-/*
+/*
* Written by Josh Dybnis and released to the public domain, as explained at
* http://creativecommons.org/licenses/publicdomain
*
* 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 won't work correctly on platforms with weaker memory
- * models if you don't add memory barriers in the right places.
+ * I've generalized the data structure to support update operations like set() and CAS() in addition to
+ * the normal add() and remove() operations.
+ *
+ * Warning: This code is written for the x86 memory-model. The algorithim depends on certain stores
+ * and loads being ordered. This code won't work correctly on platforms with weaker memory models if
+ * you don't add memory barriers in the right places.
*/
#include <stdio.h>
#include <string.h>
-#define ENABLE_TRACE
-
#include "common.h"
+#include "skiplist.h"
#include "runtime.h"
-#include "mlocal.h"
#include "mem.h"
-#include "tls.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 24
+
+enum unlink {
+ FORCE_UNLINK,
+ ASSIST_UNLINK,
+ DONT_UNLINK
+};
typedef struct node {
- void *key;
- uint64_t val;
- int top_level;
- struct node *next[];
+ map_key_t key;
+ map_val_t val;
+ unsigned num_levels;
+ markable_t next[1];
} node_t;
-struct sl {
- node_t *head;
- cmp_fun_t cmp_fun;
- clone_fun_t clone_fun;
+struct sl_iter {
+ node_t *next;
};
-static const map_impl_t sl_map_impl = {
- (map_alloc_t)sl_alloc, (map_cas_t)sl_cas, (map_get_t)sl_lookup, (map_remove_t)sl_remove,
- (map_count_t)sl_count, (map_print_t)sl_print, (map_free_t)sl_free
+struct sl {
+ node_t *head;
+ const datatype_t *key_type;
+ int high_water; // max historic number of levels
};
-const map_impl_t *MAP_TYPE_SKIPLIST = &sl_map_impl;
-
-static int random_level (void) {
- unsigned r = nbd_rand();
- if (r & 1)
- return 0;
-#if MAX_LEVEL < 31
- r |= 1 << (MAX_LEVEL+1);
+// Marking the <next> field of a node logically removes it from the list
+#if 0
+static inline markable_t MARK_NODE(node_t * x) { return TAG_VALUE((markable_t)x, 0x1); }
+static inline int HAS_MARK(markable_t x) { return (IS_TAGGED(x, 0x1) == 0x1); }
+static inline node_t * GET_NODE(markable_t x) { assert(!HAS_MARK(x)); return (node_t *)x; }
+static inline node_t * STRIP_MARK(markable_t x) { return ((node_t *)STRIP_TAG(x, 0x1)); }
+#else
+#define MARK_NODE(x) TAG_VALUE((markable_t)(x), 0x1)
+#define HAS_MARK(x) (IS_TAGGED((x), 0x1) == 0x1)
+#define GET_NODE(x) ((node_t *)(x))
+#define STRIP_MARK(x) ((node_t *)STRIP_TAG((x), 0x1))
#endif
- int n = __builtin_ctz(r)-1;
- assert(n <= MAX_LEVEL);
- return n;
+
+static int random_levels (skiplist_t *sl) {
+ uint64_t r = nbd_rand();
+ int z = __builtin_ctz(r);
+ int levels = (int)(z / 1.5);
+ if (levels == 0)
+ return 1;
+ if (levels > sl->high_water) {
+ levels = SYNC_ADD(&sl->high_water, 1);
+ TRACE("s2", "random_levels: increased high water mark to %lld", sl->high_water, 0);
+ }
+ if (levels > MAX_LEVELS) { levels = MAX_LEVELS; }
+ return levels;
}
-static node_t *node_alloc (int level, void *key, uint64_t val) {
- assert(level >= 0 && level <= MAX_LEVEL);
- size_t sz = sizeof(node_t) + (level + 1) * 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;
+ item->num_levels = num_levels;
+ TRACE("s2", "node_alloc: new node %p (%llu levels)", item, num_levels);
return item;
}
-skiplist_t *sl_alloc (cmp_fun_t cmp_fun, hash_fun_t hash_fun, clone_fun_t clone_fun) {
