-/*
+/*
* 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
*
- * I've generalized the data structure to support update operations like set() and CAS() in addition to
+ * 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
+ * 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 "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 15
+
+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;
struct sl {
node_t *head;
const datatype_t *key_type;
- int high_water; // max level of any item in the list
+ int high_water; // max historic number of levels
};
// Marking the <next> field of a node logically removes it from the list
#define STRIP_MARK(x) ((node_t *)STRIP_TAG((x), 0x1))
#endif
-static int random_level (void) {
+static int random_levels (skiplist_t *sl) {
unsigned r = nbd_rand();
- int n = __builtin_ctz(r) / 2;
- if (n > MAX_LEVEL) { n = MAX_LEVEL; }
- return n;
+ 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, 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;
}
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->high_water = 1;
+ sl->head = node_alloc(MAX_LEVELS, 0, 0);
+ memset(sl->head->next, 0, MAX_LEVELS * sizeof(skiplist_t *));
return 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 <sl> 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) {
// 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);
} 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;
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;
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;
}
}
}
- // 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);
- 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) {
}
}
- 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;
}
}
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);
+ 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.
+ // 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("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(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) {
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
+ return sl_cas(sl, key, expectation, new_val); // tail call
}
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
+ 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);
- 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 <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) {
+ 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, 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);
if (sl->key_type != NULL) {
nbd_free((void *)new_key);
}
- nbd_free(new_item);
+ 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);
- 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 <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);
+ 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;
// 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]);
+ 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;
}
}
// 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
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 <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->top_level; level >= 0; --level) {
+ for (int level = item->num_levels - 1; level >= 0; --level) {
markable_t next;
old_next = item->next[level];
do {
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)
+ 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);
+ // 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
- sl_unlink(sl, key);
+ find_preds(NULL, NULL, 0, sl, key, FORCE_UNLINK);
// free the node
if (sl->key_type != NULL) {
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) {
+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] == 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);
+ 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);
- }
- 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->top_level);
- } else {
- printf("[HEAD]");
- }
- for (int level = 1; level <= item->top_level; ++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));
}
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]);
}
projects / nbds / blobdiff