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 * init_llist_node - initialize lock-less list node
78 * @node: the node to be initialised
79 *
80 * In cases where there is a need to test if a node is on
81 * a list or not, this initialises the node to clearly
82 * not be on any list.
83 */
84static inline void init_llist_node(struct llist_node *node)
85{
86 WRITE_ONCE(node->next, node);
87}
88
89/**
90 * llist_on_list - test if a lock-list list node is on a list
91 * @node: the node to test
92 *
93 * When a node is on a list the ->next pointer will be NULL or
94 * some other node. It can never point to itself. We use that
95 * in init_llist_node() to record that a node is not on any list,
96 * and here to test whether it is on any list.
97 */
98static inline bool llist_on_list(const struct llist_node *node)
99{
100 return READ_ONCE(node->next) != node;
101}
102
103/**
104 * llist_entry - get the struct of this entry
105 * @ptr: the &struct llist_node pointer.
106 * @type: the type of the struct this is embedded in.
107 * @member: the name of the llist_node within the struct.
108 */
109#define llist_entry(ptr, type, member) \
110 container_of(ptr, type, member)
111
112/**
113 * member_address_is_nonnull - check whether the member address is not NULL
114 * @ptr: the object pointer (struct type * that contains the llist_node)
115 * @member: the name of the llist_node within the struct.
116 *
117 * This macro is conceptually the same as
118 * &ptr->member != NULL
119 * but it works around the fact that compilers can decide that taking a member
120 * address is never a NULL pointer.
121 *
122 * Real objects that start at a high address and have a member at NULL are
123 * unlikely to exist, but such pointers may be returned e.g. by the
124 * container_of() macro.
125 */
126#define member_address_is_nonnull(ptr, member) \
127 ((uintptr_t)(ptr) + offsetof(typeof(*(ptr)), member) != 0)
128
129/**
130 * llist_for_each - iterate over some deleted entries of a lock-less list
131 * @pos: the &struct llist_node to use as a loop cursor
132 * @node: the first entry of deleted list entries
133 *
134 * In general, some entries of the lock-less list can be traversed
135 * safely only after being deleted from list, so start with an entry
136 * instead of list head.
137 *
138 * If being used on entries deleted from lock-less list directly, the
139 * traverse order is from the newest to the oldest added entry. If
140 * you want to traverse from the oldest to the newest, you must
141 * reverse the order by yourself before traversing.
142 */
143#define llist_for_each(pos, node) \
144 for ((pos) = (node); pos; (pos) = (pos)->next)
145
146/**
147 * llist_for_each_safe - iterate over some deleted entries of a lock-less list
148 * safe against removal of list entry
149 * @pos: the &struct llist_node to use as a loop cursor
150 * @n: another &struct llist_node to use as temporary storage
151 * @node: the first entry of deleted list entries
152 *
153 * In general, some entries of the lock-less list can be traversed
154 * safely only after being deleted from list, so start with an entry
155 * instead of list head.
156 *
157 * If being used on entries deleted from lock-less list directly, the
158 * traverse order is from the newest to the oldest added entry. If
159 * you want to traverse from the oldest to the newest, you must
160 * reverse the order by yourself before traversing.
161 */
162#define llist_for_each_safe(pos, n, node) \
163 for ((pos) = (node); (pos) && ((n) = (pos)->next, true); (pos) = (n))
164
165/**
166 * llist_for_each_entry - iterate over some deleted entries of lock-less list of given type
167 * @pos: the type * to use as a loop cursor.
168 * @node: the fist entry of deleted list entries.
169 * @member: the name of the llist_node with the struct.
170 *
171 * In general, some entries of the lock-less list can be traversed
172 * safely only after being removed from list, so start with an entry
173 * instead of list head.
174 *
175 * If being used on entries deleted from lock-less list directly, the
176 * traverse order is from the newest to the oldest added entry. If
177 * you want to traverse from the oldest to the newest, you must
178 * reverse the order by yourself before traversing.
179 */
180#define llist_for_each_entry(pos, node, member) \
181 for ((pos) = llist_entry((node), typeof(*(pos)), member); \
182 member_address_is_nonnull(pos, member); \
183 (pos) = llist_entry((pos)->member.next, typeof(*(pos)), member))
184
185/**
186 * llist_for_each_entry_safe - iterate over some deleted entries of lock-less list of given type
187 * safe against removal of list entry
188 * @pos: the type * to use as a loop cursor.
