X-Git-Url: https://pd.if.org/git/?p=nbds;a=blobdiff_plain;f=map%2Fskiplist.c;fp=map%2Fskiplist.c;h=73bad7b03dbe6cac22b98d0845a8fab396b2f2bb;hp=70cb6e50ceb657d984b8df61d831af087131fcdf;hb=778b8c8ca708b082a1192acfb114a6751b2ad7c9;hpb=5aa9223647fbb52fa8941d92c8896ebaf148b41c diff --git a/map/skiplist.c b/map/skiplist.c index 70cb6e5..73bad7b 100644 --- a/map/skiplist.c +++ b/map/skiplist.c @@ -26,13 +26,19 @@ #include "mem.h" #include "rcu.h" -// Setting MAX_LEVEL to 0 essentially makes this data structure the Harris-Michael lock-free list (in list.c). -#define MAX_LEVEL 31 +// Setting MAX_LEVELS to 1 essentially makes this data structure the Harris-Michael lock-free list (see list.c). +#define MAX_LEVELS 32 + +enum unlink { + FORCE_UNLINK, + ASSIST_UNLINK, + DONT_UNLINK +}; typedef struct node { map_key_t key; map_val_t val; - int top_level; + unsigned num_levels; markable_t next[1]; } node_t; @@ -59,22 +65,22 @@ static inline node_t * STRIP_MARK(markable_t x) { return ((node_t *)STRIP_TAG(x, #define STRIP_MARK(x) ((node_t *)STRIP_TAG((x), 0x1)) #endif -static int random_level (void) { +static int random_levels (void) { unsigned r = nbd_rand(); - int n = __builtin_ctz(r) / 2; - if (n > MAX_LEVEL) { n = MAX_LEVEL; } + int n = __builtin_ctz(r) / 2 + 1; + if (n > MAX_LEVELS) { n = MAX_LEVELS; } 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 * sizeof(node_t *); +static node_t *node_alloc (int num_levels, map_key_t key, map_val_t val) { + assert(num_levels >= 0 && num_levels <= MAX_LEVELS); + size_t sz = sizeof(node_t) + (num_levels - 1) * 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); + item->num_levels = num_levels; + TRACE("s2", "node_alloc: new node %p (%llu levels)", item, num_levels); return item; } @@ -82,8 +88,8 @@ 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 *)); + sl->head = node_alloc(MAX_LEVELS, 0, 0); + memset(sl->head->next, 0, MAX_LEVELS * sizeof(skiplist_t *)); return sl; } @@ -111,26 +117,22 @@ size_t sl_count (skiplist_t *sl) { return count; } -static node_t *find_preds (node_t **preds, node_t **succs, int n, skiplist_t *sl, map_key_t key, int help_remove) { +static node_t *find_preds (node_t **preds, node_t **succs, int n, skiplist_t *sl, map_key_t key, enum unlink unlink) { node_t *pred = sl->head; 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 = sl->high_water; - if (EXPECT_FALSE(start_level < n)) { - start_level = n; - } // Traverse the levels of from the top level to the bottom - for (int level = start_level; level >= 0; --level) { + for (int level = sl->high_water - 1; level >= 0; --level) { markable_t next = pred->next[level]; - if (next == DOES_NOT_EXIST && level > n) + 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 + ASSERT(level == pred->num_levels - 1 || HAS_MARK(pred->next[level+1])); + return find_preds(preds, succs, n, sl, key, unlink); // retry } item = GET_NODE(next); while (item != NULL) { @@ -139,7 +141,7 @@ 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))) { TRACE("s3", "find_preds: found marked item %p (next is %p)", item, next); - if (!help_remove) { + if (unlink == DONT_UNLINK) { // Skip over logically removed items. item = STRIP_MARK(next); @@ -149,14 +151,14 @@ static node_t *find_preds (node_t **preds, node_t **succs, int n, skiplist_t *sl } else { // Unlink logically removed items. - markable_t other = SYNC_CAS(&pred->next[level], item, STRIP_MARK(next)); + markable_t other = SYNC_CAS(&pred->next[level], (markable_t)item, (markable_t)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 + return find_preds(preds, succs, n, sl, key, unlink); // retry item = GET_NODE(other); } next = (item != NULL) ? item->next[level] : DOES_NOT_EXIST; @@ -177,7 +179,9 @@ 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) + if (d > 0) + break; + if (d == 0 && unlink != FORCE_UNLINK) break; pred = item; @@ -186,8 +190,7 @@ static node_t *find_preds (node_t **preds, node_t **succs, int n, skiplist_t *sl 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 (level < n) { if (preds != NULL) { preds[level] = pred; } @@ -197,14 +200,6 @@ 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 && 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; @@ -213,72 +208,10 @@ 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); - node_t *item = find_preds(NULL, NULL, 0, sl, key, FALSE); + node_t *item = find_preds(NULL, NULL, 0, sl, key, DONT_UNLINK); // If we found an matching the return its value. if (item != NULL) { @@ -338,11 +271,15 @@ map_val_t sl_cas (skiplist_t *sl, map_key_t key, map_val_t expectation, map_val_ 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 *preds[MAX_LEVELS]; + node_t *nexts[MAX_LEVELS]; node_t *new_item = NULL; - int n = random_level(); - node_t *old_item = find_preds(preds, nexts, n, sl, key, TRUE); + int n = random_levels(); + if (n > sl->high_water) { + n = SYNC_ADD(&sl->high_water, 1); + TRACE("s2", "sl_cas: incremented high water mark to %p", n, 0); + } + node_t *old_item = find_preds(preds, nexts, n, sl, key, ASSIST_UNLINK); // If there is already an item in the skiplist that matches the key just update its value. if (old_item != NULL) { @@ -362,24 +299,18 @@ map_val_t sl_cas (skiplist_t *sl, map_key_t key, map_val_t expectation, map_val_ // 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); // 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) { + for (int level = 1; level < new_item->num_levels; ++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); + markable_t other = SYNC_CAS(&pred->next[0], next, (markable_t)new_item); if (other != next) { TRACE("s3", "sl_cas: failed to change pred's link: expected %p found %p", next, other); @@ -393,22 +324,23 @@ map_val_t sl_cas (skiplist_t *sl, map_key_t key, map_val_t expectation, map_val_ TRACE("s3", "sl_cas: successfully inserted a new item %p at the bottom level", new_item, 0); - for (int level = 1; level <= new_item->top_level; ++level) { + ASSERT(new_item->num_levels <= MAX_LEVELS); + for (int level = 1; level < new_item->num_levels; ++level) { TRACE("s3", "sl_cas: inserting the new item %p at level %p", new_item, level); do { 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); + markable_t other = SYNC_CAS(&pred->next[level], (markable_t)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); + find_preds(preds, nexts, new_item->num_levels, sl, key, ASSIST_UNLINK); - for (int i = level; i <= new_item->top_level; ++i) { + for (int i = level; i < new_item->num_levels; ++i) { markable_t old_next = new_item->next[i]; if ((markable_t)nexts[i] == old_next) continue; @@ -416,12 +348,12 @@ map_val_t sl_cas (skiplist_t *sl, map_key_t key, map_val_t expectation, map_val_ // 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]); + other = SYNC_CAS(&new_item->next[i], old_next, (markable_t)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 + find_preds(NULL, NULL, 0, sl, key, FORCE_UNLINK); // see comment below return DOES_NOT_EXIST; } } @@ -432,8 +364,8 @@ map_val_t sl_cas (skiplist_t *sl, map_key_t key, map_val_t expectation, map_val_ // 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); + if (HAS_MARK(new_item->next[new_item->num_levels - 1])) { + find_preds(NULL, NULL, 0, sl, key, FORCE_UNLINK); } return DOES_NOT_EXIST; // success, inserted a new item @@ -441,8 +373,8 @@ map_val_t sl_cas (skiplist_t *sl, map_key_t key, map_val_t expectation, map_val_ map_val_t sl_remove (skiplist_t *sl, map_key_t key) { TRACE("s1", "sl_remove: removing item with key %p from skiplist %p", key, sl); - node_t *preds[MAX_LEVEL+1]; - node_t *item = find_preds(preds, NULL, -1, sl, key, TRUE); + node_t *preds[MAX_LEVELS]; + node_t *item = find_preds(preds, NULL, sl->high_water, sl, key, ASSIST_UNLINK); 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; @@ -451,7 +383,7 @@ map_val_t sl_remove (skiplist_t *sl, map_key_t key) { // 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. markable_t old_next = 0; - for (int level = item->top_level; level >= 0; --level) { + for (int level = item->num_levels - 1; level >= 0; --level) { markable_t next; old_next = item->next[level]; do { @@ -473,7 +405,7 @@ map_val_t sl_remove (skiplist_t *sl, map_key_t key) { TRACE("s2", "sl_remove: replaced item %p's value with DOES_NOT_EXIT", item, 0); // unlink the item - sl_unlink(sl, key); + find_preds(NULL, NULL, 0, sl, key, FORCE_UNLINK); // free the node if (sl->key_type != NULL) { @@ -487,7 +419,7 @@ map_val_t sl_remove (skiplist_t *sl, map_key_t key) { void sl_print (skiplist_t *sl) { printf("high water: %d levels\n", sl->high_water); - for (int level = MAX_LEVEL; level >= 0; --level) { + for (int level = MAX_LEVELS - 1; level >= 0; --level) { node_t *item = sl->head; if (item->next[level] == DOES_NOT_EXIST) continue; @@ -511,11 +443,11 @@ void sl_print (skiplist_t *sl) { int is_marked = HAS_MARK(item->next[0]); printf("%s%p:0x%llx ", is_marked ? "*" : "", item, (uint64_t)item->key); if (item != sl->head) { - printf("[%d]", item->top_level); + printf("[%d]", item->num_levels); } else { printf("[HEAD]"); } - for (int level = 1; level <= item->top_level; ++level) { + for (int level = 1; level < item->num_levels; ++level) { node_t *next = STRIP_MARK(item->next[level]); is_marked = HAS_MARK(item->next[0]); printf(" %p%s", next, is_marked ? "*" : ""); @@ -535,7 +467,7 @@ void sl_print (skiplist_t *sl) { sl_iter_t *sl_iter_begin (skiplist_t *sl, map_key_t key) { sl_iter_t *iter = (sl_iter_t *)nbd_malloc(sizeof(sl_iter_t)); if (key != DOES_NOT_EXIST) { - find_preds(NULL, &iter->next, 0, sl, key, FALSE); + find_preds(NULL, &iter->next, 1, sl, key, DONT_UNLINK); } else { iter->next = GET_NODE(sl->head->next[0]); }