X-Git-Url: https://pd.if.org/git/?p=nbds;a=blobdiff_plain;f=struct%2Fskiplist.c;fp=struct%2Fskiplist.c;h=0000000000000000000000000000000000000000;hp=e8060ddb3e6e820282e79d8a2032d3ebba444a2d;hb=025017478bb385da88a6b185849c8bcffeb2e2aa;hpb=2d93f3b29622488bde80b6cd18661fd7eb603eee diff --git a/struct/skiplist.c b/struct/skiplist.c deleted file mode 100644 index e8060dd..0000000 --- a/struct/skiplist.c +++ /dev/null @@ -1,426 +0,0 @@ -/* - * Written by Josh Dybnis and released to the public domain, as explained at - * http://creativecommons.org/licenses/publicdomain - * - * Implementation of the lock-free skiplist data-structure created by Maurice Herlihy, Yossi Lev, - * and Nir Shavit. See Herlihy's and Shivit's book "The Art of Multiprocessor Programming". - * http://www.amazon.com/Art-Multiprocessor-Programming-Maurice-Herlihy/dp/0123705916/ - * - * See also Kir Fraser's dissertation "Practical Lock Freedom". - * www.cl.cam.ac.uk/techreports/UCAM-CL-TR-579.pdf - * - * This code is written for the x86 memory-model. The algorithim depends on certain stores and - * loads being ordered. Be careful, this code probably won't work correctly on platforms with - * weaker memory models if you don't add memory barriers in the right places. - */ -#include -#include - -#include "common.h" -#include "runtime.h" -#include "struct.h" -#include "nstring.h" -#include "mem.h" -#include "tls.h" - -// Setting MAX_LEVEL to 0 essentially makes this data structure the Harris-Michael lock-free list -// (in list.c). -#define MAX_LEVEL 31 - -typedef struct node { - nstring_t *key; - uint64_t val; - int top_level; - struct node *next[]; -} node_t; - -struct sl { - node_t *head; -}; - -static int random_level (void) { - unsigned r = nbd_rand(); - if (r & 1) - return 0; -#if MAX_LEVEL < 31 - r |= 1 << (MAX_LEVEL+1); -#endif - int n = __builtin_ctz(r)-1; - assert(n <= MAX_LEVEL); - return n; -} - -node_t *node_alloc (int level, const void *key_data, uint32_t key_len, uint64_t val) { - assert(level >= 0 && level <= MAX_LEVEL); - size_t sz = sizeof(node_t) + (level + 1) * sizeof(node_t *); - node_t *item = (node_t *)nbd_malloc(sz); - memset(item, 0, sz); - // If is -1 it indicates is an integer and not a pointer - item->key = (key_len == (unsigned)-1) - ? (void *)TAG_VALUE(key_data) - : ns_alloc(key_data, key_len); - item->val = val; - item->top_level = level; - return item; -} - -static void node_free (node_t *item) { - if (!IS_TAGGED(item->key)) { - nbd_free(item->key); - } - nbd_free(item); -} - -static void node_defer_free (node_t *item) { - if (!IS_TAGGED(item->key)) { - nbd_defer_free(item->key); - } - nbd_defer_free(item); -} - -skiplist_t *sl_alloc (void) { - skiplist_t *sl = (skiplist_t *)nbd_malloc(sizeof(skiplist_t)); - sl->head = node_alloc(MAX_LEVEL, " ", 0, 0); - memset(sl->head->next, 0, (MAX_LEVEL+1) * sizeof(skiplist_t *)); - return sl; -} - -static node_t *find_preds (node_t **preds, node_t **succs, int n, skiplist_t *sl, const void *key_data, uint32_t key_len, int help_remove) { - node_t *pred = sl->head; - node_t *item = NULL; - TRACE("s2", "find_preds: searching for key %p in sl (head is %p)", key_data, pred); - int d; - int start_level = MAX_LEVEL; -#if MAX_LEVEL > 2 - // Optimization for small lists. No need to traverse empty higher levels. - start_level = 2; - while (pred->next[start_level+1] != NULL) { - start_level += start_level - 1; - if (EXPECT_FALSE(start_level >= MAX_LEVEL)) { - start_level = MAX_LEVEL; - break; - } - } - if (EXPECT_FALSE(start_level < n)) { - start_level = n; - } -#endif - - // Traverse the levels of from the top level to the bottom - for (int level = start_level; level >= 0; --level) { - TRACE("s3", "find_preds: level %llu", level, 0); - item = pred->next[level]; - if (EXPECT_FALSE(IS_TAGGED(item))) { - TRACE("s2", "find_preds: pred %p is marked for removal (item %p); retry", pred, item); - return find_preds(preds, succs, n, sl, key_data, key_len, help_remove); // retry - } - while (item != NULL) { - node_t *next = item->next[level]; - - // A tag means an item is logically removed but not physically unlinked yet. - while (EXPECT_FALSE(IS_TAGGED(next))) { - - // Skip over logically removed items. - if (!help_remove) { - item = (node_t *)STRIP_TAG(item->next); - if (EXPECT_FALSE(item == NULL)) - break; - TRACE("s3", "find_preds: skipping marked item %p (next is %p)", item, next); - next = item->next[level]; - continue; - } - - // Unlink logically removed items. - node_t *other; - TRACE("s3", "find_preds: unlinking marked item %p; next is %p", item, next); - if ((other = SYNC_CAS(&pred->next[level], item, STRIP_TAG(next))) == item) { - item = (node_t *)STRIP_TAG(next); - if (EXPECT_FALSE(item == NULL)) - break; - next = item->next[level]; - TRACE("s3", "find_preds: now the current item is %p next is %p", item, next); - - // The thread that completes the unlink should free the memory. - if (level == 0) { node_defer_free(other); } - } else { - TRACE("s2", "find_preds: lost race to unlink item %p from pred %p", item, pred); - TRACE("s2", "find_preds: pred's link changed to %p", other, 0); - if (IS_TAGGED(other)) - return find_preds(preds, succs, n, sl, key_data, key_len, help_remove); // retry - item = other; - if (EXPECT_FALSE(item == NULL)) - break; - next = item->next[level]; - } - } - - if (EXPECT_FALSE(item == NULL)) - break; - - TRACE("s3", "find_preds: visiting item %p (next is %p)", item, next); - TRACE("s4", "find_preds: key %p val %p", STRIP_TAG(item->key), item->val); - - // A tagged key is an integer, otherwise it is a pointer to a string - if (IS_TAGGED(item->key)) { - d = (STRIP_TAG(item->key) - (uint64_t)key_data); - } else { - int item_key_len = item->key->len; - int len = (key_len < item_key_len) ? key_len : item_key_len; - d = memcmp(item->key->data, key_data, len); - if (d == 0) { d = item_key_len - key_len; } - } - - if (d >= 0) { - TRACE("s2", "find_preds: found pred %p item %p", pred, item); - break; - } - - pred = item; - item = next; - } - - // The cast to unsigned is for the case when n is -1. - if ((unsigned)level <= (unsigned)n) { - if (preds != NULL) { - preds[level] = pred; - } - if (succs != NULL) { - succs[level] = item; - } - } - } - - // fill in empty levels - if (n == -1 && item != NULL) { - for (int level = start_level + 1; level <= item->top_level; ++level) { - preds[level] = sl->head; - } - } - - if (d == 0) { - TRACE("s2", "find_preds: found matching item %p in skiplist, pred is %p", item, pred); - return item; - } - TRACE("s2", "find_preds: found proper place for key %p in skiplist, pred is %p. returning null", key_data, pred); - return NULL; -} - -// Fast find that does not help unlink partially removed nodes and does not return the node's predecessors. -uint64_t sl_lookup (skiplist_t *sl, const void *key_data, uint32_t key_len) { - TRACE("s1", "sl_lookup: searching for key %p in skiplist %p", key_data, sl); - node_t *item = find_preds(NULL, NULL, 0, sl, key_data, key_len, FALSE); - - // If we found an matching the return its value. - if (item != NULL) { - uint64_t val = item->val; - if (val != DOES_NOT_EXIST) { - TRACE("s1", "sl_lookup: found item %p. val %p. returning item", item, item->val); - return val; - } - } - - TRACE("l1", "sl_lookup: no item in the skiplist matched the key", 0, 0); - return DOES_NOT_EXIST; -} - -uint64_t sl_cas (skiplist_t *sl, const void *key_data, uint32_t key_len, uint64_t expectation, uint64_t new_val) { - TRACE("s1", "sl_cas: key %p skiplist %p", key_data, sl); - TRACE("s1", "sl_cas: expectation %p new value %p", expectation, new_val); - ASSERT((int64_t)new_val > 0); - - node_t *preds[MAX_LEVEL+1]; - node_t *nexts[MAX_LEVEL+1]; - node_t *new_item = NULL; - int n = random_level(); - do { - node_t *old_item = find_preds(preds, nexts, n, sl, key_data, key_len, TRUE); - if (old_item == NULL) { - - // There was not an item in the skiplist that matches the key. - if (EXPECT_FALSE((int64_t)expectation > 0 || expectation == EXPECT_EXISTS)) { - TRACE("l1", "sl_cas: the expectation was not met, the skiplist was not changed", 0, 0); - return DOES_NOT_EXIST; // failure - } - - ASSERT(expectation == EXPECT_DOES_NOT_EXIST || expectation == EXPECT_WHATEVER); - - // First insert into the bottom level. - TRACE("s3", "sl_cas: attempting to insert item between %p and %p", preds[0], nexts[0]); - new_item = node_alloc(n, key_data, key_len, new_val); - node_t *pred = preds[0]; - node_t *next = new_item->next[0] = nexts[0]; - for (int level = 1; level <= new_item->top_level; ++level) { - new_item->next[level] = nexts[level]; - } - node_t *other = SYNC_CAS(&pred->next[0], next, new_item); - if (other == next) { - TRACE("s3", "sl_cas: successfully inserted item %p at level 0", new_item, 0); - break; // success - } - TRACE("s3", "sl_cas: failed to change pred's link: expected %p found %p", next, other); - node_free(new_item); - continue; - } - - // Found an item in the skiplist that matches the key. - uint64_t old_item_val = old_item->val; - do { - // If the item's value is DOES_NOT_EXIST it means another thread removed the node out from under us. - if (EXPECT_FALSE(old_item_val == DOES_NOT_EXIST)) { - TRACE("s2", "sl_cas: lost a race, found an item but another thread removed it. retry", 0, 0); - break; // retry - } - - if (EXPECT_FALSE(expectation == EXPECT_DOES_NOT_EXIST)) { - TRACE("s1", "sl_cas: found an item %p in the skiplist that matched the key. the expectation was " - "not met, the skiplist was not changed", old_item, old_item_val); - return old_item_val; // failure - } - - // Use a CAS and not a SWAP. If the node is in the process of being removed and we used a SWAP, we could - // replace DOES_NOT_EXIST with our value. Then another thread that is updating the value could think it - // succeeded and return our value even though we indicated that the node has been removed. If the CAS - // fails it means another thread either removed the node or updated its value. - uint64_t ret_val = SYNC_CAS(&old_item->val, old_item_val, new_val); - if (ret_val == old_item_val) { - TRACE("s1", "sl_cas: the CAS succeeded. updated the value of the item", 0, 0); - return ret_val; // success - } - TRACE("s2", "sl_cas: lost a race. the CAS failed. another thread changed the item's value", 0, 0); - - old_item_val = ret_val; - } while (1); - } while (1); - - // Link into from the bottom up. - for (int level = 1; level <= new_item->top_level; ++level) { - node_t *pred = preds[level]; - node_t *next = nexts[level]; - do { - TRACE("s3", "sl_cas: attempting to insert item between %p and %p", pred, next); - node_t *other = SYNC_CAS(&pred->next[level], next, new_item); - if (other == next) { - TRACE("s3", "sl_cas: successfully inserted item %p at level %llu", new_item, level); - break; // success - } - TRACE("s3", "sl_cas: failed to change pred's link: expected %p found %p", next, other); - find_preds(preds, nexts, new_item->top_level, sl, key_data, key_len, TRUE); - pred = preds[level]; - next = nexts[level]; - - // Update 's next pointer - do { - // There in no need to continue linking in the item if another thread removed it. - node_t *old_next = ((volatile node_t *)new_item)->next[level]; - if (IS_TAGGED(old_next)) - return new_val; - - // Use a CAS so we do not inadvertantly stomp on a mark another