#include "tls.h"
// Setting MAX_LEVEL to 0 essentially makes this data structure the Harris-Michael lock-free list
-// in list.c
+// (in list.c).
#define MAX_LEVEL 31
typedef struct node {
nstring_t *key;
- uint64_t value;
+ uint64_t val;
int top_level;
struct node *next[];
} node_t;
static int random_level (void) {
unsigned r = nbd_rand();
- if (r&1)
+ if (r & 1)
return 0;
- int n = __builtin_ctz(r)-1;
#if MAX_LEVEL < 31
- if (n > MAX_LEVEL)
- return MAX_LEVEL;
+ r |= 1 << (MAX_LEVEL+1);
#endif
+ int n = __builtin_ctz(r)-1;
assert(n <= MAX_LEVEL);
return n;
}
-node_t *node_alloc (int level, const void *key_data, uint32_t key_len, uint64_t value) {
+node_t *node_alloc (int level, const void *key_data, uint32_t key_len, uint64_t val) {
assert(level >= 0 && level <= MAX_LEVEL);
size_t sz = sizeof(node_t) + (level + 1) * sizeof(node_t *);
node_t *item = (node_t *)nbd_malloc(sz);
memset(item, 0, sz);
- item->key = ns_alloc(key_data, key_len);
- item->value = value;
+ // If <key_len> is -1 it indicates <key_data> is an integer and not a pointer
+ item->key = (key_len == (unsigned)-1)
+ ? (void *)TAG_VALUE(key_data)
+ : ns_alloc(key_data, key_len);
+ item->val = val;
item->top_level = level;
return item;
}
+static void node_free (node_t *item) {
+ if (!IS_TAGGED(item->key)) {
+ nbd_free(item->key);
+ }
+ nbd_free(item);
+}
+
+static void node_defer_free (node_t *item) {
+ if (!IS_TAGGED(item->key)) {
+ nbd_defer_free(item->key);
+ }
+ nbd_defer_free(item);
+}
+
skiplist_t *sl_alloc (void) {
skiplist_t *sl = (skiplist_t *)nbd_malloc(sizeof(skiplist_t));
sl->head = node_alloc(MAX_LEVEL, " ", 0, 0);
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.
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", item->key, 0);
// A tag means an item is logically removed but not physically unlinked yet.
while (EXPECT_FALSE(IS_TAGGED(next))) {
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) { nbd_defer_free(other); }
+ 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;
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 = 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;
}
item = next;
}
- // The comparison is unsigned for the case when n is -1.
+ // The cast to unsigned is for the case when n is -1.
if ((unsigned)level <= (unsigned)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;
}
}
- 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 sl %p", key, 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 <item> matching the <key> return its value.
- return item != NULL ? item->value : 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 <key> if it doesn't already exist in <sl>
-uint64_t sl_add (skiplist_t *sl, const void *key_data, uint32_t key_len, uint64_t value) {
- TRACE("s3", "sl_add: inserting key %p value %p", key, value);
+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 *item = NULL;
+ node_t *new_item = NULL;
int n = random_level();
do {
- node_t *next = find_preds(preds, nexts, n, sl, key_data, key_len, TRUE);
+ node_t *old_item = find_preds(preds, nexts, n, sl, key_data, key_len, TRUE);
+ if (old_item == NULL) {
- // If a node matching <key> already exists in <sl>, return its value.
- if (next != NULL) {
- TRACE("s3", "sl_add: there is already an item %p (value %p) with the same key", nexts[0], nexts[0]->value);
- if (EXPECT_FALSE(item != NULL)) { nbd_free(item); }
- return nexts[0]->value;
- }
+ // 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 <item> into the bottom level.
- if (EXPECT_TRUE(item == NULL)) { item = node_alloc(n, key_data, key_len, value); }
- node_t *pred = preds[0];
- item->next[0] = next = nexts[0];
- TRACE("s3", "sl_add: attempting to insert item between %p and %p", pred, next);
- for (int level = 1; level <= item->top_level; ++level) {
- item->next[level] = nexts[level];
- }
- node_t *other = SYNC_CAS(&pred->next[0], next, item);
- if (other == next) {
- TRACE("s3", "sl_add: successfully inserted item %p at level 0", item, 0);
- break; // success
+ ASSERT(expectation == EXPECT_DOES_NOT_EXIST || expectation == EXPECT_WHATEVER);
+
+ // First insert <new_item> 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);
+ // 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 <item> into <sl> from the bottom level up.
- for (int level = 1; level <= item->top_level; ++level) {
+ // 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_add: attempting to insert item between %p and %p", pred, next);
- node_t *other = SYNC_CAS(&pred->next[level], next, item);
+ 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", 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);
- find_preds(preds, nexts, item->top_level, sl, key_data, key_len, TRUE);
+ 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];
- // Update <item>'s next pointer
+ // 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 *)item)->next[level];
+ node_t *old_next = ((volatile node_t *)new_item)->next[level];
if (IS_TAGGED(old_next))
- return value;
+ return new_val;
- // Use a CAS so we to not inadvertantly stomp on a mark another thread placed on the item.
- if (old_next == next || SYNC_CAS(&item->next[level], old_next, next) == old_next)
+ // 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 value;
+ 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 sl %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) {
- TRACE("s3", "sl_remove: remove failed, an item with a matching key does not exist in the sl", 0, 0);
+ TRACE("s3", "sl_remove: remove failed, an item with a matching key does not exist in the skiplist", 0, 0);
return DOES_NOT_EXIST;
}
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 value = item->value;
+ // 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 <item> from the top down.
+ // Unlink <item> from <sl>. 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);
- // By marking the item earlier, we logically removed it. It is safe to leave the item partially
- // unlinked. Another thread will finish physically removing it from <sl>.
- return value;
+ 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.
- nbd_defer_free(item);
- return value;
+ TRACE("s1", "sl_remove: successfully unlinked item %p from the skiplist", item, 0);
+ node_defer_free(item);
+ return val;
}
void sl_print (skiplist_t *sl) {
node_t *item = sl->head;
while (item) {
int is_marked = IS_TAGGED(item->next[0]);
- printf("%s%p:%s ", is_marked ? "*" : "", item, (char *)ns_data(item->key));
+
+ if (IS_TAGGED(item->key)) {
+ printf("%s%p:%llx ", is_marked ? "*" : "", item, STRIP_TAG(item->key));
+ } else {
+ printf("%s%p:%s ", is_marked ? "*" : "", item, (char *)item->key->data);
+ }
if (item != sl->head) {
printf("[%d]", item->top_level);
} else {
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