189 * @n: another type * to use as temporary storage
190 * @node: the first entry of deleted list entries.
191 * @member: the name of the llist_node with the struct.
192 *
193 * In general, some entries of the lock-less list can be traversed
194 * safely only after being removed from list, so start with an entry
195 * instead of list head.
196 *
197 * If being used on entries deleted from lock-less list directly, the
198 * traverse order is from the newest to the oldest added entry. If
199 * you want to traverse from the oldest to the newest, you must
200 * reverse the order by yourself before traversing.
201 */
202#define llist_for_each_entry_safe(pos, n, node, member) \
203 for (pos = llist_entry((node), typeof(*pos), member); \
204 member_address_is_nonnull(pos, member) && \
205 (n = llist_entry(pos->member.next, typeof(*n), member), true); \
206 pos = n)
207
208/**
209 * llist_empty - tests whether a lock-less list is empty
210 * @head: the list to test
211 *
212 * Not guaranteed to be accurate or up to date. Just a quick way to
213 * test whether the list is empty without deleting something from the
214 * list.
215 */
216static inline bool llist_empty(const struct llist_head *head)
217{
218 return READ_ONCE(head->first) == NULL;
219}
220
221static inline struct llist_node *llist_next(struct llist_node *node)
222{
223 return READ_ONCE(node->next);
224}
225
226/**
227 * llist_add_batch - add several linked entries in batch
228 * @new_first: first entry in batch to be added
229 * @new_last: last entry in batch to be added
230 * @head: the head for your lock-less list
231 *
232 * Return whether list is empty before adding.
233 */
234static inline bool llist_add_batch(struct llist_node *new_first,
235 struct llist_node *new_last,
236 struct llist_head *head)
237{
238 struct llist_node *first = READ_ONCE(head->first);
239
240 do {
241 new_last->next = first;
242 } while (!try_cmpxchg(&head->first, &first, new_first));
243
244 return !first;
245}
246
247static inline bool __llist_add_batch(struct llist_node *new_first,
248 struct llist_node *new_last,
249 struct llist_head *head)
250{
251 new_last->next = head->first;
252 head->first = new_first;
253 return new_last->next == NULL;
254}
255
256/**
257 * llist_add - add a new entry
258 * @new: new entry to be added
259 * @head: the head for your lock-less list
260 *
261 * Returns true if the list was empty prior to adding this entry.
262 */
263static inline bool llist_add(struct llist_node *new, struct llist_head *head)
264{
265 return llist_add_batch(new_first: new, new_last: new, head);
266}
267
268static inline bool __llist_add(struct llist_node *new, struct llist_head *head)
269{
270 return __llist_add_batch(new_first: new, new_last: new, head);
271}
272
273/**
274 * llist_del_all - delete all entries from lock-less list
275 * @head: the head of lock-less list to delete all entries
276 *
277 * If list is empty, return NULL, otherwise, delete all entries and
278 * return the pointer to the first entry. The order of entries
279 * deleted is from the newest to the oldest added one.
280 */
281static inline struct llist_node *llist_del_all(struct llist_head *head)
282{
283 return xchg(&head->first, NULL);
284}
285
286static inline struct llist_node *__llist_del_all(struct llist_head *head)
287{
288 struct llist_node *first = head->first;
289
290 head->first = NULL;
291 return first;
292}
293
294extern struct llist_node *llist_del_first(struct llist_head *head);
295
296/**
297 * llist_del_first_init - delete first entry from lock-list and mark is as being off-list
298 * @head: the head of lock-less list to delete from.
299 *
300 * This behave the same as llist_del_first() except that llist_init_node() is called
301 * on the returned node so that llist_on_list() will report false for the node.
302 */
303static inline struct llist_node *llist_del_first_init(struct llist_head *head)
304{
305 struct llist_node *n = llist_del_first(head);
306
307 if (n)
308 init_llist_node(node: n);
309 return n;
310}
311
312extern bool llist_del_first_this(struct llist_head *head,
313 struct llist_node *this);
314
315struct llist_node *llist_reverse_order(struct llist_node *head);
316
317#endif /* LLIST_H */
318