2 * Written by Josh Dybnis and released to the public domain, as explained at
3 * http://creativecommons.org/licenses/publicdomain
5 * Harris-Michael lock-free list-based set
6 * http://www.research.ibm.com/people/m/michael/spaa-2002.pdf
19 uint64_t next; // next node
28 const datatype_t *key_type;
31 static node_t *node_alloc (map_key_t key, map_val_t val) {
32 node_t *item = (node_t *)nbd_malloc(sizeof(node_t));
38 list_t *ll_alloc (const datatype_t *key_type) {
39 list_t *ll = (list_t *)nbd_malloc(sizeof(list_t));
40 ll->key_type = key_type;
41 ll->head = node_alloc(0, 0);
42 ll->head->next = DOES_NOT_EXIST;
46 void ll_free (list_t *ll) {
47 node_t *item = (node_t *)(size_t)ll->head->next; // the head can't be tagged
49 node_t *next = (node_t *)(size_t)STRIP_TAG(item->next, TAG1);
55 uint64_t ll_count (list_t *ll) {
57 node_t *item = (node_t *)(size_t)ll->head->next;
59 if (!IS_TAGGED(item->next, TAG1)) {
62 item = (node_t *)(size_t)STRIP_TAG(item->next, TAG1);
67 static int find_pred (node_t **pred_ptr, node_t **item_ptr, list_t *ll, map_key_t key, int help_remove) {
68 node_t *pred = ll->head;
69 node_t *item = (node_t *)(size_t)pred->next;
70 TRACE("l2", "find_pred: searching for key %p in list (head is %p)", key, pred);
72 while (item != NULL) {
73 uint64_t next = item->next;
75 // A tag means an item is logically removed but not physically unlinked yet.
76 while (EXPECT_FALSE(IS_TAGGED(next, TAG1))) {
78 // Skip over logically removed items.
80 item = (node_t *)(size_t)STRIP_TAG(item->next, TAG1);
81 if (EXPECT_FALSE(item == NULL))
83 TRACE("l3", "find_pred: skipping marked item %p (next is %p)", item, next);
88 // Unlink logically removed items.
89 TRACE("l3", "find_pred: unlinking marked item %p next is %p", item, next);
91 uint64_t other = SYNC_CAS(&pred->next, (uint64_t)(size_t)item, STRIP_TAG(next, TAG1));
92 if (other == (uint64_t)(size_t)item) {
93 TRACE("l2", "find_pred: unlinked item %p from pred %p", item, pred);
94 item = (node_t *)(size_t)STRIP_TAG(next, TAG1);
95 next = (item != NULL) ? item->next : DOES_NOT_EXIST;
96 TRACE("l3", "find_pred: now current item is %p next is %p", item, next);
98 // The thread that completes the unlink should free the memory.
99 if (ll->key_type != NULL) {
100 nbd_defer_free((void *)(size_t)((node_t *)(size_t)other)->key);
102 nbd_defer_free(((node_t *)(size_t)other));
104 TRACE("l2", "find_pred: lost a race to unlink item %p from pred %p", item, pred);
105 TRACE("l2", "find_pred: pred's link changed to %p", other, 0);
106 if (IS_TAGGED(other, TAG1))
107 return find_pred(pred_ptr, item_ptr, ll, key, help_remove); // retry
108 item = (node_t *)(size_t)other;
109 next = (item != NULL) ? item->next : DOES_NOT_EXIST;
113 if (EXPECT_FALSE(item == NULL))
116 TRACE("l3", "find_pred: visiting item %p (next is %p)", item, next);
117 TRACE("l4", "find_pred: key %p val %p", item->key, item->val);
120 if (EXPECT_TRUE(ll->key_type == NULL)) {
121 d = (uint64_t)item->key - (uint64_t)key;
123 d = ll->key_type->cmp((void *)(size_t)item->key, (void *)(size_t)key);
126 if (next != DOES_NOT_EXIST && ((node_t *)next)->key < item->key) {
131 // If we reached the key (or passed where it should be), we found the right predesssor
133 if (pred_ptr != NULL) {
138 TRACE("l2", "find_pred: found matching item %p in list, pred is %p", item, pred);
141 TRACE("l2", "find_pred: found proper place for key %p in list, pred is %p", key, pred);
146 item = (node_t *)(size_t)next;
149 // <key> is not in <ll>.
