From 2d93f3b29622488bde80b6cd18661fd7eb603eee Mon Sep 17 00:00:00 2001 From: jdybnis Date: Sat, 29 Nov 2008 06:17:06 +0000 Subject: [PATCH] generalize skiplist from a set to an updatable map --- include/struct.h | 2 +- struct/list.c | 11 ++- struct/skiplist.c | 182 +++++++++++++++++++++++++++++++--------------- test/sl_test.c | 4 +- 4 files changed, 133 insertions(+), 66 deletions(-) diff --git a/include/struct.h b/include/struct.h index 359c4cf..9d12f73 100644 --- a/include/struct.h +++ b/include/struct.h @@ -25,7 +25,7 @@ void ll_print (list_t *ll); typedef struct sl skiplist_t; skiplist_t * sl_alloc (void); uint64_t sl_lookup (skiplist_t *sl, const void *key_data, uint32_t key_len); -uint64_t sl_add (skiplist_t *sl, const void *key_data, uint32_t key_len, uint64_t value); +uint64_t sl_cas (skiplist_t *sl, const void *key_data, uint32_t key_len, uint64_t expected_val, uint64_t new_val); uint64_t sl_remove (skiplist_t *sl, const void *key_data, uint32_t key_len); void sl_print (skiplist_t *sl); diff --git a/struct/list.c b/struct/list.c index b9ec91f..5849247 100644 --- a/struct/list.c +++ b/struct/list.c @@ -128,7 +128,7 @@ static int find_pred (node_t **pred_ptr, node_t **item_ptr, list_t *ll, const vo TRACE("l2", "find_pred: found matching item %p in list, pred is %p", item, pred); return TRUE; } - TRACE("l2", "find_pred: found proper place for key %p in list, pred is %p. returning null", key_data, pred); + TRACE("l2", "find_pred: found proper place for key %p in list, pred is %p", key_data, pred); return FALSE; } @@ -169,11 +169,11 @@ uint64_t ll_cas (list_t *ll, const void *key_data, uint32_t key_len, uint64_t ex TRACE("l1", "ll_cas: expectation %p new value %p", expectation, new_val); ASSERT((int64_t)new_val > 0); - node_t *pred, *old_item; do { + node_t *pred, *old_item; if (!find_pred(&pred, &old_item, ll, key_data, key_len, TRUE)) { - // There is no existing item in the list that matches the key. + // There was not an item in the list that matches the key. if (EXPECT_FALSE((int64_t)expectation > 0 || expectation == EXPECT_EXISTS)) { TRACE("l1", "ll_cas: the expectation was not met, the list was not changed", 0, 0); return DOES_NOT_EXIST; // failure @@ -251,11 +251,10 @@ uint64_t ll_remove (list_t *ll, const void *key_data, uint32_t key_len) { } } while (next != old_next); TRACE("l2", "ll_remove: logically removed item %p", item, 0); - ASSERT(!IS_TAGGED(item->next)); + ASSERT(IS_TAGGED(item->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("l2", "ll_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 @@ -269,7 +268,7 @@ uint64_t ll_remove (list_t *ll, const void *key_data, uint32_t key_len) { } // The thread that completes the unlink should free the memory. - node_defer_free((node_t *)item); + node_defer_free(item); TRACE("l1", "ll_remove: successfully unlinked item %p from the list", item, 0); return val; } diff --git a/struct/skiplist.c b/struct/skiplist.c index 826751c..e8060dd 100644 --- a/struct/skiplist.c +++ b/struct/skiplist.c @@ -88,8 +88,8 @@ skiplist_t *sl_alloc (void) { 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("s3", "find_preds: searching for key %p in sl (head is %p)", key_data, pred); - int x; + 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. @@ -111,13 +111,11 @@ static node_t *find_preds (node_t **preds, node_t **succs, int n, skiplist_t *sl TRACE("s3", "find_preds: level %llu", level, 0); item = pred->next[level]; if (EXPECT_FALSE(IS_TAGGED(item))) { - TRACE("s3", "find_preds: pred %p is marked for removal (item %p); retry", pred, 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]; - TRACE("s3", "find_preds: visiting item %p (next %p)", item, next); - TRACE("s3", "find_preds: key %p", STRIP_TAG(item->key), item->val); // A tag means an item is logically removed but not physically unlinked yet. while (EXPECT_FALSE(IS_TAGGED(next))) { @@ -127,24 +125,26 @@ static node_t *find_preds (node_t **preds, node_t **succs, int n, skiplist_t *sl 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: unlinked item %p from pred %p", item, pred); - TRACE("s3", "find_preds: now item is %p next is %p", item, next); + 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("s3", "find_preds: lost race to unlink from pred %p; its link changed to %p", pred, other); + 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; @@ -157,12 +157,21 @@ static node_t *find_preds (node_t **preds, node_t **succs, int n, skiplist_t *sl if (EXPECT_FALSE(item == NULL)) break; - // If we reached the key (or passed where