+++ /dev/null
-/*
- * Written by Josh Dybnis and released to the public domain, as explained at
- * http://creativecommons.org/licenses/publicdomain
- *
- * Implementation of the lock-free skiplist data-structure created by Maurice Herlihy, Yossi Lev,
- * and Nir Shavit. See Herlihy's and Shivit's book "The Art of Multiprocessor Programming".
- * http://www.amazon.com/Art-Multiprocessor-Programming-Maurice-Herlihy/dp/0123705916/
- *
- * 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 probably 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 <string.h>
-
-#include "common.h"
-#include "runtime.h"
-#include "struct.h"
-#include "nstring.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).
-#define MAX_LEVEL 31
-
-typedef struct node {
- nstring_t *key;
- uint64_t val;
- int top_level;
- struct node *next[];
-} node_t;
-
-struct sl {
- node_t *head;
-};
-
-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);
- return n;
-}
-
-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);
- // 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);
- memset(sl->head->next, 0, (MAX_LEVEL+1) * sizeof(skiplist_t *));
- return sl;
-}
-
-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("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.
- start_level = 2;
- while (pred->next[start_level+1] != NULL) {
- start_level += start_level - 1;
- if (EXPECT_FALSE(start_level >= MAX_LEVEL)) {
- start_level = MAX_LEVEL;
- break;
- }
- }
- 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);
- 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
- }
- while (item != NULL) {
- node_t *next = item->next[level];
-
- // A tag means an item is logically removed but not physically unlinked yet.
- while (EXPECT_FALSE(IS_TAGGED(next))) {
-
- // Skip over logically removed items.
- if (!help_remove) {
- 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: 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); }
- } 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);
- 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;
- next = item->next[level];
- }
- }
-
- if (EXPECT_FALSE(item == NULL))
- break;
-
- 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;
- }
-
- pred = item;
- item = next;
- }
-
- // The cast to unsigned is for the case when n is -1.
- if ((unsigned)level <= (unsigned)n) {
- if (preds != NULL) {
- preds[level] = pred;
- }
- if (succs != NULL) {
- succs[level] = item;
- }
- }
- }
-
- // 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);
- 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);
-
- // If we found an <item> matching the <key> return its value.
- 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;
-}
-
-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 *new_item = NULL;
- int n = random_level();
- do {
- node_t *old_item = find_preds(preds, nexts, n, sl, key_data, key_len, TRUE);
- if (old_item == NULL) {
-
- // 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
- }
-
- 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;
- }
-
- // 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);
-
- // 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_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_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);
- find_preds(preds, nexts, new_item->top_level, sl, key_data, key_len, TRUE);
- pred = preds[level];
- next = nexts[level];
-
- // 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 *)new_item)->next[level];
- if (IS_TAGGED(old_next))
- return new_val;
-
- // 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 new_val;
-}
-
-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);
- 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 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;
- }
- 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);
- }
-
- // 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 <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));
- 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;
- }
- --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);
- return val;
-}
-
-void sl_print (skiplist_t *sl) {
- for (int level = MAX_LEVEL; level >= 0; --level) {
- node_t *item = sl->head;
- if (item->next[level] == NULL)
- continue;
- printf("(%d) ", level);
- while (item) {
- node_t *next = item->next[level];
- printf("%s%p ", IS_TAGGED(next) ? "*" : "", item);
- item = (node_t *)STRIP_TAG(next);
- }
- printf("\n");
- fflush(stdout);
- }
-
- printf("\n");
- node_t *item = sl->head;
- 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);
- }
- if (item != sl->head) {
- printf("[%d]", item->top_level);
- } else {
- printf("[*]");
- }
- for (int level = 1; level <= item->top_level; ++level) {
- node_t *next = (node_t *)STRIP_TAG(item->next[level]);
- is_marked = IS_TAGGED(item->next[0]);
- printf(" %p%s", next, is_marked ? "*" : "");
- if (item == sl->head && item->next[level] == NULL)
- break;
- }
- printf("\n");
- fflush(stdout);
- item = (node_t *)STRIP_TAG(item->next[0]);
- }
-}
projects / nbds / blobdiff