+skiplist_t *sl_alloc (const datatype_t *key_type) {
skiplist_t *sl = (skiplist_t *)nbd_malloc(sizeof(skiplist_t));
- sl->cmp_fun = cmp_fun;
- sl->clone_fun = clone_fun;
- sl->head = node_alloc(MAX_LEVEL, NULL, 0);
- memset(sl->head->next, 0, (MAX_LEVEL+1) * sizeof(skiplist_t *));
+ sl->key_type = key_type;
+ sl->high_water = 1;
+ sl->head = node_alloc(MAX_LEVELS, 0, 0);
+ memset(sl->head->next, 0, MAX_LEVELS * sizeof(skiplist_t *));
return sl;
}
void sl_free (skiplist_t *sl) {
- node_t *item = sl->head->next[0];
+ node_t *item = GET_NODE(sl->head->next[0]);
while (item) {
- node_t *next = (node_t *)STRIP_TAG(item->next[0]);
+ node_t *next = STRIP_MARK(item->next[0]);
+ if (sl->key_type != NULL) {
+ nbd_free((void *)item->key);
+ }
nbd_free(item);
item = next;
}
}
-uint64_t sl_count (skiplist_t *sl) {
- uint64_t count = 0;
- node_t *item = sl->head->next[0];
+size_t sl_count (skiplist_t *sl) {
+ size_t count = 0;
+ node_t *item = GET_NODE(sl->head->next[0]);
while (item) {
- if (!IS_TAGGED(item->next[0])) {
+ if (!HAS_MARK(item->next[0])) {
count++;
}
- item = (node_t *)STRIP_TAG(item->next[0]);
+ item = STRIP_MARK(item->next[0]);
}
return count;
}
-static node_t *find_preds (node_t **preds, node_t **succs, int n, skiplist_t *sl, void *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;
- 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
+ int d = 0;
// Traverse the levels of <sl> 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, help_remove); // retry
+ for (int level = sl->high_water - 1; level >= 0; --level) {
+ 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->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) {
- node_t *next = item->next[level];
+ next = item->next[level];
// A tag means an item is logically removed but not physically unlinked yet.
- while (EXPECT_FALSE(IS_TAGGED(next))) {
+ while (EXPECT_FALSE(HAS_MARK(next))) {
+ TRACE("s3", "find_preds: found marked item %p (next is %p)", item, next);
+ if (unlink == DONT_UNLINK) {
- // Skip over logically removed items.
- if (!help_remove) {
- item = (node_t *)STRIP_TAG(item->next);
+ // Skip over logically removed items.
+ item = STRIP_MARK(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;
- }
+ } else {
- // 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) {
- if (sl->clone_fun != NULL) {
- nbd_defer_free((void*)other->key);
- }
- nbd_defer_free(other);
+ // Unlink logically removed items.
+ 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, unlink); // 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 (IS_TAGGED(other))
- return find_preds(preds, succs, n, sl, key, help_remove); // retry
- item = other;
- if (EXPECT_FALSE(item == NULL))
- break;
- next = item->next[level];
+ 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_TAG(item->key), item->val);
+ TRACE("s4", "find_preds: key %p val %p", STRIP_MARK(item->key), item->val);
- if (EXPECT_TRUE(sl->cmp_fun == NULL)) {
- d = (uint64_t)item->key - (uint64_t)key;
+ if (EXPECT_TRUE(sl->key_type == NULL)) {
+ d = item->key - key;
} else {
- d = sl->cmp_fun(item->key, key);
+ 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;
+ if (d == 0 && unlink != FORCE_UNLINK)
break;
- }
pred = item;
- item = next;
+ item = GET_NODE(next);
}
- // The cast to unsigned is for the case when n is -1.
