONCE(entry->next);
68 } while (!try_cmpxchg(&head->first, &entry, next));
69
70 return true;
71 }
72 EXPORT_SYMBOL_GPL(llist_del_first_this);
73
74 /**
75 * llist_reverse_order - reverse order of a llist chain
76 * @head: first item of the list to be reversed
77 *
78 * Reverse the order of a chain of llist entries and return the
79 * new first entry.
80 */
llist_reverse_order(struct llist_node * head)81 struct llist_node *llist_reverse_order(struct llist_node *head)
82 {
83 struct llist_node *new_head = NULL;
84
85 while (head) {
86 struct llist_node *tmp = head;
87 head = head->next;
88 tmp->next = new_head;
89 new_head = tmp;
90 }
91
92 return new_head;
93 }
94 EXPORT_SYMBOL_GPL(llist_reverse_order);
95