X-Git-Url: https://pd.if.org/git/?p=nbds;a=blobdiff_plain;f=struct%2Fskiplist.c;fp=struct%2Fskiplist.c;h=7686b76f37db9d643601bdcf660d48d21b3e7c33;hp=0000000000000000000000000000000000000000;hb=7899dce86342c50be9a52d148fa27375bdb5d218;hpb=269f52d6f8cd031692c83afaa05c389115d05bd0;ds=sidebyside

diff --git a/struct/skiplist.c b/struct/skiplist.c
new file mode 100644
index 0000000..7686b76
--- /dev/null
+++ b/struct/skiplist.c
@@ -0,0 +1,373 @@
+/* 
+ * 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