X-Git-Url: https://pd.if.org/git/?p=nbds;a=blobdiff_plain;f=map%2Fskiplist.c;h=70cb6e50ceb657d984b8df61d831af087131fcdf;hp=6e02e12600673267a80291b2005ae1e90662d4b3;hb=2b107655a1df8ae7703b44ef8cf1430a7250a5c3;hpb=a1d0b3ca99552878b1becf561d8f3291992aaa67 diff --git a/map/skiplist.c b/map/skiplist.c index 6e02e12..70cb6e5 100644 --- a/map/skiplist.c +++ b/map/skiplist.c @@ -33,7 +33,7 @@ typedef struct node { map_key_t key; map_val_t val; int top_level; - markable_t next[]; + markable_t next[1]; } node_t; struct sl_iter { @@ -43,6 +43,7 @@ struct sl_iter { struct sl { node_t *head; const datatype_t *key_type; + int high_water; // max level of any item in the list }; // Marking the field of a node logically removes it from the list @@ -60,30 +61,27 @@ static inline node_t * STRIP_MARK(markable_t x) { return ((node_t *)STRIP_TAG(x, 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); + int n = __builtin_ctz(r) / 2; + if (n > MAX_LEVEL) { n = MAX_LEVEL; } return n; } static node_t *node_alloc (int level, map_key_t key, map_val_t val) { assert(level >= 0 && level <= MAX_LEVEL); - size_t sz = sizeof(node_t) + (level + 1) * sizeof(node_t *); + size_t sz = sizeof(node_t) + level * sizeof(node_t *); node_t *item = (node_t *)nbd_malloc(sz); memset(item, 0, sz); item->key = key; item->val = val; item->top_level = level; + TRACE("s2", "node_alloc: new node %p (%llu levels)", item, level); return item; } skiplist_t *sl_alloc (const datatype_t *key_type) { skiplist_t *sl = (skiplist_t *)nbd_malloc(sizeof(skiplist_t)); sl->key_type = key_type; + sl->high_water = 0; sl->head = node_alloc(MAX_LEVEL, 0, 0); memset(sl->head->next, 0, (MAX_LEVEL+1) * sizeof(skiplist_t *)); return sl; @@ -118,28 +116,20 @@ static node_t *find_preds (node_t **preds, node_t **succs, int n, skiplist_t *sl node_t *item = NULL; TRACE("s2", "find_preds: searching for key %p in skiplist (head is %p)", key, pred); int d = 0; - 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] != DOES_NOT_EXIST) { - start_level += start_level - 1; - if (EXPECT_FALSE(start_level >= MAX_LEVEL)) { - start_level = MAX_LEVEL; - break; - } - } + int start_level = sl->high_water; 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); markable_t next = pred->next[level]; + if (next == DOES_NOT_EXIST && level > n) + continue; + TRACE("s3", "find_preds: traversing level %p starting at %p", level, pred); if (EXPECT_FALSE(HAS_MARK(next))) { TRACE("s2", "find_preds: pred %p is marked for removal (next %p); retry", pred, next); + ASSERT(level == pred->top_level || HAS_MARK(pred->next[level+1])); return find_preds(preds, succs, n, sl, key, help_remove); // retry } item = GET_NODE(next); @@ -148,45 +138,35 @@ static node_t *find_preds (node_t **preds, node_t **succs, int n, skiplist_t *sl // A tag means an item is logically removed but not physically unlinked yet. while (EXPECT_FALSE(HAS_MARK(next))) { - - // Skip over logically removed items. + TRACE("s3", "find_preds: found marked item %p (next is %p)", item, next); if (!help_remove) { + + // Skip over logically removed items. item = STRIP_MARK(next); if (EXPECT_FALSE(item == NULL)) break; next = item->next[level]; - TRACE("s3", "find_preds: skipping marked item %p (next is 0x%llx)", item, next); - continue; - } + } else { - // Unlink logically removed items. - TRACE("s3", "find_preds: unlinking marked item %p; next is 0x%llx", item, next); - markable_t other = SYNC_CAS(&pred->next[level], item, STRIP_MARK(next)); - if (other == (markable_t)item) { - item = STRIP_MARK(next); - next = (item != NULL) ? item->next[level] : DOES_NOT_EXIST; - TRACE("s3", "find_preds: now the current item is %p next is 0x%llx", item, next); - - // The thread that completes the unlink should free the memory. - if (level == 0) { - node_t *unlinked = GET_NODE(other); - if (sl->key_type != NULL) { - rcu_defer_free((void *)unlinked->key); - } - rcu_defer_free(unlinked); + // Unlink logically removed items. + markable_t other = SYNC_CAS(&pred->next[level], item, STRIP_MARK(next)); + if (other == (markable_t)item) { + TRACE("s3", "find_preds: unlinked item from pred %p", pred, 0); + item = STRIP_MARK(next); + } else { + TRACE("s3", "find_preds: lost race to unlink item pred %p's link changed to %p", pred, other); + if (HAS_MARK(other)) + return find_preds(preds, succs, n, sl, key, help_remove); // retry + item = GET_NODE(other); } - } else { - TRACE("s3", "find_preds: lost race to unlink item %p from pred %p", item, pred); - TRACE("s3", "find_preds: pred's link changed to %p", other, 0); - if (HAS_MARK(other)) - return find_preds(preds, succs, n, sl, key, help_remove); // retry - item = GET_NODE(other); next = (item != NULL) ? item->next[level] : DOES_NOT_EXIST; } } - if (EXPECT_FALSE(item == NULL)) + if (EXPECT_FALSE(item == NULL)) { + TRACE("s3", "find_preds: past the last item in the skiplist", 0, 0); break; + } TRACE("s4", "find_preds: visiting item %p (next is %p)", item, next); TRACE("s4", "find_preds: key %p val %p", STRIP_MARK(item->key), item->val); @@ -197,15 +177,15 @@ static node_t *find_preds (node_t **preds, node_t **succs, int n, skiplist_t *sl d = sl->key_type->cmp((void *)item->key, (void *)key); } - if (d >= 0) { - TRACE("s4", "find_preds: found pred %p item %p", pred, item); + if (d >= 0) break; - } pred = item; item = GET_NODE(next); } + TRACE("s3", "find_preds: found pred %p next %p", pred, item); + // The cast to unsigned is for the case when n is -1. if ((unsigned)level <= (unsigned)n) { if (preds != NULL) { @@ -217,14 +197,14 @@ 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) { - assert(item->top_level <= MAX_LEVEL); - for (int level = start_level + 1; level <= item->top_level; ++level) { - preds[level] = sl->head; - } - } - + // fill in empty levels + if (n == -1 && item != NULL && preds != NULL) { + assert(item->top_level <= MAX_LEVEL); + 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; @@ -233,6 +213,68 @@ static node_t *find_preds (node_t **preds, node_t **succs, int n, skiplist_t *sl return NULL; } +static void sl_unlink (skiplist_t *sl, map_key_t key) { + node_t *pred = sl->head; + node_t *item = NULL; + TRACE("s2", "sl_unlink: unlinking marked item with key %p", key, 0); + int d = 0; + + // Traverse the levels of from the top level to the bottom + for (int level = sl->high_water; level >= 0; --level) { + markable_t next = pred->next[level]; + if (next == DOES_NOT_EXIST) + continue; + TRACE("s3", "sl_unlink: traversing level %p starting at %p", level, pred); + if (EXPECT_FALSE(HAS_MARK(next))) { + TRACE("s2", "sl_unlink: lost a race; pred %p is marked for removal (next %p); retry", pred, next); + ASSERT(level == pred->top_level || HAS_MARK(pred->next[level+1])); + return sl_unlink(sl, key); // retry + } + item = GET_NODE(next); + while (item != NULL) { + next = item->next[level]; + + while (HAS_MARK(next)) { + TRACE("s3", "sl_unlink: found marked item %p (next is %p)", item, next); + + markable_t other = SYNC_CAS(&pred->next[level], item, STRIP_MARK(next)); + if (other == (markable_t)item) { + TRACE("s3", "sl_unlink: unlinked item from pred %p", pred, 0); + item = STRIP_MARK(next); + } else { + TRACE("s3", "sl_unlink: lost race to unlink item, pred %p's link changed to %p", pred, other); + if (HAS_MARK(other)) + return sl_unlink(sl, key); // retry + item = GET_NODE(other); + } + next = (item != NULL) ? item->next[level] : DOES_NOT_EXIST; + } + + if (EXPECT_FALSE(item == NULL)) { + TRACE("s3", "sl_unlink: past the last item in the skiplist", 0, 0); + break; + } + + TRACE("s4", "sl_unlink: visiting item %p (next is %p)", item, next); + TRACE("s4", "sl_unlink: key %p val %p", STRIP_MARK(item->key), item->val); + + if (EXPECT_TRUE(sl->key_type == NULL)) { + d = item->key - key; + } else { + d = sl->key_type->cmp((void *)item->key, (void *)key); + } + + if (d > 0) + break; + + pred = item; + item = GET_NODE(next); + } + + TRACE("s3", "sl_unlink: at pred %p next %p", pred, item); + } +} + // Fast find that does not help unlink partially