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/kernel.h>
53
54 struct llist_head {
55 struct llist_node *first;
56 };
57
58 struct llist_node {
59 struct llist_node *next;
60 };
61
62 #define LLIST_HEAD_INIT(name) { NULL }
63 #define LLIST_HEAD(name) struct llist_head name = LLIST_HEAD_INIT(name)
64
65 /**
66 * init_llist_head - initialize lock-less list head
67 * @head: the head for your lock-less list
68 */
69 static inline void init_llist_head(struct llist_head *list)
70 {
71 list->first = NULL;
72 }
73
74 /**
75 * llist_entry - get the struct of this entry
76 * @ptr: the &struct llist_node pointer.
77 * @type: the type of the struct this is embedded in.
78 * @member: the name of the llist_node within the struct.
79 */
80 #define llist_entry(ptr, type, member) \
81 container_of(ptr, type, member)
82
83 /**
84 * member_address_is_nonnull - check whether the member address is not NULL
85 * @ptr: the object pointer (struct type * that contains the llist_node)
86 * @member: the name of the llist_node within the struct.
87 *
88 * This macro is conceptually the same as
89 * &ptr->member != NULL
90 * but it works around the fact that compilers can decide that taking a member
91 * address is never a NULL pointer.
92 *
93 * Real objects that start at a high address and have a member at NULL are
94 * unlikely to exist, but such pointers may be returned e.g. by the
95 * container_of() macro.
96 */
97 #define member_address_is_nonnull(ptr, member) \
98 ((uintptr_t)(ptr) + offsetof(typeof(*(ptr)), member) != 0)
99
100 /**
101 * llist_for_each - iterate over some deleted entries of a lock-less list
102 * @pos: the &struct llist_node to use as a loop cursor
103 * @node: the first entry of deleted list entries
104 *
105 * In general, some entries of the lock-less list can be traversed
106 * safely only after being deleted from list, so start with an entry
107 * instead of list head.
108 *
109 * If being used on entries deleted from lock-less list directly, the
110 * traverse order is from the newest to the oldest added entry. If
111 * you want to traverse from the oldest to the newest, you must
112 * reverse the order by yourself before traversing.
113 */
114 #define llist_for_each(pos, node) \
115 for ((pos) = (node); pos; (pos) = (pos)->next)
116
117 /**
118 * llist_for_each_safe - iterate over some deleted entries of a lock-less list
119 * safe against removal of list entry
120 * @pos: the &struct llist_node to use as a loop cursor
121 * @n: another &struct llist_node to use as temporary storage
122 * @node: the first entry of deleted list entries
123 *
124 * In general, some entries of the lock-less list can be traversed
125 * safely only after being deleted from list, so start with an entry
126 * instead of list head.
127 *
128 * If being used on entries deleted from lock-less list directly, the
129 * traverse order is from the newest to the oldest added entry. If
130 * you want to traverse from the oldest to the newest, you must
131 * reverse the order by yourself before traversing.
132 */
133 #define llist_for_each_safe(pos, n, node) \
134 for ((pos) = (node); (pos) && ((n) = (pos)->next, true); (pos) = (n))
135
136 /**
137 * llist_for_each_entry - iterate over some deleted entries of lock-less list of given type
138 * @pos: the type * to use as a loop cursor.
139 * @node: the fist entry of deleted list entries.
140 * @member: the name of the llist_node with the struct.
141 *
142 * In general, some entries of the lock-less list can be traversed
143 * safely only after being removed from list, so start with an entry
144 * instead of list head.
145 *
146 * If being used on entries deleted from lock-less list directly, the
147 * traverse order is from the newest to the oldest added entry. If
148 * you want to traverse from the oldest to the newest, you must
149 * reverse the order by yourself before traversing.
150 */
151 #define llist_for_each_entry(pos, node, member) \
152 for ((pos) = llist_entry((node), typeof(*(pos)), member); \
153 member_address_is_nonnull(pos, member); \
154 (pos) = llist_entry((pos)->member.next, typeof(*(pos)), member))
155
156 /**
157 * llist_for_each_entry_safe - iterate over some deleted entries of lock-less list of given type
158 * safe against removal of list entry
159 * @pos: the type * to use as a loop cursor.
160 * @n: another type * to use as temporary storage
161 * @node: the first entry of deleted list entries.
162 * @member: the name of the llist_node with the struct.
163 *
164 * In general, some entries of the lock-less list can be traversed
165 * safely only after being removed from list, so start with an entry
166 * instead of list head.
167 *
168 * If being used on entries deleted from lock-less list directly, the
169 * traverse order is from the newest to the oldest added entry. If
170 * you want to traverse from the oldest to the newest, you must
171 * reverse the order by yourself before traversing.
172 */
173 #define llist_for_each_entry_safe(pos, n, node, member) \
174 for (pos = llist_entry((node), typeof(*pos), member); \
175 member_address_is_nonnull(pos, member) && \
176 (n = llist_entry(pos->member.next, typeof(*n), member), true); \
177 pos = n)
178
179 /**
180 * llist_empty - tests whether a lock-less list is empty
181 * @head: the list to test
182 *
183 * Not guaranteed to be accurate or up to date. Just a quick way to
184 * test whether the list is empty without deleting something from the
185 * list.
186 */
187 static inline bool llist_empty(const struct llist_head *head)
188 {
189 return READ_ONCE(head->first) == NULL;
190 }
191
192 static inline struct llist_node *llist_next(struct llist_node *node)
193 {
194 return node->next;
195 }
196
197 extern bool llist_add_batch(struct llist_node *new_first,
198 struct llist_node *new_last,
199 struct llist_head *head);
200 /**
201 * llist_add - add a new entry
202 * @new: new entry to be added
203 * @head: the head for your lock-less list
204 *
205 * Returns true if the list was empty prior to adding this entry.
206 */
207 static inline bool llist_add(struct llist_node *new, struct llist_head *head)
208 {
209 return llist_add_batch(new, new, head);
210 }
211
212 /**
213 * llist_del_all - delete all entries from lock-less list
214 * @head: the head of lock-less list to delete all entries
215 *
216 * If list is empty, return NULL, otherwise, delete all entries and
217 * return the pointer to the first entry. The order of entries
218 * deleted is from the newest to the oldest added one.
219 */
220 static inline struct llist_node *llist_del_all(struct llist_head *head)
221 {
222 return xchg(&head->first, NULL);
223 }
224
225 extern struct llist_node *llist_del_first(struct llist_head *head);
226
227 struct llist_node *llist_reverse_order(struct llist_node *head);
228
229 #endif /* LLIST_H */