- if ((unsigned)level <= (unsigned)n) {
+ TRACE("s3", "find_preds: found pred %p next %p", pred, item);
+
+ if (level < n) {
if (preds != NULL) {
preds[level] = pred;
}
}
}
- // 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;
}
// 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, void *key) {
+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 <item> matching the <key> return its value.
if (item != NULL) {
- uint64_t val = item->val;
+ map_val_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);
+ TRACE("s1", "sl_lookup: no item in the skiplist matched the key", 0, 0);
+ return DOES_NOT_EXIST;
+}
+
+map_key_t sl_min_key (skiplist_t *sl) {
+ node_t *item = GET_NODE(sl->head->next[0]);
+ while (item != NULL) {
+ markable_t next = item->next[0];
+ if (!HAS_MARK(next))
+ return item->key;
+ item = STRIP_MARK(next);
+ }
return DOES_NOT_EXIST;
}
-uint64_t sl_cas (skiplist_t *sl, void *key, uint64_t expectation, uint64_t new_val) {
+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: the expectation was not met; the skiplist was not changed", 0, 0);
+ 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);
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();
- 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((int64_t)expectation > 0 || expectation == CAS_EXPECT_EXISTS)) {
- TRACE("l1", "sl_cas: the expectation was not met, the skiplist was not changed", 0, 0);
- return DOES_NOT_EXIST; // failure
- }
+ int n = random_levels(sl);
+ node_t *old_item = find_preds(preds, nexts, n, sl, key, ASSIST_UNLINK);
- ASSERT(expectation == CAS_EXPECT_DOES_NOT_EXIST || expectation == CAS_EXPECT_WHATEVER);
+ // 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;
- // First insert <new_item> into the bottom level.
- TRACE("s3", "sl_cas: attempting to insert item between %p and %p", preds[0], nexts[0]);
- void *new_key = (sl->clone_fun == NULL) ? key : sl->clone_fun(key);
- new_item = node_alloc(n, new_key, 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);
- if (sl->clone_fun != NULL) {
- nbd_free(new_key);
- }
- nbd_free(new_item);
- continue;
+ // 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 && expectation != CAS_EXPECT_WHATEVER)) {
+ TRACE("s1", "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 <key> to already exist
+ }
+
+ // 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);
+ new_item = node_alloc(n, new_key, new_val);
+
+ // Set <new_item>'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->num_levels; ++level) {
+ new_item->next[level] = (markable_t)nexts[level];
+ }
+
+ // Link <new_item> into <sl> from the bottom level up. After <new_item> 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, (markable_t)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);
- // Found an item in the skiplist that matches the key.
- uint64_t old_item_val = old_item->val;
+ 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 {
- // 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
- }
+ 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]);
- 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
- }
+ 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 <new_item> into the skiplist at the current <level>
+ TRACE("s3", "sl_cas: lost a race. failed to change pred's link. expected %p found %p", nexts[level], other);
- // 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);
+ // Find <new_item>'s new preds and nexts.
+ find_preds(preds, nexts, new_item->num_levels, sl, key, ASSIST_UNLINK);
- old_item_val = ret_val;
- } while (1);
- } while (1);
+ 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;
- // Link <new_item> into <sl> 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
+ // Update <new_item>'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, (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)) {
+ find_preds(NULL, NULL, 0, sl, key, FORCE_UNLINK); // see comment below
+ return DOES_NOT_EXIST;
+ }
}
- 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, TRUE);
- pred = preds[level];
- next = nexts[level];
-
- // Update <new_item>'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 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);
} while (1);
}
- return DOES_NOT_EXIST; // success
+
+ // In case another thread was in the process of removing the <new_item> 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->num_levels - 1])) {
+ find_preds(NULL, NULL, 0, sl, key, FORCE_UNLINK);
+ }
+
+ return DOES_NOT_EXIST; // success, inserted a new item
}
-uint64_t sl_remove (skiplist_t *sl, void *key) {
+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;
}
- // Mark and unlink <item> at each level of <sl> from the top down. If multiple threads try to concurrently remove
+ // Mark <item> at each level of <sl> 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) {
- node_t *next;
- node_t *old_next = item->next[level];
+ markable_t old_next = 0;
+ for (int level = item->num_levels - 1; level >= 0; --level) {
+ markable_t next;
+ 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, TAG_VALUE(next));
- if (IS_TAGGED(old_next)) {
- TRACE("s2", "sl_remove: %p is already marked for removal by another thread at level %llu", item, level);
+ 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 (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_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);
- // 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 == NULL)
- continue; // <pred> points past <item> to the end of the list; go on to the next level.
