X-Git-Url: https://pd.if.org/git/?p=nbds;a=blobdiff_plain;f=map%2Fskiplist.c;h=a0e9c60769a2ce5847471fbbaf9fc6894c8828db;hp=c0e869c25e584656c33cf452d50bcacd1797b8f6;hb=d26bac75802a324ed98c8d3d88cfb9eb87b3b35a;hpb=025017478bb385da88a6b185849c8bcffeb2e2aa diff --git a/map/skiplist.c b/map/skiplist.c index c0e869c..a0e9c60 100644 --- a/map/skiplist.c +++ b/map/skiplist.c @@ -9,26 +9,28 @@ * 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 #include #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[]; @@ -36,8 +38,16 @@ typedef struct node { struct sl { node_t *head; + const datatype_t *key_type; +}; + +static const map_impl_t sl_map_impl = { + (map_alloc_t)sl_alloc, (map_cas_t)sl_cas, (map_get_t)sl_lookup, (map_remove_t)sl_remove, + (map_count_t)sl_count, (map_print_t)sl_print, (map_free_t)sl_free }; +const map_impl_t *MAP_TYPE_SKIPLIST = &sl_map_impl; + static int random_level (void) { unsigned r = nbd_rand(); if (r & 1) @@ -50,48 +60,50 @@ static int random_level (void) { 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 is -1 it indicates 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; } -void ll_free (list_t *ll) { +void sl_free (skiplist_t *sl) { + node_t *item = sl->head->next[0]; + while (item) { + node_t *next = (node_t *)STRIP_TAG(item->next[0]); + nbd_free(item); + item = next; + } +} + +uint64_t sl_count (skiplist_t *sl) { + uint64_t count = 0; + node_t *item = sl->head->next[0]; + while (item) { + 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 @@ -115,7 +127,7 @@ static node_t *find_preds (node_t **preds, node_t **succs, int n, skiplist_t *sl 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]; @@ -144,12 +156,17 @@ static node_t *find_preds (node_t **preds, node_t **succs, int n, skiplist_t *sl 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; @@ -160,21 +177,17 @@ static node_t *find_preds (node_t **preds, node_t **succs, int n, skiplist_t *sl 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; } @@ -193,25 +206,25 @@ static node_t *find_preds (node_t **preds, node_t **succs, int n, skiplist_t *sl } } - // 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 matching the return its value. if (item != NULL) { @@ -226,8 +239,8 @@ uint64_t sl_lookup (skiplist_t *sl, const void *key_data, uint32_t key_len) { 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); @@ -236,7 +249,7 @@ uint64_t sl_cas (skiplist_t *sl, const void *key_data, uint32_t key_len, uint64_ 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. @@ -249,7 +262,8 @@ uint64_t sl_cas (skiplist_t *sl, const void *key_data, uint32_t key_len, uint64_ // First insert 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) { @@ -261,7 +275,10 @@ uint64_t sl_cas (skiplist_t *sl, const void *key_data, uint32_t key_len, uint64_ 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; } @@ -307,7 +324,7 @@ uint64_t sl_cas (skiplist_t *sl, const void *key_data, uint32_t key_len, uint64_ 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]; @@ -316,7 +333,7 @@ uint64_t sl_cas (skiplist_t *sl, const void *key_data, uint32_t key_len, uint64_ // 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) @@ -324,64 +341,86 @@ uint64_t sl_cas (skiplist_t *sl, const void *key_data, uint32_t key_len, uint64_ } 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 removed at each level of 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 at each level of 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); - } + } while (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 from . 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; // points past 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; } @@ -391,29 +430,28 @@ void sl_print (skiplist_t *sl) { 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]); @@ -425,5 +463,9 @@ void sl_print (skiplist_t *sl) { printf("\n"); fflush(stdout); item = (node_t *)STRIP_TAG(item->next[0]); + if (i++ > 30) { + printf("...\n"); + break; + } } }