* Written by Josh Dybnis and released to the public domain, as explained at
* http://creativecommons.org/licenses/publicdomain
*
- * C implementation of the lock-free skiplist data-structure created by Maurice Herlihy,
- * Yossi Lev, and Nir Shavit. See "The Art of Multiprocessor Programming"
+ * 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"
+ * See also Kir Fraser's dissertation "Practical Lock Freedom".
* www.cl.cam.ac.uk/techreports/UCAM-CL-TR-579.pdf
*
- * This code depends on certain stores and loads being ordered. Be careful on non-x86 platforms
- * with weaker memory-models. This code probably won't work without adding some memory barriers.
+ * 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 "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 {
struct node *next[];
} node_t;
-typedef struct skiplist {
+struct sl {
node_t *head;
- node_t *last;
- int top_level;
-} skiplist_t;
+};
-static int random_level (int r) {
+static int random_level (void) {
+ unsigned r = nbd_rand();
if (r&1)
return 0;
- int n = __builtin_ctz(r);
- if (n < MAX_LEVEL)
- return n;
- return MAX_LEVEL;
+ int n = __builtin_ctz(r)-1;
+#if MAX_LEVEL < 31
+ if (n > MAX_LEVEL)
+ return MAX_LEVEL;
+#endif
+ assert(n <= MAX_LEVEL);
+ return n;
}
-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 *node_alloc (int level, uint64_t key, uint64_t value) {
+ 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);
item->key = key;
item->value = value;
- item->top_level = top_level;
+ item->top_level = level;
return item;
}
skiplist_t *sl_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;
+ 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, skiplist_t *skiplist, uint64_t key, int help_remove) {
- node_t *pred = skiplist->head;
+static node_t *find_preds (node_t *preds[MAX_LEVEL+1], int n, skiplist_t *sl, uint64_t key, int help_remove) {
+ node_t *pred = sl->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;
+ TRACE("s3", "find_preds: searching for key %p in sl (head is %p)", key, pred);
+
+ 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 the skiplist from the top level to the bottom
- for (int level = MAX_LEVEL; level >= 0; --level) {
+ // 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 (IS_TAGGED(item)) {
+ if (EXPECT_FALSE(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
+ return find_preds(preds, n, sl, key, help_remove); // retry
}
- do {
+ while (item != NULL) {
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);
// Skip over partially removed items.
if (!help_remove) {
item = (node_t *)STRIP_TAG(item->next);
+ if (EXPECT_FALSE(item == NULL))
+ break;
next = item->next[level];
continue;
}
node_t *other;
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: 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);
+ TRACE("s3", "find_preds: lost race to unlink from pred %p; its link changed to %p", pred, other);
if (IS_TAGGED(other))
- return find_preds(preds, skiplist, key, help_remove); // retry
+ return find_preds(preds, n, sl, key, help_remove); // retry
item = other;
+ if (EXPECT_FALSE(item == NULL))
+ break;
next = item->next[level];
}
}
+ if (EXPECT_FALSE(item == NULL))
+ break;
+
// 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);
+ }
+ if (preds != NULL) {
+ preds[level] = pred;
+ }
+ }
+ if (n == -1 && item != NULL) {
+ assert(preds != NULL);
+ for (int level = start_level + 1; level <= item->top_level; ++level) {
+ preds[level] = sl->head;
+ }
}
return item;
}
// Fast find that does not help unlink partially removed nodes and does not return the node's predecessors.
-uint64_t sl_lookup (skiplist_t *skiplist, uint64_t key) {
- TRACE("s3", "sl_lookup: searching for key %p in skiplist %p", key, skiplist);
- node_t *item = find_preds(NULL, skiplist, key, FALSE);
+uint64_t sl_lookup (skiplist_t *sl, uint64_t key) {
+ TRACE("s3", "sl_lookup: searching for key %p in sl %p", key, sl);
+ node_t *item = find_preds(NULL, 0, sl, key, FALSE);
// If we found an <item> matching the <key> return its value.
