-/*
+/*
* Written by Josh Dybnis and released to the public domain, as explained at
* http://creativecommons.org/licenses/publicdomain
*
* See also Kir Fraser's dissertation "Practical Lock Freedom".
* www.cl.cam.ac.uk/techreports/UCAM-CL-TR-579.pdf
*
- * I've generalized the data structure to support update operations like set() and CAS() in addition to
+ * 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
+ * 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 "rcu.h"
// Setting MAX_LEVELS to 1 essentially makes this data structure the Harris-Michael lock-free list (see list.c).
-#define MAX_LEVELS 32
+#define MAX_LEVELS 15
enum unlink {
FORCE_UNLINK,
struct sl {
node_t *head;
const datatype_t *key_type;
- int high_water; // max level of any item in the list
+ int high_water; // max historic number of levels
};
// Marking the <next> field of a node logically removes it from the list
#define STRIP_MARK(x) ((node_t *)STRIP_TAG((x), 0x1))
#endif
-static int random_levels (void) {
+static int random_levels (skiplist_t *sl) {
unsigned r = nbd_rand();
- int n = __builtin_ctz(r) / 2 + 1;
- if (n > MAX_LEVELS) { n = MAX_LEVELS; }
- return n;
+ int z = __builtin_ctz(r);
+ int levels = (int)(z / 1.5);
+ if (levels == 0)
+ return 1;
+ if (levels > sl->high_water) {
+ levels = SYNC_ADD(&sl->high_water, 1);
+ TRACE("s2", "random_levels: increased high water mark to %lld", sl->high_water, 0);
+ }
+ if (levels > MAX_LEVELS) { levels = MAX_LEVELS; }
+ return levels;
}
static node_t *node_alloc (int num_levels, map_key_t key, map_val_t val) {
skiplist_t *sl_alloc (const datatype_t *key_type) {
skiplist_t *sl = (skiplist_t *)nbd_malloc(sizeof(skiplist_t));
sl->key_type = key_type;
- sl->high_water = 0;
+ sl->high_water = 1;
sl->head = node_alloc(MAX_LEVELS, 0, 0);
memset(sl->head->next, 0, MAX_LEVELS * sizeof(skiplist_t *));
return sl;
TRACE("s3", "find_preds: found pred %p next %p", pred, item);
- if (level < n) {
+ if (level < n) {
if (preds != NULL) {
preds[level] = pred;
}
}
}
- TRACE("l1", "sl_lookup: no item in the skiplist matched the key", 0, 0);
+ TRACE("s1", "sl_lookup: no item in the skiplist matched the key", 0, 0);
return DOES_NOT_EXIST;
}
}
if (EXPECT_FALSE(expectation == CAS_EXPECT_DOES_NOT_EXIST)) {
- TRACE("s1", "update_item: found an item %p in the skiplist that matched the key. the expectation was "
- "not met, the skiplist was not changed", item, old_val);
+ TRACE("s1", "update_item: the expectation was not met; the skiplist was not changed", 0, 0);
return old_val; // failure
}
// Use a CAS and not a SWAP. If the CAS fails it means another thread removed the node or updated its
// value. If another thread removed the node but it is not unlinked yet 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 it should return DOES_NOT_EXIST.
+ // succeeded and return our value even though it should return DOES_NOT_EXIST.
if (old_val == SYNC_CAS(&item->val, old_val, new_val)) {
TRACE("s1", "update_item: the CAS succeeded. updated the value of the item", 0, 0);
return old_val; // success
node_t *preds[MAX_LEVELS];
node_t *nexts[MAX_LEVELS];
node_t *new_item = NULL;
- int n = random_levels();
- if (n > sl->high_water) {
- n = SYNC_ADD(&sl->high_water, 1);
- TRACE("s2", "sl_cas: incremented high water mark to %p", n, 0);
- }
+ int n = random_levels(sl);
node_t *old_item = find_preds(preds, nexts, n, sl, key, ASSIST_UNLINK);
// If there is already an item in the skiplist that matches the key just update its value.
return ret_val;
// If we lose a race with a thread removing the item we tried to update then we have to retry.
