+/*
+ * Written by Josh Dybnis and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ *
+ * lock-free skiplist
+ */
+#include <stdio.h>
+#include <string.h>
+
+#include "common.h"
+#include "struct.h"
+#include "mem.h"
+
+#define MAX_LEVEL 3
+
+typedef struct node {
+ uint64_t key;
+ uint64_t value;
+ int top_level;
+ struct node *next[];
+} node_t;
+
+typedef struct skiplist {
+ node_t *head;
+ node_t *last;
+ int top_level;
+} skiplist_t;
+
+static int random_level (int r) {
+ if (r&1)
+ return 0;
+ int n = __builtin_ctz(r);
+ if (n < MAX_LEVEL)
+ return n;
+ return MAX_LEVEL;
+}
+
+node_t *node_alloc (int top_level, uint64_t key, uint64_t value) {
+ assert(top_level >= 0 && top_level <= MAX_LEVEL);
+ size_t sz = sizeof(node_t) + (top_level + 1) * sizeof(node_t *);
+ node_t *item = (node_t *)nbd_malloc(sz);
+ memset(item, 0, sz);
+ item->key = key;
+ item->value = value;
+ item->top_level = top_level;
+ return item;
+}
+
+skiplist_t *skiplist_alloc (void) {
+ skiplist_t *skiplist = (skiplist_t *)nbd_malloc(sizeof(skiplist_t));
+ skiplist->head = node_alloc(MAX_LEVEL, 0, 0);
+ skiplist->last = node_alloc(MAX_LEVEL, (uint64_t)-1, 0);
+ for (int level = 0; level <= MAX_LEVEL; ++level) {
+ skiplist->head->next[level] = skiplist->last;
+ }
+ return skiplist;
+}
+
+static node_t *find_preds (node_t **preds, skiplist_t *skiplist, uint64_t key, int help_remove) {
+ node_t *pred = skiplist->head;
+ node_t *item = NULL;
+ TRACE("s3", "find_preds: searching for key %p in skiplist (head is %p)", key, pred);
+#ifndef NDEBUG
+ int count = 0;
+#endif
+
+ // Traverse the levels of the skiplist from the top level to the bottom
+ for (int level = MAX_LEVEL; level >= 0; --level) {
+ TRACE("s3", "find_preds: level %llu", level, 0);
+ item = pred->next[level];
+ if (IS_TAGGED(item)) {
+ TRACE("s3", "find_preds: pred %p is marked for removal (item %p); retry", pred, item);
+ return find_preds(preds, skiplist, key, help_remove); // retry
+ }
+ do {
+ node_t *next = item->next[level];
+ TRACE("s3", "find_preds: visiting item %p (next %p)", item, next);
+ TRACE("s3", "find_preds: key %p", item->key, 0);
+
+ // Marked items are logically removed, but not fully unlinked yet.
+ while (EXPECT_FALSE(IS_TAGGED(next))) {
+
+ // Skip over partially removed items.
+ if (!help_remove) {
+ item = (node_t *)STRIP_TAG(item->next);
+ next = item->next[level];
+ continue;
+ }
+
+ // Unlink partially removed items.
+ node_t *other;
+ if ((other = SYNC_CAS(&pred->next[level], item, STRIP_TAG(next))) == item) {
+ item = (node_t *)STRIP_TAG(next);
+ next = item->next[level];
+ TRACE("s3", "find_preds: unlinked item %p from pred %p", item, pred);
+ TRACE("s3", "find_preds: now item is %p next is %p", item, next);
+
+ // The thread that completes the unlink should free the memory.
+ if (level == 0) { nbd_defer_free(other); }
+ } else {
+ TRACE("s3", "find_preds: lost race to unlink item from pred %p; its link changed to %p",pred,other);
+ if (IS_TAGGED(other))
+ return find_preds(preds, skiplist, key, help_remove); // retry
+ item = other;
+ next = item->next[level];
+ }
+ }
+
+ // If we reached the key (or passed where it should be), we found a pred. Save it and continue down.
+ if (item->key >= key) {
+ TRACE("s3", "find_preds: found pred %p item %p", pred, item);
+ if (preds != NULL) {
+ preds[level] = pred;
+ }
+ break;
+ }
+
+ assert(count++ < 18);
+ pred = item;
+ item = next;
+
+ } while (1);
+ }
+ return item;
+}
+
+// Fast find that does not help unlink partially removed nodes and does not return the node's predecessors.
