llist.h

  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, new, head);
266}
267
268static inline bool __llist_add(struct llist_node *new, struct llist_head *head)
269{
270	return __llist_add_batch(new, 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(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 */