map_key_t key;
map_val_t val;
int top_level;
- markable_t next[];
+ markable_t next[1];
} node_t;
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 <next> field of a node logically removes it from the list
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;
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 <sl> 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);
// 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);
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) {
}
}
- // 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;
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 <sl> 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);
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);
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 <new_item> 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 <key> 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 <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->top_level; ++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, 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 <new_item> into <sl> 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 <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);
+
+ // Find <new_item>'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 <new_item>'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 <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, 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 <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->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) {
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) {
+ 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; // <pred> points past <item> 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)
projects / nbds / blobdiff