2 * Written by Josh Dybnis and released to the public domain, as explained at
3 * http://creativecommons.org/licenses/publicdomain
5 * C implementation of the lock-free skiplist data-structure created by Maurice Herlihy,
6 * Yossi Lev, and Nir Shavit. See "The Art of Multiprocessor Programming"
7 * http://www.amazon.com/Art-Multiprocessor-Programming-Maurice-Herlihy/dp/0123705916/
9 * See also Kir Fraser's dissertation "Practical Lock Freedom"
10 * www.cl.cam.ac.uk/techreports/UCAM-CL-TR-579.pdf
29 typedef struct skiplist {
35 static int random_level (int r) {
38 int n = __builtin_ctz(r);
44 node_t *node_alloc (int top_level, uint64_t key, uint64_t value) {
45 assert(top_level >= 0 && top_level <= MAX_LEVEL);
46 size_t sz = sizeof(node_t) + (top_level + 1) * sizeof(node_t *);
47 node_t *item = (node_t *)nbd_malloc(sz);
51 item->top_level = top_level;
55 skiplist_t *skiplist_alloc (void) {
56 skiplist_t *skiplist = (skiplist_t *)nbd_malloc(sizeof(skiplist_t));
57 skiplist->head = node_alloc(MAX_LEVEL, 0, 0);
58 skiplist->last = node_alloc(MAX_LEVEL, (uint64_t)-1, 0);
59 for (int level = 0; level <= MAX_LEVEL; ++level) {
60 skiplist->head->next[level] = skiplist->last;
65 static node_t *find_preds (node_t **preds, skiplist_t *skiplist, uint64_t key, int help_remove) {
66 node_t *pred = skiplist->head;
68 TRACE("s3", "find_preds: searching for key %p in skiplist (head is %p)", key, pred);
73 // Traverse the levels of the skiplist from the top level to the bottom
74 for (int level = MAX_LEVEL; level >= 0; --level) {
75 TRACE("s3", "find_preds: level %llu", level, 0);
76 item = pred->next[level];
77 if (IS_TAGGED(item)) {
78 TRACE("s3", "find_preds: pred %p is marked for removal (item %p); retry", pred, item);
79 return find_preds(preds, skiplist, key, help_remove); // retry
82 node_t *next = item->next[level];
83 TRACE("s3", "find_preds: visiting item %p (next %p)", item, next);
84 TRACE("s3", "find_preds: key %p", item->key, 0);
86 // Marked items are logically removed, but not fully unlinked yet.
87 while (EXPECT_FALSE(IS_TAGGED(next))) {
89 // Skip over partially removed items.
91 item = (node_t *)STRIP_TAG(item->next);
92 next = item->next[level];
96 // Unlink partially removed items.
98 if ((other = SYNC_CAS(&pred->next[level], item, STRIP_TAG(next))) == item) {
99 item = (node_t *)STRIP_TAG(next);
100 next = item->next[level];
101 TRACE("s3", "find_preds: unlinked item %p from pred %p", item, pred);
102 TRACE("s3", "find_preds: now item is %p next is %p", item, next);
104 // The thread that completes the unlink should free the memory.
105 if (level == 0) { nbd_defer_free(other); }
107 TRACE("s3", "find_preds: lost race to unlink item from pred %p; its link changed to %p",pred,other);
108 if (IS_TAGGED(other))
109 return find_preds(preds, skiplist, key, help_remove); // retry
111 next = item->next[level];
115 // If we reached the key (or passed where it should be), we found a pred. Save it and continue down.
116 if (item->key >= key) {
117 TRACE("s3", "find_preds: found pred %p item %p", pred, item);
124 assert(count++ < 18);
133 // Fast find that does not help unlink partially removed nodes and does not return the node's predecessors.
134 uint64_t skiplist_lookup (skiplist_t *skiplist, uint64_t key) {
135 TRACE("s3", "skiplist_lookup: searching for key %p in skiplist %p", key, skiplist);
136 node_t *item = find_preds(NULL, skiplist, key, FALSE);
138 // If we found an <item> matching the <key> return its value.
