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;
int i = 0;
while (item) {
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("...\n");
break;
}
}
}
printf("levels:%-2d count:%-6lld \n", sl->high_water, (uint64_t)sl_count(sl));
}
int l_find(node_t **pred_ptr, node_t **item_ptr, node_t *head, key_t key) {
node_t *pred = head;
node_t *item = head->next;
/* pred和next会被使用，所以进行标记 */
hazard_t *hp1 = haz_get(0);
hazard_t *hp2 = haz_get(1);
while (item) {
node_t *sitem = STRIP_MARK(item);
haz_set_ptr(hp1, STRIP_MARK(pred));
haz_set_ptr(hp2, STRIP_MARK(item));
/*
如果已被标记，那么紧接着item可能被移除链表甚至释放，所以需要重头查找
*/
if (HAS_MARK(sitem->next)) {
trace("item->next %p %p marked, retry\n", item, sitem->next);
return l_find(pred_ptr, item_ptr, head, key);
}
int d = KEY_CMP(sitem->key, key);
if (d >= 0) {
trace("item %p match key %d, pred %p\n", item, key, pred);
*pred_ptr = pred;
*item_ptr = item;
return d == 0 ? TRUE : FALSE;
}
pred = item;
item = sitem->next;
}
trace("not found key %d\n", key);
*pred_ptr = pred;
*item_ptr = NULL;
return FALSE;
}
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;
}
size_t sl_count (skiplist_t *sl) {
size_t count = 0;
node_t *item = GET_NODE(sl->head->next[0]);
while (item) {
if (!HAS_MARK(item->next[0])) {
count++;
}
item = STRIP_MARK(item->next[0]);
}
return count;
}
int l_count(node_t *head) {
int cnt = 0;
node_t *item = STRIP_MARK(head->next);
while (item) {
if (!HAS_MARK(item->next)) {
cnt ++;
}
item = STRIP_MARK(item->next);
}
return cnt;
}
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;
}
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_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 <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.
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, 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);
}
// 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);
// 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;
}
static inline node_t * GET_NODE(markable_t x) { assert(!HAS_MARK(x)); return (node_t *)x; }
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_LEVELS];
node_t *nexts[MAX_LEVELS];
node_t *new_item = NULL;
int n = random_levels(sl);
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) {
map_val_t ret_val = update_item(old_item, expectation, new_val);
if (ret_val != DOES_NOT_EXIST)
return ret_val;
// 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);
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], (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);
// Find <new_item>'s new preds and nexts.
find_preds(preds, nexts, new_item->num_levels, sl, key, ASSIST_UNLINK);
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;
// 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;
}
}
} while (1);
}
// 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
}
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;
// Traverse the levels of <sl> from the top level to the bottom
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) {
next = item->next[level];
// 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 (unlink == DONT_UNLINK) {
// Skip over logically removed items.
item = STRIP_MARK(next);
if (EXPECT_FALSE(item == NULL))
break;
next = item->next[level];
} else {
// 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);
}
next = (item != NULL) ? item->next[level] : DOES_NOT_EXIST;
}
}
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);
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;
if (d == 0 && unlink != FORCE_UNLINK)
break;
pred = item;
item = GET_NODE(next);
}
TRACE("s3", "find_preds: found pred %p next %p", pred, item);
if (level < n) {
if (preds != NULL) {
preds[level] = pred;
}
if (succs != NULL) {
succs[level] = item;
}
}
}
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, pred);
return NULL;
}