+uint64_t skiplist_lookup (skiplist_t *skiplist, uint64_t key) {
+ TRACE("s3", "skiplist_lookup: searching for key %p in skiplist %p", key, skiplist);
+ node_t *item = find_preds(NULL, skiplist, key, FALSE);
+
+ // If we found an <item> matching the <key> return its value.
+ return (item->key == key) ? item->value : DOES_NOT_EXIST;
+}
+
+// Insert the <key> if it doesn't already exist in the <skiplist>
+uint64_t skiplist_add_i (int n, skiplist_t *skiplist, uint64_t key, uint64_t value) {
+ TRACE("s3", "skiplist_add: inserting key %p value %p", key, value);
+ node_t *preds[MAX_LEVEL+1];
+ node_t *item = NULL;
+ do {
+ node_t *next = find_preds(preds, skiplist, key, TRUE);
+
+ // If a node matching <key> already exists in the skiplist, return its value.
+ if (next->key == key) {
+ TRACE("s3", "skiplist_add: there is already an item %p (value %p) with the same key", next, next->value);
+ if (EXPECT_FALSE(item != NULL)) { nbd_free(item); }
+ return next->value;
+ }
+
+ // First insert <item> into the bottom level.
+ if (EXPECT_TRUE(item == NULL)) { item = node_alloc(random_level(n), key, value); }
+ TRACE("s3", "skiplist_add: attempting to insert item between %p and %p", preds[0], next);
+ item->next[0] = next;
+ for (int level = 1; level <= item->top_level; ++level) {
+ item->next[level] = preds[level]->next[level];
+ }
+ node_t *other = SYNC_CAS(&preds[0]->next[0], next, item);
+ if (other == next) {
+ TRACE("s3", "skiplist_add: successfully inserted item %p at level 0", item, 0);
+ break; // success
+ }
+ TRACE("s3", "skiplist_add: failed to change pred's link: expected %p found %p", next, other);
+
+ } while (1);
+
+ // Insert <item> into the skiplist from the bottom level up.
+ for (int level = 1; level <= item->top_level; ++level) {
+ do {
+ node_t *pred = preds[level];
+ node_t *next = pred->next[level];
+ while (EXPECT_FALSE(IS_TAGGED(next) || next->key < key)) {
+ find_preds(preds, skiplist, key, TRUE);
+ pred = preds[level];
+ next = pred->next[level];
+ }
+
+ do {
+ // There in no need to continue linking in the item if another thread removed it.
+ node_t *old_next = ((volatile node_t *)item)->next[level];
+ if (IS_TAGGED(old_next))
+ return DOES_NOT_EXIST; // success
+
+ // Use a CAS so we to not inadvertantly remove a mark another thread placed on the item.
+ if (next == old_next || SYNC_CAS(&item->next[level], old_next, next) == old_next)
+ break;
+ } while (1);
+
+ TRACE("s3", "skiplist_add: attempting to insert item between %p and %p", pred, next);
+ node_t *other = SYNC_CAS(&pred->next[level], next, item);
+ if (other == next) {
+ TRACE("s3", "skiplist_add: successfully inserted item %p at level %llu", item, level);
+ break; // success
+ }
+ TRACE("s3", "skiplist_add: failed to change pred's link: expected %p found %p", next, other);
+
+ } while (1);
+ }
+ return value;
+}
+
+uint64_t skiplist_remove (skiplist_t *skiplist, uint64_t key) {
+ TRACE("s3", "skiplist_remove: removing item with key %p from skiplist %p", key, skiplist);
+ node_t *preds[MAX_LEVEL+1];
+ node_t *item = find_preds(preds, skiplist, key, TRUE);
+ if (item->key != key) {
+ TRACE("s3", "skiplist_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 the skiplist 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", "skiplist_remove: %p is already marked for removal by another thread", item, 0);
+ if (level == 0)
+ return DOES_NOT_EXIST;
+ continue;
+ }
+ node_t *next = SYNC_FETCH_AND_OR(&item->next[level], TAG);
+ if (EXPECT_FALSE(IS_TAGGED(next))) {
+ TRACE("s3", "skiplist_remove: lost race -- %p is already marked for removal by another thread", item, 0);
+ if (level == 0)
+ return DOES_NOT_EXIST;
+ continue;
+ }
+ }
+
+ uint64_t value = item->value;
+
+ // Unlink <item> from the top down.
+ int level = item->top_level;
+ while (level >= 0) {
+ node_t *pred = preds[level];
+ node_t *next = item->next[level];
+ TRACE("s3", "skiplist_remove: link item's 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("s3", "skiplist_remove: unlink failed; pred's link changed from %p to %p", item, other);
+ // By marking the item earlier, we logically removed it. It is safe to leave the item partially
+ // unlinked. Another thread will finish physically removing it from the skiplist.
