* 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 "common.h"
#include "runtime.h"
#include "mlocal.h"
-#include "nstring.h"
+#include "skiplist.h"
#include "mem.h"
-#include "tls.h"
-// Setting MAX_LEVEL to 0 essentially makes this data structure the Harris-Michael lock-free list
-// (in list.c).
+// Setting MAX_LEVEL to 0 essentially makes this data structure the Harris-Michael lock-free list (in list.c).
#define MAX_LEVEL 31
typedef struct node {
- nstring_t *key;
+ void *key;
uint64_t val;
int top_level;
struct node *next[];
struct sl {
node_t *head;
+ const datatype_t *key_type;
};
static const map_impl_t sl_map_impl = {
return n;
}
-static node_t *node_alloc (int level, const void *key_data, uint32_t key_len, uint64_t val) {
+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 *);
node_t *item = (node_t *)nbd_malloc(sz);
memset(item, 0, sz);
- // 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->key = key;
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_alloc (const datatype_t *key_type) {
skiplist_t *sl = (skiplist_t *)nbd_malloc(sizeof(skiplist_t));
- sl->head = node_alloc(MAX_LEVEL, " ", 0, 0);
+ sl->key_type = key_type;
+ sl->head = node_alloc(MAX_LEVEL, NULL, 0);
memset(sl->head->next, 0, (MAX_LEVEL+1) * sizeof(skiplist_t *));
return sl;
}
uint64_t count = 0;
node_t *item = sl->head->next[0];
while (item) {
- count++;
+ if (!IS_TAGGED(item->next[0])) {
+ count++;
+ }
item = (node_t *)STRIP_TAG(item->next[0]);
}
return count;
}
-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) {
+static node_t *find_preds (node_t **preds, node_t **succs, int n, skiplist_t *sl, void *key, int help_remove) {
node_t *pred = sl->head;
node_t *item = NULL;
- TRACE("s2", "find_preds: searching for key %p in sl (head is %p)", key_data, pred);
+ 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
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_data, key_len, help_remove); // retry
+ return find_preds(preds, succs, n, sl, key, help_remove); // retry
}
while (item != NULL) {
node_t *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) { node_defer_free(other); }
+ if (level == 0) {
+ if (sl->key_type != NULL) {
+ nbd_defer_free((void*)other->key);
+ }
+ nbd_defer_free(other);
+ }
} else {
- 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);
+ 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_data, key_len, help_remove); // retry
+ return find_preds(preds, succs, n, sl, key, help_remove); // retry
item = other;
if (EXPECT_FALSE(item == NULL))
break;
if (EXPECT_FALSE(item == NULL))
break;
- TRACE("s3", "find_preds: visiting item %p (next is %p)", item, next);
+ 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);
- // 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);
+ if (EXPECT_TRUE(sl->key_type == NULL)) {
+ d = (uint64_t)item->key - (uint64_t)key;
} 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; }
+ d = sl->key_type->cmp(item->key, key);
}
if (d >= 0) {
- TRACE("s2", "find_preds: found pred %p item %p", pred, item);
+ TRACE("s4", "find_preds: found pred %p item %p", pred, item);
break;
}
}
}
- // fill in empty levels
- if (n == -1 && item != NULL) {
- for (int level = start_level + 1; level <= item->top_level; ++level) {
- preds[level] = sl->head;
- }
- }
-
+ // 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;
}
- TRACE("s2", "find_preds: found proper place for key %p in skiplist, pred is %p. returning null", key_data, pred);
+ TRACE("s2", "find_preds: found proper place for key %p in skiplist, pred is %p. returning null", key, 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("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);
+uint64_t sl_lookup (skiplist_t *sl, void *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);
// If we found an <item> matching the <key> return its value.
if (item != NULL) {
return DOES_NOT_EXIST;
}
-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);
+uint64_t sl_cas (skiplist_t *sl, void *key, uint64_t expectation, uint64_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 *new_item = NULL;
int n = random_level();
do {
- node_t *old_item = find_preds(preds, nexts, n, sl, key_data, key_len, TRUE);
+ 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.
// 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);
+ void *new_key = (sl->key_type == NULL) ? key : sl->key_type->clone(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) {
break; // success
}
TRACE("s3", "sl_cas: failed to change pred's link: expected %p found %p", next, other);
- node_free(new_item);
+ if (sl->key_type != NULL) {
+ nbd_free(new_key);
+ }
+ nbd_free(new_item);
continue;
}
break; // success
}
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);
+ find_preds(preds, nexts, new_item->top_level, sl, key, TRUE);
pred = preds[level];
next = nexts[level];
// 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 new_val;
+ 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)
} while (1);
} while (1);
}
- return new_val;
+ return DOES_NOT_EXIST; // success
}
-uint64_t sl_remove (skiplist_t *sl, const void *key_data, uint32_t key_len) {
- TRACE("s1", "sl_remove: removing item with key %p from skiplist %p", key_data, sl);
+uint64_t sl_remove (skiplist_t *sl, void *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_data, key_len, TRUE);
+ node_t *item = find_preds(preds, NULL, -1, sl, key, TRUE);
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> removed at each level of <sl> from the top down. This must be atomic. If multiple threads
- // try to 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) {
- if (EXPECT_FALSE(IS_TAGGED(item->next[level]))) {
- TRACE("s3", "sl_remove: %p is already marked for removal by another thread", item, 0);
- if (level == 0)
- return DOES_NOT_EXIST;
- continue;
- }
+ // Mark and unlink <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];
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;
+ TRACE("s2", "sl_remove: %p is already marked for removal by another thread at level %llu", item, level);
+ break;
}
- } while (!IS_TAGGED(old_next) || next != old_next);
- }
-
- // 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);
+ } while (next != old_next);
- // 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("s2", "sl_remove: unlink the item by linking its pred %p to it's successor %p", pred, STRIP_TAG(next));
+ 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);
- return val;
+ // 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->key_type == NULL)) {
+ d = (uint64_t)item->key - (uint64_t)other->key;
+ } else {
+ d = sl->key_type->cmp(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.
+ }
}
- --level;
}
- // The thread that completes the unlink should free the memory.
- TRACE("s1", "sl_remove: successfully unlinked item %p from the skiplist", item, 0);
- node_defer_free(item);
+ 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 operation occurs first logically, the update or the remove.
+ uint64_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->key_type != NULL) {
+ nbd_defer_free(item->key);
+ }
+ nbd_defer_free(item);
+ }
return val;
}
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);
+ if (i++ > 30) {
+ printf("...");
+ break;
+ }
}
printf("\n");
fflush(stdout);
}
-
- printf("\n");
node_t *item = sl->head;
+ int i = 0;
while (item) {
int is_marked = IS_TAGGED(item->next[0]);
-
- 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);
- }
+ printf("%s%p:%p ", is_marked ? "*" : "", item, item->key);
if (item != sl->head) {
printf("[%d]", item->top_level);
} else {
- printf("[*]");
+ printf("[HEAD]");
}
for (int level = 1; level <= item->top_level; ++level) {
node_t *next = (node_t *)STRIP_TAG(item->next[level]);
printf("\n");
fflush(stdout);
item = (node_t *)STRIP_TAG(item->next[0]);
+ if (i++ > 30) {
+ printf("...\n");
+ break;
+ }
}
}
projects / nbds / blobdiff