llist.c

 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 */
31struct 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}
44EXPORT_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 */
57bool 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}
72EXPORT_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 */
81struct 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}
94EXPORT_SYMBOL_GPL(llist_reverse_order);