- return sl_cas(sl, key, expectation, new_val); // tail call
+ return sl_cas(sl, key, expectation, new_val); // tail call
}
if (EXPECT_FALSE(expectation != CAS_EXPECT_DOES_NOT_EXIST && expectation != CAS_EXPECT_WHATEVER)) {
- TRACE("l1", "sl_cas: the expectation was not met, the skiplist was not changed", 0, 0);
- return DOES_NOT_EXIST; // failure, the caller expected an item for the <key> to already exist
+ TRACE("s1", "sl_cas: the expectation was not met, the skiplist was not changed", 0, 0);
+ return DOES_NOT_EXIST; // failure, the caller expected an item for the <key> to already exist
}
// Create a new node and insert it into the skiplist.
if (sl->key_type != NULL) {
nbd_free((void *)new_key);
}
- nbd_free(new_item);
+ nbd_free(new_item);
return sl_cas(sl, key, expectation, new_val); // tail call
}
TRACE("s3", "sl_cas: attempting to update the new item's link from %p to %p", old_next, nexts[i]);
other = SYNC_CAS(&new_item->next[i], old_next, (markable_t)nexts[i]);
ASSERT(other == old_next || other == MARK_NODE(old_next));
-
+
// If another thread is removing this item we can stop linking it into to skiplist
if (HAS_MARK(other)) {
find_preds(NULL, NULL, 0, sl, key, FORCE_UNLINK); // see comment below
old_next = SYNC_CAS(&item->next[level], next, MARK_NODE((node_t *)next));
if (HAS_MARK(old_next)) {
TRACE("s2", "sl_remove: %p is already marked for removal by another thread (next %p)", item, old_next);
- if (level == 0)
+ if (level == 0)
return DOES_NOT_EXIST;
break;
}
} while (next != old_next);
}
- // 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.
- map_val_t val = SYNC_SWAP(&item->val, DOES_NOT_EXIST);
+ // 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.
+ map_val_t val = SYNC_SWAP(&item->val, DOES_NOT_EXIST);
TRACE("s2", "sl_remove: replaced item %p's value with DOES_NOT_EXIT", item, 0);
// unlink the item
return val;
}
-void sl_print (skiplist_t *sl) {
-
- printf("high water: %d levels\n", sl->high_water);
- for (int level = MAX_LEVELS - 1; level >= 0; --level) {
+void sl_print (skiplist_t *sl, int verbose) {
+
+ if (verbose) {
+ for (int level = MAX_LEVELS - 1; level >= 0; --level) {
+ node_t *item = sl->head;
+ if (item->next[level] == DOES_NOT_EXIST)
+ continue;
+ printf("(%d) ", level);
+ int i = 0;
+ while (item) {
+ markable_t next = item->next[level];
+ printf("%s%p ", HAS_MARK(next) ? "*" : "", item);
+ item = STRIP_MARK(next);
+ if (i++ > 30) {
+ printf("...");
+ break;
+ }
+ }
+ printf("\n");
+ fflush(stdout);
+ }
node_t *item = sl->head;
- if (item->next[level] == DOES_NOT_EXIST)
- continue;
- printf("(%d) ", level);
int i = 0;
while (item) {
- markable_t next = item->next[level];
- printf("%s%p ", HAS_MARK(next) ? "*" : "", item);
- item = STRIP_MARK(next);
+ int is_marked = HAS_MARK(item->next[0]);
+ printf("%s%p:0x%llx ", is_marked ? "*" : "", item, (uint64_t)item->key);
+ if (item != sl->head) {
+ printf("[%d]", item->num_levels);
+ } else {
+ printf("[HEAD]");
+ }
+ for (int level = 1; level < item->num_levels; ++level) {
+ node_t *next = STRIP_MARK(item->next[level]);
+ is_marked = HAS_MARK(item->next[0]);
+ printf(" %p%s", next, is_marked ? "*" : "");
+ if (item == sl->head && item->next[level] == DOES_NOT_EXIST)
+ break;
+ }
+ printf("\n");
+ fflush(stdout);
+ item = STRIP_MARK(item->next[0]);
if (i++ > 30) {
- printf("...");
+ printf("...\n");
break;
}
}
- printf("\n");
- fflush(stdout);
- }
- node_t *item = sl->head;
- int i = 0;
- while (item) {
- int is_marked = HAS_MARK(item->next[0]);
- printf("%s%p:0x%llx ", is_marked ? "*" : "", item, (uint64_t)item->key);
- if (item != sl->head) {
- printf("[%d]", item->num_levels);
- } else {
- printf("[HEAD]");
- }
- for (int level = 1; level < item->num_levels; ++level) {
- node_t *next = STRIP_MARK(item->next[level]);
- is_marked = HAS_MARK(item->next[0]);
- printf(" %p%s", next, is_marked ? "*" : "");
- if (item == sl->head && item->next[level] == DOES_NOT_EXIST)
- break;
- }
- printf("\n");
- fflush(stdout);
- item = STRIP_MARK(item->next[0]);
- if (i++ > 30) {
- printf("...\n");
- break;
- }
}
+ printf("levels:%-2d count:%-6lld \n", sl->high_water, (uint64_t)sl_count(sl));
}
sl_iter_t *sl_iter_begin (skiplist_t *sl, map_key_t key) {
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