removed nodes and does not return the node's predecessors. map_val_t sl_lookup (skiplist_t *sl, map_key_t key) { TRACE("s1", "sl_lookup: searching for key %p in skiplist %p", key, sl); @@ -262,6 +304,35 @@ map_key_t sl_min_key (skiplist_t *sl) { return DOES_NOT_EXIST; } +static map_val_t update_item (node_t *item, map_val_t expectation, map_val_t new_val) { + map_val_t old_val = item->val; + + // If the item's value is DOES_NOT_EXIST it means another thread removed the node out from under us. + if (EXPECT_FALSE(old_val == DOES_NOT_EXIST)) { + TRACE("s2", "update_item: lost a race to another thread removing the item. retry", 0, 0); + return DOES_NOT_EXIST; // retry + } + + if (EXPECT_FALSE(expectation == CAS_EXPECT_DOES_NOT_EXIST)) { + TRACE("s1", "update_item: found an item %p in the skiplist that matched the key. the expectation was " + "not met, the skiplist was not changed", item, old_val); + return old_val; // failure + } + + // Use a CAS and not a SWAP. If the CAS fails it means another thread removed the node or updated its + // value. If another thread removed the node but it is not unlinked yet 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 it should return DOES_NOT_EXIST. + if (old_val == SYNC_CAS(&item->val, old_val, new_val)) { + TRACE("s1", "update_item: the CAS succeeded. updated the value of the item", 0, 0); + return old_val; // success + } + TRACE("s2", "update_item: lost a race. the CAS failed. another thread changed the item's value", 0, 0); + + // retry + return update_item(item, expectation, new_val); // tail call +} + map_val_t sl_cas (skiplist_t *sl, map_key_t key, map_val_t expectation, map_val_t new_val) { TRACE("s1", "sl_cas: key %p skiplist %p", key, sl); TRACE("s1", "sl_cas: expectation %p new value %p", expectation, new_val); @@ -271,98 +342,101 @@ map_val_t sl_cas (skiplist_t *sl, map_key_t key, map_val_t expectation, map_val_ 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, TRUE); - if (old_item == NULL) { - - // There was not an item in the skiplist that matches the key. - if (EXPECT_FALSE(expectation != CAS_EXPECT_DOES_NOT_EXIST && expectation != CAS_EXPECT_WHATEVER)) { - TRACE("l1", "sl_cas: the expectation was not met, the skiplist was not changed", 0, 0); - return DOES_NOT_EXIST; // failure - } + node_t *old_item = find_preds(preds, nexts, n, sl, key, TRUE); - // First insert into the bottom level. - TRACE("s3", "sl_cas: attempting to insert item between %p and %p", preds[0], nexts[0]); - map_key_t new_key = sl->key_type == NULL ? key : (map_key_t)sl->key_type->clone((void *)key); - new_item = node_alloc(n, new_key, new_val); - node_t *pred = preds[0]; - markable_t next = new_item->next[0] = (markable_t)nexts[0]; - for (int level = 1; level <= new_item->top_level; ++level) { - new_item->next[level] = (markable_t)nexts[level]; - } - markable_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); - if (sl->key_type != NULL) { - nbd_free((void *)new_key); - } - nbd_free(new_item); - continue; - } + // If there is already an item in the skiplist that matches the key just update its value. + if (old_item != NULL) { + map_val_t ret_val = update_item(old_item, expectation, new_val); + if (ret_val != DOES_NOT_EXIST) + return ret_val; - // Found an item in the skiplist that matches the key. - map_val_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 we lose a race with a thread removing the item we tried to update then we have to retry. + return sl_cas(sl, key, expectation, new_val); // tail call + } - if (EXPECT_FALSE(expectation == CAS_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 - } + if (EXPECT_FALSE(expectation != CAS_EXPECT_DOES_NOT_EXIST && expectation != CAS_EXPECT_WHATEVER)) { + TRACE("l1", "sl_cas: the expectation was not met, the skiplist was not changed", 0, 0); + return DOES_NOT_EXIST; // failure, the caller expected an item for the to already exist + } - // 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. - map_val_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); + // Create