thread placed on the item. - if (old_next == next || SYNC_CAS(&new_item->next[level], old_next, next) == old_next) - break; - } while (1); - } while (1); - } - return new_val; -} - -uint64_t sl_remove (skiplist_t *sl, const void *key_data, uint32_t key_len) { - TRACE("s1", "sl_remove: removing item with key %p from skiplist %p", key_data, sl); - node_t *preds[MAX_LEVEL+1]; - node_t *item = find_preds(preds, NULL, -1, sl, key_data, key_len, TRUE); - if (item == NULL) { - TRACE("s3", "sl_remove: remove failed, an item with a matching key does not exist in the skiplist", 0, 0); - return DOES_NOT_EXIST; - } - - // Mark removed at each level of from the top down. This must be atomic. If multiple threads - // try to remove the same item only one of them should succeed. Marking the bottom level establishes which of - // them succeeds. - for (int level = item->top_level; level >= 0; --level) { - if (EXPECT_FALSE(IS_TAGGED(item->next[level]))) { - TRACE("s3", "sl_remove: %p is already marked for removal by another thread", item, 0); - if (level == 0) - return DOES_NOT_EXIST; - continue; - } - node_t *next; - node_t *old_next = item->next[level]; - do { - next = old_next; - old_next = SYNC_CAS(&item->next[level], next, TAG_VALUE(next)); - if (IS_TAGGED(old_next)) { - TRACE("s2", "sl_remove: lost race -- %p is already marked for removal by another thread", item, 0); - if (level == 0) - return DOES_NOT_EXIST; - } - } while (!IS_TAGGED(old_next) || next != old_next); - } - - // This has to be an atomic swap in case another thread is updating the item while we are removing it. - uint64_t val = SYNC_SWAP(&item->val, DOES_NOT_EXIST); - TRACE("s2", "sl_remove: replaced item's val %p with DOES_NOT_EXIT", val, 0); - - // Unlink from . If we lose a race to another thread just back off. It is safe to leave the - // item partially unlinked for a later call (or some other thread) to physically unlink. By marking the - // item earlier, we logically removed it. - int level = item->top_level; - while (level >= 0) { - node_t *pred = preds[level]; - node_t *next = item->next[level]; - TRACE("s2", "sl_remove: unlink the item by linking its pred %p to it's successor %p", pred, STRIP_TAG(next)); - node_t *other = NULL; - if ((other = SYNC_CAS(&pred->next[level], item, STRIP_TAG(next))) != item) { - TRACE("s1", "sl_remove: unlink failed; pred's link changed from %p to %p", item, other); - return val; - } - --level; - } - - // The thread that completes the unlink should free the memory. - TRACE("s1", "sl_remove: successfully unlinked item %p from the skiplist", item, 0); - node_defer_free(item); - return val; -} - -void sl_print (skiplist_t *sl) { - for (int level = MAX_LEVEL; level >= 0; --level) { - node_t *item = sl->head; - if (item->next[level] == NULL) - continue; - printf("(%d) ", level); - while (item) { - node_t *next = item->next[level]; - printf("%s%p ", IS_TAGGED(next) ? "*" : "", item); - item = (node_t *)STRIP_TAG(next); - } - printf("\n"); - fflush(stdout); - } - - printf("\n"); - node_t *item = sl->head; - while (item) { - int is_marked = IS_TAGGED(item->next[0]); - - if (IS_TAGGED(item->key)) { - printf("%s%p:%llx ", is_marked ? "*" : "", item, STRIP_TAG(item->key)); - } else { - printf("%s%p:%s ", is_marked ? "*" : "", item, (char *)item->key->data); - } - if (item != sl->head) { - printf("[%d]", item->top_level); - } else { - printf("[*]"); - } - for (int level = 1; level <= item->top_level; ++level) { - node_t *next = (node_t *)STRIP_TAG(item->next[level]); - is_marked = IS_TAGGED(item->next[0]); - printf(" %p%s", next, is_marked ? "*" : ""); - if (item == sl->head && item->next[level] == NULL) - break; - } - printf("\n"); - fflush(stdout); - item = (node_t *)STRIP_TAG(item->next[0]); - } -}