1/* SPDX-License-Identifier: GPL-2.0-only */
2#ifndef LLIST_H
3#define LLIST_H
4/*
5 * Lock-less NULL terminated single linked list
6 *
7 * Cases where locking is not needed:
8 * If there are multiple producers and multiple consumers, llist_add can be
9 * used in producers and llist_del_all can be used in consumers simultaneously
10 * without locking. Also a single consumer can use llist_del_first while
11 * multiple producers simultaneously use llist_add, without any locking.
12 *
13 * Cases where locking is needed:
14 * If we have multiple consumers with llist_del_first used in one consumer, and
15 * llist_del_first or llist_del_all used in other consumers, then a lock is
16 * needed. This is because llist_del_first depends on list->first->next not
17 * changing, but without lock protection, there's no way to be sure about that
18 * if a preemption happens in the middle of the delete operation and on being
19 * preempted back, the list->first is the same as before causing the cmpxchg in
20 * llist_del_first to succeed. For example, while a llist_del_first operation
21 * is in progress in one consumer, then a llist_del_first, llist_add,
22 * llist_add (or llist_del_all, llist_add, llist_add) sequence in another
23 * consumer may cause violations.
24 *
25 * This can be summarized as follows:
26 *
27 * | add | del_first | del_all
28 * add | - | - | -
29 * del_first | | L | L
30 * del_all | | | -
31 *
32 * Where, a particular row's operation can happen concurrently with a column's
33 * operation, with "-" being no lock needed, while "L" being lock is needed.
34 *
35 * The list entries deleted via llist_del_all can be traversed with
36 * traversing function such as llist_for_each etc. But the list
37 * entries can not be traversed safely before deleted from the list.
38 * The order of deleted entries is from the newest to the oldest added
39 * one. If you want to traverse from the oldest to the newest, you
40 * must reverse the order by yourself before traversing.
41 *
42 * The basic atomic operation of this list is cmpxchg on long. On
43 * architectures that don't have NMI-safe cmpxchg implementation, the
44 * list can NOT be used in NMI handlers. So code that uses the list in
45 * an NMI handler should depend on CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG.
46 *
47 * Copyright 2010,2011 Intel Corp.
48 * Author: Huang Ying <ying.huang@intel.com>
49 */
50
51#include <linux/atomic.h>
52#include <linux/container_of.h>
53#include <linux/stddef.h>
54#include <linux/types.h>
55
56struct llist_head {
57 struct llist_node *first;
58};
59
60struct llist_node {
61 struct llist_node *next;
62};
63
64#define LLIST_HEAD_INIT(name) { NULL }
65#define LLIST_HEAD(name) struct llist_head name = LLIST_HEAD_INIT(name)
66
67/**
68 * init_llist_head - initialize lock-less list head
69 * @head: the head for your lock-less list
70 */
71static inline void init_llist_head(struct llist_head *list)
72{
73 list->first = NULL;
74}
75
76/**
77 * llist_entry - get the struct of this entry
78 * @ptr: the &struct llist_node pointer.
79 * @type: the type of the struct this is embedded in.
80 * @member: the name of the llist_node within the struct.
81 */
82#define llist_entry(ptr, type, member) \
83 container_of(ptr, type, member)
84
85/**
86 * member_address_is_nonnull - check whether the member address is not NULL
87 * @ptr: the object pointer (struct type * that contains the llist_node)
88 * @member: the name of the llist_node within the struct.
89 *
90 * This macro is conceptually the same as
91 * &ptr->member != NULL
92 * but it works around the fact that compilers can decide that taking a member
93 * address is never a NULL pointer.
94 *
95 * Real objects that start at a high address and have a member at NULL are
96 * unlikely to exist, but such pointers may be returned e.g. by the
97 * container_of() macro.
98 */
99#define member_address_is_nonnull(ptr, member) \
100 ((uintptr_t)(ptr) + offsetof(typeof(*(ptr)), member) != 0)
101
102/**
103 * llist_for_each - iterate over some deleted entries of a lock-less list
104 * @pos: the &struct llist_node to use as a loop cursor
105 * @node: the first entry of deleted list entries
106 *
107 * In general, some entries of the lock-less list can be traversed
108 * safely only after being deleted from list, so start with an entry
109 * instead of list head.
110 *
111 * If being used on entries deleted from lock-less list directly, the
112 * traverse order is from the newest to the oldest added entry. If
113 * you want to traverse from the oldest to the newest, you must
114 * reverse the order by yourself before traversing.
115 */
116#define llist_for_each(pos, node) \
117 for ((pos) = (node); pos; (pos) = (pos)->next)
118
119/**
120 * llist_for_each_safe - iterate over some deleted entries of a lock-less list
121 * safe against removal of list entry
122 * @pos: the &struct llist_node to use as a loop cursor
123 * @n: another &struct llist_node to use as temporary storage
124 * @node: the first entry of deleted list entries
125 *
126 * In general, some entries of the lock-less list can be traversed
127 * safely only after being deleted from list, so start with an entry
128 * instead of list head.