150 if (pred_ptr != NULL) {
154 TRACE("l2", "find_pred: reached end of list. last item is %p", pred, 0);
158 // Fast find. Do not help unlink partially removed nodes and do not return the found item's predecessor.
159 map_val_t ll_lookup (list_t *ll, map_key_t key) {
160 TRACE("l1", "ll_lookup: searching for key %p in list %p", key, ll);
162 int found = find_pred(NULL, &item, ll, key, FALSE);
164 // If we found an <item> matching the key return its value.
166 map_val_t val = item->val;
167 if (val != DOES_NOT_EXIST) {
168 TRACE("l1", "ll_lookup: found item %p. val %p. returning item", item, item->val);
173 TRACE("l1", "ll_lookup: no item in the list matched the key", 0, 0);
174 return DOES_NOT_EXIST;
177 map_val_t ll_cas (list_t *ll, map_key_t key, map_val_t expectation, map_val_t new_val) {
178 TRACE("l1", "ll_cas: key %p list %p", key, ll);
179 TRACE("l1", "ll_cas: expectation %p new value %p", expectation, new_val);
180 ASSERT((int64_t)new_val > 0);
183 node_t *pred, *old_item;
184 int found = find_pred(&pred, &old_item, ll, key, TRUE);
187 // There was not an item in the list that matches the key.
188 if (EXPECT_FALSE((int64_t)expectation > 0 || expectation == CAS_EXPECT_EXISTS)) {
189 TRACE("l1", "ll_cas: the expectation was not met, the list was not changed", 0, 0);
190 return DOES_NOT_EXIST; // failure
193 ASSERT(expectation == CAS_EXPECT_DOES_NOT_EXIST || expectation == CAS_EXPECT_WHATEVER);
195 // Create a new item and insert it into the list.
196 TRACE("l2", "ll_cas: attempting to insert item between %p and %p", pred, pred->next);
197 map_key_t new_key = (ll->key_type == NULL)
199 : (map_key_t)(size_t)ll->key_type->clone((void *)(size_t)key);
200 node_t *new_item = node_alloc(new_key, new_val);
201 uint64_t next = new_item->next = (uint64_t)(size_t)old_item;
202 uint64_t other = SYNC_CAS(&pred->next, next, new_item);
204 TRACE("l1", "ll_cas: successfully inserted new item %p", new_item, 0);
205 return DOES_NOT_EXIST; // success
208 // Lost a race. Failed to insert the new item into the list.
209 TRACE("l1", "ll_cas: lost a race. CAS failed. expected pred's link to be %p but found %p", next, other);
210 if (ll->key_type != NULL) {
211 nbd_free((void *)(size_t)new_key);
217 // Found an item in the list that matches the key.
218 map_val_t old_item_val = old_item->val;
220 // If the item's value is DOES_NOT_EXIST it means another thread removed the node out from under us.
221 if (EXPECT_FALSE(old_item_val == DOES_NOT_EXIST)) {
222 TRACE("l2", "ll_cas: lost a race, found an item but another thread removed it. retry", 0, 0);
226 if (EXPECT_FALSE(expectation == CAS_EXPECT_DOES_NOT_EXIST)) {
227 TRACE("l1", "ll_cas: found an item %p in the list that matched the key. the expectation was "
228 "not met, the list was not changed", old_item, old_item_val);
229 return old_item_val; // failure
232 // Use a CAS and not a SWAP. If the node is in the process of being removed and we used a SWAP, we could
233 // replace DOES_NOT_EXIST with our value. Then another thread that is updating the value could think it
234 // succeeded and return our value even though we indicated that the node has been removed. If the CAS
235 // fails it means another thread either removed the node or updated its value.