+ return value;
+ }
+ --level;
+ }
+
+ // The thread that completes the unlink should free the memory.
+ nbd_defer_free(item);
+ return value;
+}
+
+void skiplist_print (skiplist_t *skiplist) {
+ for (int level = MAX_LEVEL; level >= 0; --level) {
+ printf("(%d) ", level);
+ node_t *item = skiplist->head;
+ while (item) {
+ node_t *next = item->next[level];
+ printf("%s%p:0x%llx ", IS_TAGGED(next) ? "*" : "", item, item->key);
+ item = (node_t *)STRIP_TAG(next);
+ }
+ printf("\n");
+ fflush(stdout);
+ }
+
+ node_t *item = skiplist->head;
+ while (item) {
+ assert(item->top_level <= MAX_LEVEL);
+ int is_marked = IS_TAGGED(item->next[0]);
+ printf("%s%p:0x%llx (%d", is_marked ? "*" : "", item, item->key, item->top_level);
+ for (int i = 1; i <= item->top_level; ++i) {
+ node_t *next = (node_t *)STRIP_TAG(item->next[i]);
+ printf(" %p:0x%llx", item->next[i], next ? next->key : 0);
+ }
+ printf(")\n");
+ fflush(stdout);
+ item = (node_t *)STRIP_TAG(item->next[0]);
+ }
+}
+
+#ifdef MAKE_skiplist_test
+#include <errno.h>
+#include <pthread.h>
+#include <sys/time.h>
+
+#include "runtime.h"
+
+#define NUM_ITERATIONS 10000000
+
+static volatile int wait_;
+static long num_threads_;
+static skiplist_t *skiplist_;
+
+void *worker (void *arg) {
+ int id = (int)(size_t)arg;
+
+ unsigned int rand_seed = id+1;//rdtsc_l();
+
+ // Wait for all the worker threads to be ready.
+ SYNC_ADD(&wait_, -1);
+ do {} while (wait_);
+
+ for (int i = 0; i < NUM_ITERATIONS/num_threads_; ++i) {
+ int n = rand_r(&rand_seed);
+ int key = (n & 0xF) + 1;
+ if (n & (1 << 8)) {
+ skiplist_add_i(n, skiplist_, key, 1);
+ } else {
+ skiplist_remove(skiplist_, key);
+ }
+
+ rcu_update();
+ }
+
+ return NULL;
+}
+
+int main (int argc, char **argv) {
+ nbd_init();
+ lwt_set_trace_level("s3");
+
+ char* program_name = argv[0];
+ pthread_t thread[MAX_NUM_THREADS];
+
+ if (argc > 2) {
+ fprintf(stderr, "Usage: %s num_threads\n", program_name);
+ return -1;
+ }
+
+ num_threads_ = 2;
+ if (argc == 2)
+ {
+ errno = 0;
+ num_threads_ = strtol(argv[1], NULL, 10);
+ if (errno) {
+ fprintf(stderr, "%s: Invalid argument for number of threads\n", program_name);
+ return -1;
+ }
+ if (num_threads_ <= 0) {
+ fprintf(stderr, "%s: Number of threads must be at least 1\n", program_name);
+ return -1;
+ }
+ if (num_threads_ > MAX_NUM_THREADS) {
+ fprintf(stderr, "%s: Number of threads cannot be more than %d\n", program_name, MAX_NUM_THREADS);
+ return -1;
+ }
+ }
+
+ skiplist_ = skiplist_alloc();
+
+ struct timeval tv1, tv2;
+ gettimeofday(&tv1, NULL);
+
+ wait_ = num_threads_;
+
+ for (int i = 0; i < num_threads_; ++i) {
+ int rc = nbd_thread_create(thread + i, i, worker, (void*)(size_t)i);
+ if (rc != 0) { perror("pthread_create"); return rc; }
+ }
+
+ for (int i = 0; i < num_threads_; ++i) {
+ pthread_join(thread[i], NULL);
+ }
+
+ gettimeofday(&tv2, NULL);
+ int ms = (int)(1000000*(tv2.tv_sec - tv1.tv_sec) + tv2.tv_usec - tv1.tv_usec) / 1000;
+ printf("Th:%ld Time:%dms\n", num_threads_, ms);
+ skiplist_print(skiplist_);
+ lwt_dump("lwt.out");
+
+ return 0;
+}
+#endif//skiplist_test
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