xref: /openbmc/linux/lib/llist.c (revision aa5b395b)
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 /**
19  * llist_add_batch - add several linked entries in batch
20  * @new_first:	first entry in batch to be added
21  * @new_last:	last entry in batch to be added
22  * @head:	the head for your lock-less list
23  *
24  * Return whether list is empty before adding.
25  */
26 bool llist_add_batch(struct llist_node *new_first, struct llist_node *new_last,
27 		     struct llist_head *head)
28 {
29 	struct llist_node *first;
30 
31 	do {
32 		new_last->next = first = READ_ONCE(head->first);
33 	} while (cmpxchg(&head->first, first, new_first) != first);
34 
35 	return !first;
36 }
37 EXPORT_SYMBOL_GPL(llist_add_batch);
38 
39 /**
40  * llist_del_first - delete the first entry of lock-less list
41  * @head:	the head for your lock-less list
42  *
43  * If list is empty, return NULL, otherwise, return the first entry
44  * deleted, this is the newest added one.
45  *
46  * Only one llist_del_first user can be used simultaneously with
47  * multiple llist_add users without lock.  Because otherwise
48  * llist_del_first, llist_add, llist_add (or llist_del_all, llist_add,
49  * llist_add) sequence in another user may change @head->first->next,
50  * but keep @head->first.  If multiple consumers are needed, please
51  * use llist_del_all or use lock between consumers.
52  */
53 struct llist_node *llist_del_first(struct llist_head *head)
54 {
55 	struct llist_node *entry, *old_entry, *next;
56 
57 	entry = smp_load_acquire(&head->first);
58 	for (;;) {
59 		if (entry == NULL)
60 			return NULL;
61 		old_entry = entry;
62 		next = READ_ONCE(entry->next);
63 		entry = cmpxchg(&head->first, old_entry, next);
64 		if (entry == old_entry)
65 			break;
66 	}
67 
68 	return entry;
69 }
70 EXPORT_SYMBOL_GPL(llist_del_first);
71 
72 /**
73  * llist_reverse_order - reverse order of a llist chain
74  * @head:	first item of the list to be reversed
75  *
76  * Reverse the order of a chain of llist entries and return the
77  * new first entry.
78  */
79 struct llist_node *llist_reverse_order(struct llist_node *head)
80 {
81 	struct llist_node *new_head = NULL;
82 
83 	while (head) {
84 		struct llist_node *tmp = head;
85 		head = head->next;
86 		tmp->next = new_head;
87 		new_head = tmp;
88 	}
89 
90 	return new_head;
91 }
92 EXPORT_SYMBOL_GPL(llist_reverse_order);
93