139 return (item->key == key) ? item->value : DOES_NOT_EXIST;
142 // Insert the <key> if it doesn't already exist in the <skiplist>
143 uint64_t skiplist_add_i (int n, skiplist_t *skiplist, uint64_t key, uint64_t value) {
144 TRACE("s3", "skiplist_add: inserting key %p value %p", key, value);
145 node_t *preds[MAX_LEVEL+1];
148 node_t *next = find_preds(preds, skiplist, key, TRUE);
150 // If a node matching <key> already exists in the skiplist, return its value.
151 if (next->key == key) {
152 TRACE("s3", "skiplist_add: there is already an item %p (value %p) with the same key", next, next->value);
153 if (EXPECT_FALSE(item != NULL)) { nbd_free(item); }
157 // First insert <item> into the bottom level.
158 if (EXPECT_TRUE(item == NULL)) { item = node_alloc(random_level(n), key, value); }
159 TRACE("s3", "skiplist_add: attempting to insert item between %p and %p", preds[0], next);
160 item->next[0] = next;
161 for (int level = 1; level <= item->top_level; ++level) {
162 item->next[level] = preds[level]->next[level];
164 node_t *other = SYNC_CAS(&preds[0]->next[0], next, item);
166 TRACE("s3", "skiplist_add: successfully inserted item %p at level 0", item, 0);
169 TRACE("s3", "skiplist_add: failed to change pred's link: expected %p found %p", next, other);
173 // Insert <item> into the skiplist from the bottom level up.
174 for (int level = 1; level <= item->top_level; ++level) {
176 node_t *pred = preds[level];
177 node_t *next = pred->next[level];
178 while (EXPECT_FALSE(IS_TAGGED(next) || next->key < key)) {
179 find_preds(preds, skiplist, key, TRUE);
181 next = pred->next[level];
185 // There in no need to continue linking in the item if another thread removed it.
186 node_t *old_next = ((volatile node_t *)item)->next[level];
187 if (IS_TAGGED(old_next))
188 return DOES_NOT_EXIST; // success
190 // Use a CAS so we to not inadvertantly remove a mark another thread placed on the item.
191 if (next == old_next || SYNC_CAS(&item->next[level], old_next, next) == old_next)
195 TRACE("s3", "skiplist_add: attempting to insert item between %p and %p", pred, next);
196 node_t *other = SYNC_CAS(&pred->next[level], next, item);
198 TRACE("s3", "skiplist_add: successfully inserted item %p at level %llu", item, level);
201 TRACE("s3", "skiplist_add: failed to change pred's link: expected %p found %p", next, other);
208 uint64_t skiplist_remove (skiplist_t *skiplist, uint64_t key) {
209 TRACE("s3", "skiplist_remove: removing item with key %p from skiplist %p", key, skiplist);
210 node_t *preds[MAX_LEVEL+1];
211 node_t *item = find_preds(preds, skiplist, key, TRUE);
212 if (item->key != key) {
213 TRACE("s3", "skiplist_remove: remove failed, an item with a matching key does not exist in the skiplist", 0, 0);
214 return DOES_NOT_EXIST;
217 // Mark <item> removed at each level of the skiplist from the top down. This must be atomic. If multiple threads
218 // try to remove the same item only one of them should succeed. Marking the bottom level establishes which of
220 for (int level = item->top_level; level >= 0; --level) {
221 if (EXPECT_FALSE(IS_TAGGED(item->next[level]))) {
222 TRACE("s3", "skiplist_remove: %p is already marked for removal by another thread", item, 0);
224 return DOES_NOT_EXIST;
227 node_t *next = SYNC_FETCH_AND_OR(&item->next[level], TAG);
228 if (EXPECT_FALSE(IS_TAGGED(next))) {
229 TRACE("s3", "skiplist_remove: lost race -- %p is already marked for removal by another thread", item, 0);
231 return DOES_NOT_EXIST;
236 uint64_t value = item->value;
238 // Unlink <item> from the top down.