a new node and insert it into the skiplist. + TRACE("s3", "sl_cas: attempting to insert a new item between %p and %p", preds[0], nexts[0]); + map_key_t new_key = sl->key_type == NULL ? key : (map_key_t)sl->key_type->clone((void *)key); + if (n > sl->high_water) { + n = sl->high_water + 1; + int x = SYNC_ADD(&sl->high_water, 1); + x = x; + TRACE("s2", "sl_cas: incremented high water mark to %p", x, 0); + } + new_item = node_alloc(n, new_key, new_val); - old_item_val = ret_val; - } while (1); - } while (1); + // Set 's next pointers to their proper values + markable_t next = new_item->next[0] = (markable_t)nexts[0]; + for (int level = 1; level <= new_item->top_level; ++level) { + new_item->next[level] = (markable_t)nexts[level]; + } + + // Link into from the bottom level up. After is inserted into the bottom level + // it is officially part of the skiplist. + node_t *pred = preds[0]; + markable_t other = SYNC_CAS(&pred->next[0], next, new_item); + if (other != next) { + TRACE("s3", "sl_cas: failed to change pred's link: expected %p found %p", next, other); + + // Lost a race to another thread modifying the skiplist. Free the new item we allocated and retry. + if (sl->key_type != NULL) { + nbd_free((void *)new_key); + } + nbd_free(new_item); + return sl_cas(sl, key, expectation, new_val); // tail call + } + + TRACE("s3", "sl_cas: successfully inserted a new item %p at the bottom level", new_item, 0); - // Link into from the bottom up. for (int level = 1; level <= new_item->top_level; ++level) { - node_t *pred = preds[level]; - markable_t next = (markable_t)nexts[level]; + TRACE("s3", "sl_cas: inserting the new item %p at level %p", new_item, level); do { - TRACE("s3", "sl_cas: attempting to insert item between %p and %p", pred, next); - markable_t other = SYNC_CAS(&pred->next[level], next, (markable_t)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); + node_t * pred = preds[level]; + ASSERT(new_item->next[level]==(markable_t)nexts[level] || new_item->next[level]==MARK_NODE(nexts[level])); + TRACE("s3", "sl_cas: attempting to to insert the new item between %p and %p", pred, nexts[level]); + + markable_t other = SYNC_CAS(&pred->next[level], nexts[level], (markable_t)new_item); + if (other == (markable_t)nexts[level]) + break; // successfully linked into the skiplist at the current + TRACE("s3", "sl_cas: lost a race. failed to change pred's link. expected %p found %p", nexts[level], other); + + // Find 's new preds and nexts. find_preds(preds, nexts, new_item->top_level, sl, key, TRUE); - pred = preds[level]; - next = (markable_t)nexts[level]; - - // Update 's next pointer - do { - // There in no need to continue linking in the item if another thread removed it. - markable_t old_next = ((volatile node_t *)new_item)->next[level]; - if (HAS_MARK(old_next)) - return DOES_NOT_EXIST; // success - - // 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); + + for (int i = level; i <= new_item->top_level; ++i) { + markable_t old_next = new_item->next[i]; + if ((markable_t)nexts[i] == old_next) + continue; + + // Update 's inconsistent next pointer before trying again. Use a CAS so if another thread + // is trying to remove the new item concurrently we do not stomp on the mark it places on the item. + TRACE("s3", "sl_cas: attempting to update the new item's link from %p to %p", old_next, nexts[i]); + other = SYNC_CAS(&new_item->next[i], old_next, nexts[i]); + ASSERT(other == old_next || other == MARK_NODE(old_next)); + + // If another thread is removing this item we can stop linking it into to skiplist + if (HAS_MARK(other)) { + sl_unlink(sl, key); // see comment below + return DOES_NOT_EXIST; + } + } } while (1); } - return DOES_NOT_EXIST; // success + + // In case another thread was in the process of removing the while we were added it, we have to + // make sure it is completely unlinked before we return. We might have lost a race and inserted the new item + // at some level after