236 map_val_t ret_val = SYNC_CAS(&old_item->val, old_item_val, new_val);
237 if (ret_val == old_item_val) {
238 TRACE("l1", "ll_cas: the CAS succeeded. updated the value of the item", 0, 0);
239 return ret_val; // success
241 TRACE("l2", "ll_cas: lost a race. the CAS failed. another thread changed the item's value", 0, 0);
243 old_item_val = ret_val;
248 map_val_t ll_remove (list_t *ll, map_key_t key) {
249 TRACE("l1", "ll_remove: removing item with key %p from list %p", key, ll);
252 int found = find_pred(&pred, &item, ll, key, TRUE);
254 TRACE("l1", "ll_remove: remove failed, an item with a matching key does not exist in the list", 0, 0);
255 return DOES_NOT_EXIST;
258 // Mark <item> removed. If multiple threads try to remove the same item only one of them should succeed.
260 uint64_t old_next = item->next;
263 old_next = SYNC_CAS(&item->next, next, TAG_VALUE(next, TAG1));
264 if (IS_TAGGED(old_next, TAG1)) {
265 TRACE("l1", "ll_remove: lost a race -- %p is already marked for removal by another thread", item, 0);
266 return DOES_NOT_EXIST;
268 } while (next != old_next);
269 TRACE("l2", "ll_remove: logically removed item %p", item, 0);
270 ASSERT(IS_TAGGED(((volatile node_t *)item)->next, TAG1));
272 // Atomically swap out the item's value in case another thread is updating the item while we are
273 // removing it. This establishes which operation occurs first logically, the update or the remove.
274 map_val_t val = SYNC_SWAP(&item->val, DOES_NOT_EXIST);
275 TRACE("l2", "ll_remove: replaced item's val %p with DOES_NOT_EXIT", val, 0);
277 // Unlink <item> from <ll>. If we lose a race to another thread just back off. It is safe to leave the
278 // item logically removed for a later call (or some other thread) to physically unlink. By marking the
279 // item earlier, we logically removed it.
280 TRACE("l2", "ll_remove: unlink the item by linking its pred %p to its successor %p", pred, next);
282 if ((other = SYNC_CAS(&pred->next, (uint64_t)(size_t)item, next)) != (uint64_t)(size_t)item) {
283 TRACE("l1", "ll_remove: unlink failed; pred's link changed from %p to %p", item, other);
287 // The thread that completes the unlink should free the memory.
288 if (ll->key_type != NULL) {
289 nbd_defer_free((void *)(size_t)item->key);
291 nbd_defer_free(item);
292 TRACE("l1", "ll_remove: successfully unlinked item %p from the list", item, 0);
296 void ll_print (list_t *ll) {
297 uint64_t next = ll->head->next;
299 while (next != DOES_NOT_EXIST) {
300 if (IS_TAGGED(next, TAG1)) {
303 node_t *item = (node_t *)(size_t)STRIP_TAG(next, TAG1);
306 printf("%p:0x%llx ", item, item->key);
317 ll_iter_t *ll_iter_begin (list_t *ll, map_key_t key) {
318 ll_iter_t *iter = (ll_iter_t *)nbd_malloc(sizeof(ll_iter_t));
319 find_pred(NULL, &iter->next, ll, key, FALSE);
323 map_val_t ll_iter_next (ll_iter_t *iter, map_key_t *key_ptr) {
325 node_t *item = iter->next;
326 while (item != NULL && IS_TAGGED(item->next, TAG1)) {
327 item = (node_t *)(size_t)STRIP_TAG(item->next, TAG1);
331 return DOES_NOT_EXIST;
333 iter->next = (node_t *)(size_t)STRIP_TAG(item->next, TAG1);
334 if (key_ptr != NULL) {
335 *key_ptr = item->key;
340 void ll_iter_free (ll_iter_t *iter) {