129 *
130 * If being used on entries deleted from lock-less list directly, the
131 * traverse order is from the newest to the oldest added entry. If
132 * you want to traverse from the oldest to the newest, you must
133 * reverse the order by yourself before traversing.
134 */
135#define llist_for_each_safe(pos, n, node) \
136 for ((pos) = (node); (pos) && ((n) = (pos)->next, true); (pos) = (n))
137
138/**
139 * llist_for_each_entry - iterate over some deleted entries of lock-less list of given type
140 * @pos: the type * to use as a loop cursor.
141 * @node: the fist entry of deleted list entries.
142 * @member: the name of the llist_node with the struct.
143 *
144 * In general, some entries of the lock-less list can be traversed
145 * safely only after being removed from list, so start with an entry
146 * instead of list head.
147 *
148 * If being used on entries deleted from lock-less list directly, the
149 * traverse order is from the newest to the oldest added entry. If
150 * you want to traverse from the oldest to the newest, you must
151 * reverse the order by yourself before traversing.
152 */
153#define llist_for_each_entry(pos, node, member) \
154 for ((pos) = llist_entry((node), typeof(*(pos)), member); \
155 member_address_is_nonnull(pos, member); \
156 (pos) = llist_entry((pos)->member.next, typeof(*(pos)), member))
157
158/**
159 * llist_for_each_entry_safe - iterate over some deleted entries of lock-less list of given type
160 * safe against removal of list entry
161 * @pos: the type * to use as a loop cursor.
162 * @n: another type * to use as temporary storage
163 * @node: the first entry of deleted list entries.
164 * @member: the name of the llist_node with the struct.
165 *
166 * In general, some entries of the lock-less list can be traversed
167 * safely only after being removed from list, so start with an entry
168 * instead of list head.
169 *
170 * If being used on entries deleted from lock-less list directly, the
171 * traverse order is from the newest to the oldest added entry. If
172 * you want to traverse from the oldest to the newest, you must
173 * reverse the order by yourself before traversing.
174 */
175#define llist_for_each_entry_safe(pos, n, node, member) \
176 for (pos = llist_entry((node), typeof(*pos), member); \
177 member_address_is_nonnull(pos, member) && \
178 (n = llist_entry(pos->member.next, typeof(*n), member), true); \
179 pos = n)
180
181/**
182 * llist_empty - tests whether a lock-less list is empty
183 * @head: the list to test
184 *
185 * Not guaranteed to be accurate or up to date. Just a quick way to
186 * test whether the list is empty without deleting something from the
187 * list.
188 */
189static inline bool llist_empty(const struct llist_head *head)
190{
191 return READ_ONCE(head->first) == NULL;
192}
193
194static inline struct llist_node *llist_next(struct llist_node *node)
195{
196 return node->next;
197}
198
199extern bool llist_add_batch(struct llist_node *new_first,
200 struct llist_node *new_last,
201 struct llist_head *head);
202
203static inline bool __llist_add_batch(struct llist_node *new_first,
204 struct llist_node *new_last,
205 struct llist_head *head)
206{
207 new_last->next = head->first;
208 head->first = new_first;
209 return new_last->next == NULL;
210}
211
212/**
213 * llist_add - add a new entry
214 * @new: new entry to be added
215 * @head: the head for your lock-less list
216 *
217 * Returns true if the list was empty prior to adding this entry.
218 */
219static inline bool llist_add(struct llist_node *new, struct llist_head *head)
220{
221 return llist_add_batch(new, new, head);
222}
223
224static inline bool __llist_add(struct llist_node *new, struct llist_head *head)
225{
226 return __llist_add_batch(new, new, head);
227}
228
229/**
230 * llist_del_all - delete all entries from lock-less list
231 * @head: the head of lock-less list to delete all entries
232 *
233 * If list is empty, return NULL, otherwise, delete all entries and
234 * return the pointer to the first entry. The order of entries
235 * deleted is from the newest to the oldest added one.
236 */
237static inline struct llist_node *llist_del_all(struct llist_head *head)
238{
239 return xchg(&head->first, NULL);
240}
241
242static inline struct llist_node *__llist_del_all(struct llist_head *head)
243{
244 struct llist_node *first = head->first;
245
246 head->first = NULL;
247 return first;
248}
249
250extern struct llist_node *llist_del_first(struct llist_head *head);
251
252struct llist_node *llist_reverse_order(struct llist_node *head);
253
254#endif /* LLIST_H */
255

source code of linux/include/linux/llist.h