the other thread thought it was fully removed. That is a problem because once a thread + // thinks it completely unlinks a node it queues it to be freed + if (HAS_MARK(new_item->next[new_item->top_level])) { + sl_unlink(sl, key); + } + + return DOES_NOT_EXIST; // success, inserted a new item } map_val_t sl_remove (skiplist_t *sl, map_key_t key) { @@ -374,79 +448,45 @@ map_val_t sl_remove (skiplist_t *sl, map_key_t key) { return DOES_NOT_EXIST; } - // Mark and unlink at each level of from the top down. If multiple threads try to concurrently remove + // Mark at each level of from the top down. If multiple threads try to concurrently 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) { + markable_t old_next = 0; + for (int level = item->top_level; level >= 0; --level) { markable_t next; - markable_t old_next = item->next[level]; + old_next = item->next[level]; do { + TRACE("s3", "sl_remove: marking item at level %p (next %p)", level, old_next); next = old_next; old_next = SYNC_CAS(&item->next[level], next, MARK_NODE((node_t *)next)); if (HAS_MARK(old_next)) { - TRACE("s2", "sl_remove: %p is already marked for removal by another thread at level %llu", item, level); + TRACE("s2", "sl_remove: %p is already marked for removal by another thread (next %p)", item, old_next); + if (level == 0) + return DOES_NOT_EXIST; break; } } while (next != old_next); - - node_t *pred = preds[level]; - TRACE("s2", "sl_remove: linking the item's pred %p to the item's successor %p", pred, STRIP_MARK(next)); - markable_t other = SYNC_CAS(&pred->next[level], item, STRIP_MARK(next)); - if (other != (markable_t)item) { - TRACE("s1", "sl_remove: unlink failed; pred's link changed from %p to %p", item, other); - // If our former predecessor now points past us we know another thread unlinked us. Otherwise, we need - // to search for a new set of preds. - if (other == DOES_NOT_EXIST) - continue; // points past to the end of the list; go on to the next level. - - int d = -1; - if (!HAS_MARK(other)) { - map_key_t other_key = GET_NODE(other)->key; - if (EXPECT_TRUE(sl->key_type == NULL)) { - d = item->key - other_key; - } else { - d = sl->key_type->cmp((void *)item->key, (void *)other_key); - } - } - if (d > 0) { - node_t *temp = find_preds(preds, NULL, level, sl, key, TRUE); - if (temp != item) - return DOES_NOT_EXIST; // Another thread removed the item we were targeting. - level++; // Redo this level. - } - } } - markable_t next; - markable_t old_next = item->next[0]; - do { - next = old_next; - old_next = SYNC_CAS(&item->next[0], next, MARK_NODE((node_t *)next)); - if (HAS_MARK(old_next)) { - TRACE("s2", "sl_remove: %p is already marked for removal by another thread at level 0", item, 0); - return DOES_NOT_EXIST; - } - } while (next != old_next); - TRACE("s1", "sl_remove: marked item %p removed at level 0", item, 0); - // Atomically swap out the item's value in case another thread is updating the item while we are // removing it. This establishes which operation occurs first logically, the update or the remove. map_val_t val = SYNC_SWAP(&item->val, DOES_NOT_EXIST); TRACE("s2", "sl_remove: replaced item %p's value with DOES_NOT_EXIT", item, 0); - node_t *pred = preds[0]; - TRACE("s2", "sl_remove: linking the item's pred %p to the item's successor %p", pred, STRIP_MARK(next)); - if (SYNC_CAS(&pred->next[0], item, STRIP_MARK(next))) { - TRACE("s2", "sl_remove: unlinked item %p from the skiplist at level 0", item, 0); - // The thread that completes the unlink should free the memory. - if (sl->key_type != NULL) { - rcu_defer_free((void *)item->key); - } - rcu_defer_free(item); + // unlink the item + sl_unlink(sl, key); + + // free the node + if (sl->key_type != NULL) { + rcu_defer_free((void *)item->key); } + rcu_defer_free(item); + return val; } void sl_print (skiplist_t *sl) { + + printf("high water: %d levels\n", sl->high_water); for (int level = MAX_LEVEL; level >= 0; --level) { node_t *item = sl->head; if (item->next[level] == DOES_NOT_EXIST)