239 int level = item->top_level;
241 node_t *pred = preds[level];
242 node_t *next = item->next[level];
243 TRACE("s3", "skiplist_remove: link item's pred %p to it's successor %p", pred, STRIP_TAG(next));
244 node_t *other = NULL;
245 if ((other = SYNC_CAS(&pred->next[level], item, STRIP_TAG(next))) != item) {
246 TRACE("s3", "skiplist_remove: unlink failed; pred's link changed from %p to %p", item, other);
247 // By marking the item earlier, we logically removed it. It is safe to leave the item partially
248 // unlinked. Another thread will finish physically removing it from the skiplist.
254 // The thread that completes the unlink should free the memory.
255 nbd_defer_free(item);
259 void skiplist_print (skiplist_t *skiplist) {
260 for (int level = MAX_LEVEL; level >= 0; --level) {
261 printf("(%d) ", level);
262 node_t *item = skiplist->head;
264 node_t *next = item->next[level];
265 printf("%s%p:0x%llx ", IS_TAGGED(next) ? "*" : "", item, item->key);
266 item = (node_t *)STRIP_TAG(next);
272 node_t *item = skiplist->head;
274 assert(item->top_level <= MAX_LEVEL);
275 int is_marked = IS_TAGGED(item->next[0]);
276 printf("%s%p:0x%llx (%d", is_marked ? "*" : "", item, item->key, item->top_level);
277 for (int i = 1; i <= item->top_level; ++i) {
278 node_t *next = (node_t *)STRIP_TAG(item->next[i]);
279 printf(" %p:0x%llx", item->next[i], next ? next->key : 0);
283 item = (node_t *)STRIP_TAG(item->next[0]);
287 #ifdef MAKE_skiplist_test
290 #include <sys/time.h>
294 #define NUM_ITERATIONS 10000000
296 static volatile int wait_;
297 static long num_threads_;
298 static skiplist_t *skiplist_;
300 void *worker (void *arg) {
301 int id = (int)(size_t)arg;
303 unsigned int rand_seed = id+1;//rdtsc_l();
305 // Wait for all the worker threads to be ready.
306 SYNC_ADD(&wait_, -1);
309 for (int i = 0; i < NUM_ITERATIONS/num_threads_; ++i) {
310 int n = rand_r(&rand_seed);
311 int key = (n & 0xF) + 1;
313 skiplist_add_i(n, skiplist_, key, 1);
315 skiplist_remove(skiplist_, key);
324 int main (int argc, char **argv) {
326 lwt_set_trace_level("s3");
328 char* program_name = argv[0];
329 pthread_t thread[MAX_NUM_THREADS];
332 fprintf(stderr, "Usage: %s num_threads\n", program_name);
340 num_threads_ = strtol(argv[1], NULL, 10);
342 fprintf(stderr, "%s: Invalid argument for number of threads\n", program_name);
345 if (num_threads_ <= 0) {
346 fprintf(stderr, "%s: Number of threads must be at least 1\n", program_name);
349 if (num_threads_ > MAX_NUM_THREADS) {
350 fprintf(stderr, "%s: Number of threads cannot be more than %d\n", program_name, MAX_NUM_THREADS);
355 skiplist_ = skiplist_alloc();
357 struct timeval tv1, tv2;
358 gettimeofday(&tv1, NULL);
360 wait_ = num_threads_;
362 for (int i = 0; i < num_threads_; ++i) {
363 int rc = nbd_thread_create(thread + i, i, worker, (void*)(size_t)i);
364 if (rc != 0) { perror("pthread_create"); return rc; }
367 for (int i = 0; i < num_threads_; ++i) {
368 pthread_join(thread[i], NULL);
371 gettimeofday(&tv2, NULL);
372 int ms = (int)(1000000*(tv2.tv_sec - tv1.tv_sec) + tv2.tv_usec - tv1.tv_usec) / 1000;
373 printf("Th:%ld Time:%dms\n", num_threads_, ms);
374 skiplist_print(skiplist_);
379 #endif//skiplist_test