-
- int d = -1;
- if (!IS_TAGGED(other)) {
- if (EXPECT_TRUE(sl->cmp_fun == NULL)) {
- d = (uint64_t)item->key - (uint64_t)other->key;
- } else {
- d = sl->cmp_fun(item->key, 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.
- }
- }
}
- node_t *next;
- node_t *old_next = item->next[0];
- do {
- next = old_next;
- old_next = SYNC_CAS(&item->next[0], next, TAG_VALUE(next));
- if (IS_TAGGED(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 one occurs first, the update or the remove.
- uint64_t val = SYNC_SWAP(&item->val, DOES_NOT_EXIST);
+ // 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_TAG(next));
- if (SYNC_CAS(&pred->next[0], item, STRIP_TAG(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->clone_fun != NULL) {
- nbd_defer_free(item->key);
- }
- nbd_defer_free(item);
+ // unlink the item
+ find_preds(NULL, NULL, 0, sl, key, FORCE_UNLINK);
+
+ // 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) {
- for (int level = MAX_LEVEL; level >= 0; --level) {
+void sl_print (skiplist_t *sl, int verbose) {
+
+ if (verbose) {
+ for (int level = MAX_LEVELS - 1; level >= 0; --level) {
+ node_t *item = sl->head;
+ if (item->next[level] == DOES_NOT_EXIST)
+ continue;
+ printf("(%d) ", level);
+ int i = 0;
+ while (item) {
+ markable_t next = item->next[level];
+ printf("%s%p ", HAS_MARK(next) ? "*" : "", item);
+ item = STRIP_MARK(next);
+ if (i++ > 30) {
+ printf("...");
+ break;
+ }
+ }
+ printf("\n");
+ fflush(stdout);
+ }
node_t *item = sl->head;
- if (item->next[level] == NULL)
- continue;
- printf("(%d) ", level);
int i = 0;
while (item) {
- node_t *next = item->next[level];
- printf("%s%p ", IS_TAGGED(next) ? "*" : "", item);
- item = (node_t *)STRIP_TAG(next);
+ 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->num_levels);
+ } else {
+ printf("[HEAD]");
+ }
+ 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 ? "*" : "");
+ if (item == sl->head && item->next[level] == DOES_NOT_EXIST)
+ break;
+ }
+ printf("\n");
+ fflush(stdout);
+ item = STRIP_MARK(item->next[0]);
if (i++ > 30) {
- printf("...");
+ printf("...\n");
break;
}
}
- printf("\n");
- fflush(stdout);
}
+ printf("levels:%-2d count:%-6lld \n", sl->high_water, (uint64_t)sl_count(sl));
+}
- node_t *item = sl->head;
- int i = 0;
- while (item) {
- int is_marked = IS_TAGGED(item->next[0]);
- printf("%s%p:%p ", is_marked ? "*" : "", item, item->key);
- if (item != sl->head) {
- printf("[%d]", item->top_level);
- } else {
- printf("[HEAD]");
- }
- 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]);
- if (i++ > 30) {
- printf("...\n");
- break;
- }
+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, 1, sl, key, DONT_UNLINK);
+ } else {
+ iter->next = GET_NODE(sl->head->next[0]);
+ }
+ return iter;
+}
+
+map_val_t sl_iter_next (sl_iter_t *iter, map_key_t *key_ptr) {
+ assert(iter);
+ node_t *item = iter->next;
+ while (item != NULL && HAS_MARK(item->next[0])) {
+ item = STRIP_MARK(item->next[0]);
}
+ if (item == NULL) {
+ iter->next = NULL;
+ return DOES_NOT_EXIST;
+ }
+ iter->next = STRIP_MARK(item->next[0]);
+ if (key_ptr != NULL) {
+ *key_ptr = item->key;
+ }
+ return item->val;
+}
+
+void sl_iter_free (sl_iter_t *iter) {
+ nbd_free(iter);
}