it should be), we found a pred. Save it and continue down. - x = (IS_TAGGED(item->key)) - ? (STRIP_TAG(item->key) - (uint64_t)key_data) - : ns_cmp_raw(item->key, key_data, key_len); - if (x >= 0) { - TRACE("s3", "find_preds: found pred %p item %p", pred, item); + 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; } @@ -180,69 +189,121 @@ static node_t *find_preds (node_t **preds, node_t **succs, int n, skiplist_t *sl } } } + + // fill in empty levels if (n == -1 && item != NULL) { for (int level = start_level + 1; level <= item->top_level; ++level) { preds[level] = sl->head; } } - return x == 0 ? item : NULL; + + 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("s3", "sl_lookup: searching for key %p in skiplist %p", key_data, sl); + 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. - return item != NULL ? item->val : DOES_NOT_EXIST; + 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; } -// Insert the if it doesn't already exist in -uint64_t sl_add (skiplist_t *sl, const void *key_data, uint32_t key_len, uint64_t val) { - TRACE("s3", "sl_add: inserting key %p val %p", key_data, val); +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) { - // If a node matching already exists in , return its value. - if (old_item != NULL) { - TRACE("s3", "sl_add: there is already an item %p (value %p) with the same key", nexts[0], nexts[0]->val); - return nexts[0]->val; - } + // 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 + } - // First insert into the bottom level. - TRACE("s3", "sl_add: attempting to insert item between %p and %p", preds[0], nexts[0]); - new_item = node_alloc(n, key_data, key_len, 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_add: successfully inserted item %p at level 0", new_item, 0); - break; // success + 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; } - TRACE("s3", "sl_add: failed to change pred's link: expected %p found %p", next, other); - node_free(new_item); + // 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); - // Insert into from the bottom level up. + // 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_add: attempting to insert item between %p and %p", pred, next); + 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_add: successfully inserted item %p at level %llu", new_item, level); + TRACE("s3", "sl_cas: successfully inserted item %p at level %llu", new_item, level); break; // success } - TRACE("s3", "sl_add: failed to change pred's link: expected %p found %p", next, other); + 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]; @@ -252,7 +313,7 @@ uint64_t sl_add (skiplist_t *sl, const void *key_data, uint32_t key_len, uint64_ // 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 val; + 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) @@ -260,11 +321,11 @@ uint64_t sl_add (skiplist_t *sl, const void *key_data, uint32_t key_len, uint64_ } while (1); } while (1); } - return val; + return new_val; } uint64_t sl_remove (skiplist_t *sl, const void *key_data, uint32_t key_len) { - TRACE("s3", "sl_remove: removing item with key %p from skiplist %p", key_data, sl); + 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) { @@ -282,34 +343,41 @@ uint64_t sl_remove (skiplist_t *sl, const void *key_data, uint32_t key_len) { return DOES_NOT_EXIST; continue; } - node_t *next = SYNC_FETCH_AND_OR(&item->next[level], TAG); - if (EXPECT_FALSE(IS_TAGGED(next))) { - TRACE("s3", "sl_remove: lost race -- %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); } - uint64_t val = item->val; + // 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 + // 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("s3", "sl_remove: link item's pred %p to it's successor %p", pred, STRIP_TAG(next)); + 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("s3", "sl_remove: unlink failed; pred's link changed from %p to %p", item, other); + 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; } diff --git a/test/sl_test.c b/test/sl_test.c index 6c2d4ab..195a5dd 100644 --- a/test/sl_test.c +++ b/test/sl_test.c @@ -26,13 +26,13 @@ void *worker (void *arg) { char key_str[10]; sprintf(key_str, "%llX", key); if (r & (1 << 8)) { - sl_add(sl_, key_str, strlen(key_str) + 1, 1); + sl_cas(sl_, key_str, strlen(key_str) + 1, EXPECT_WHATEVER, (r & 0xFF)+1); } else { sl_remove(sl_, key_str, strlen(key_str) + 1); } #else if (r & (1 << 8)) { - sl_add(sl_, (void *)key, -1, 1); + sl_cas(sl_, (void *)key, -1, EXPECT_WHATEVER, (r & 0xFF)+1); } else { sl_remove(sl_, (void *)key, -1); } -- 2.40.0