- return (item->key == key) ? item->value : DOES_NOT_EXIST;
+ return (item && item->key == key) ? item->value : DOES_NOT_EXIST;
}
-// Insert the <key> if it doesn't already exist in the <skiplist>
-uint64_t sl_add_r (int r, skiplist_t *skiplist, uint64_t key, uint64_t value) {
+// Insert the <key> if it doesn't already exist in <sl>
+uint64_t sl_add (skiplist_t *sl, uint64_t key, uint64_t value) {
TRACE("s3", "sl_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);
+ int n = random_level();
+ node_t *next = find_preds(preds, n, sl, key, TRUE);
- // If a node matching <key> already exists in the skiplist, return its value.
- if (next->key == key) {
+ // If a node matching <key> already exists in <sl>, return its value.
+ if (next != NULL && next->key == key) {
TRACE("s3", "sl_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(r), key, value); }
+ if (EXPECT_TRUE(item == NULL)) { item = node_alloc(n, key, value); }
TRACE("s3", "sl_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 *pred = preds[level];
+ item->next[level] = pred->next[level];
}
- node_t *other = SYNC_CAS(&preds[0]->next[0], next, item);
+ node_t *pred = preds[0];
+ node_t *other = SYNC_CAS(&pred->next[0], next, item);
if (other == next) {
TRACE("s3", "sl_add: successfully inserted item %p at level 0", item, 0);
break; // success
} while (1);
- // Insert <item> into the skiplist from the bottom level up.
+ // Insert <item> into <sl> 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);
+ node_t *pred;
+ node_t *next;
+ do {
pred = preds[level];
next = pred->next[level];
- }
+ if (next == NULL) // item goes at the end of the list
+ break;
+ if (!IS_TAGGED(next) && next->key > key) // pred's link changed
+ break;
+ find_preds(preds, item->top_level, sl, key, TRUE);
+ } while (1);
do {
// There in no need to continue linking in the item if another thread removed it.
return value;
}
-uint64_t sl_remove (skiplist_t *skiplist, uint64_t key) {
- TRACE("s3", "sl_remove: removing item with key %p from skiplist %p", key, skiplist);
+uint64_t sl_remove (skiplist_t *sl, uint64_t key) {
+ TRACE("s3", "sl_remove: removing item with key %p from sl %p", key, sl);
node_t *preds[MAX_LEVEL+1];
- node_t *item = find_preds(preds, skiplist, key, TRUE);
- if (item->key != key) {
- TRACE("s3", "sl_remove: remove failed, an item with a matching key does not exist in the skiplist", 0, 0);
+ node_t *item = find_preds(preds, -1, sl, key, TRUE);
+ if (item == NULL || item->key != key) {
+ TRACE("s3", "sl_remove: remove failed, an item with a matching key does not exist in the sl", 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
+ // 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 ((other = SYNC_CAS(&pred->next[level], item, STRIP_TAG(next))) != item) {
TRACE("s3", "sl_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.
+ // unlinked. Another thread will finish physically removing it from <sl>.
return value;
}
--level;
return value;
}
-void sl_print (skiplist_t *skiplist) {
+void sl_print (skiplist_t *sl) {
for (int level = MAX_LEVEL; level >= 0; --level) {
- node_t *item = skiplist->head;
- if (item->next[level] == skiplist->last)
+ 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:0x%llx ", IS_TAGGED(next) ? "*" : "", item, item->key);
+ printf("%s%p ", IS_TAGGED(next) ? "*" : "", item);
item = (node_t *)STRIP_TAG(next);
}
printf("\n");
}
printf("\n");
- node_t *item = skiplist->head;
+ node_t *item = sl->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);
+ printf("%s%p:0x%llx ", is_marked ? "*" : "", item, item->key);
+ 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]);
- printf(" %p:0x%llx", item->next[level], next ? next->key : 0);
- if (item == skiplist->head && item->next[level] == skiplist->last)
- break;
- if (item == skiplist->last && item->next[level] == NULL)
+ is_marked = IS_TAGGED(item->next[0]);
+ printf(" %p%s", next, is_marked ? "*" : "");
+ if (item == sl->head && item->next[level] == NULL)
break;
}
- printf(")\n");
+ 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 *sl_;
-
-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)) {
- sl_add_r(n, sl_, key, 1);
- } else {
- sl_remove(sl_, 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;
- }
- }
-
- sl_ = sl_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);
- sl_print(sl_);
- lwt_dump("lwt.out");
-
- return 0;
-}
-#endif//skiplist_test
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