2 * Written by Josh Dybnis and released to the public domain, as explained at
3 * http://creativecommons.org/licenses/publicdomain
10 #define UNDETERMINED_VERSION 0
11 #define ABORTED_VERSION TAG_VALUE(0, TAG1)
12 #define INITIAL_WRITES_SIZE 4
14 typedef struct update_rec update_t;
19 map_val_t next; // an earlier update
22 typedef struct write_rec {
33 uint64_t writes_count;
38 static txn_state_e txn_validate (txn_t *txn);
40 static uint64_t version_ = 1;
42 static skiplist_t *active_ = NULL;
44 void txn_init (void) {
45 active_ = sl_alloc(NULL);
48 // Validate the updates for <key>. Validation fails if there is a write-write conflict. That is if after our
49 // read version another transaction committed a change to an entry we are also trying to change.
51 // If we encounter a potential conflict with a transaction that is in the process of validating, we help it
52 // complete validating. It must be finished before we can decide to rollback or commit.
54 static txn_state_e validate_key (txn_t *txn, map_key_t key) {
55 assert(txn->state != TXN_RUNNING);
57 map_val_t val = map_get(txn->map, key);
58 update_t *update = NULL;
59 for (; val != DOES_NOT_EXIST; val = update->next) {
61 // If the update or its version is not tagged it means the update is committed.
63 // We can stop at the first committed record we find that is at least as old as our read version. All
64 // the other committed records following it will be older. And all the uncommitted records following it
65 // will eventually conflict with it and abort.
66 if (!IS_TAGGED(val, TAG2))
68 update = (update_t *)(size_t)STRIP_TAG(val, TAG2);
69 if (!IS_TAGGED(update->version, TAG1))
70 return (update->version <= txn->rv) ? TXN_VALIDATED : TXN_ABORTED;
72 // If the update's version is tagged then either the update was aborted or the the version number is
73 // actually a pointer to a running transaction's txn_t.
75 // Skip aborted transactions.
76 if (EXPECT_FALSE(update->version == ABORTED_VERSION))
79 // The update's transaction is still in progress. Access its txn_t.
80 txn_t *writer = (txn_t *)(size_t)STRIP_TAG(update->version, TAG1);
82 continue; // Skip our own updates.
83 txn_state_e writer_state = writer->state;
85 // Any running transaction will only be able to acquire a wv greater than ours. A transaction changes its
86 // state to validating before aquiring a wv. We can ignore an unvalidated transaction if its version is
87 // greater than ours. See next comment below for why.
88 if (writer_state == TXN_RUNNING)
91 // If <writer> has a later version than us we can safely ignore its updates. It will not commit until
92 // we have completed validation (in order to remain non-blocking it will help us validate if necessary).
93 // This protocol ensures a deterministic resolution to every conflict and avoids infinite ping-ponging
94 // between validating two conflicting transactions.
95 if (writer_state == TXN_VALIDATING) {
96 if (writer->wv > txn->wv)
98 // Help <writer> commit. We need to know if <writer> aborts or commits before we can decide what to
99 // do. But we don't want to block, so we assist.
100 writer_state = txn_validate(writer);
103 // Skip updates from aborted transactions.
104 if (writer_state == TXN_ABORTED)
107 assert(writer_state == TXN_VALIDATED);
108 return (writer->wv <= txn->rv) ? TXN_VALIDATED : TXN_ABORTED;
111 return TXN_VALIDATED;
114 static txn_state_e txn_validate (txn_t *txn) {
115 assert(txn->state != TXN_RUNNING);
117 switch (txn->state) {
120 if (txn->wv == UNDETERMINED_VERSION) {
121 uint64_t wv = SYNC_ADD(&version_, 1);
122 SYNC_CAS(&txn->wv, UNDETERMINED_VERSION, wv);
125 for (i = 0; i < txn->writes_count; ++i) {
126 txn_state_e s = validate_key(txn, txn->writes[i].key);
127 if (s == TXN_ABORTED) {
128 txn->state = TXN_ABORTED;
131 assert(s == TXN_VALIDATED);
133 if (txn->state == TXN_VALIDATING) {
134 txn->state = TXN_VALIDATED;
149 static update_t *alloc_update_rec (void) {
150 update_t *u = (update_t *)nbd_malloc(sizeof(update_t));
151 memset(u, 0, sizeof(update_t));
155 txn_t *txn_begin (map_t *map) {
156 txn_t *txn = (txn_t *)nbd_malloc(sizeof(txn_t));
157 memset(txn, 0, sizeof(txn_t));
158 txn->wv = UNDETERMINED_VERSION;
159 txn->state = TXN_RUNNING;
161 txn->writes = nbd_malloc(sizeof(*txn->writes) * INITIAL_WRITES_SIZE);
162 txn->writes_size = INITIAL_WRITES_SIZE;
164 // acquire the read version for txn. must be careful to avoid a race
172 temp = (uint64_t)sl_cas(active_, (map_key_t)txn->rv, old_count, old_count + 1);
173 } while (temp != old_count);
175 if (txn->rv == version_)
181 temp = sl_cas(active_, (map_key_t)txn->rv, old_count, old_count - 1);
182 } while (temp != old_count);
188 void txn_abort (txn_t *txn) {
189 if (txn->state != TXN_RUNNING)
190 return; // TODO: return some sort of error code
193 for (i = 0; i < txn->writes_count; ++i) {
194 update_t *update = (update_t *)txn->writes[i].rec;
195 update->version = ABORTED_VERSION;
198 nbd_defer_free(txn->writes);
202 txn_state_e txn_commit (txn_t *txn) {
203 if (txn->state != TXN_RUNNING)
204 return txn->state; // TODO: return some sort of error code
206 assert(txn->state == TXN_RUNNING);
207 txn->state = TXN_VALIDATING;
208 txn_state_e state = txn_validate(txn);
210 // Detach <txn> from its updates.
211 uint64_t wv = (txn->state == TXN_ABORTED) ? ABORTED_VERSION : txn->wv;
213 for (i = 0; i < txn->writes_count; ++i) {
214 update_t *update = (update_t *)txn->writes[i].rec;
215 update->version = wv;
218 // Lower the reference count for <txn>'s read version
223 temp = sl_cas(active_, (map_key_t)txn->rv, old_count, old_count - 1);
224 if (temp == 1 && txn->rv != version_) {
225 sl_remove(active_, (map_key_t)txn->rv);
228 } while (old_count != temp);
230 nbd_defer_free(txn->writes);
236 // Get most recent committed version prior to our read version.
237 uint64_t txn_map_get (txn_t *txn, map_key_t key) {
238 if (txn->state != TXN_RUNNING)
239 return ERROR_TXN_NOT_RUNNING;
241 // Iterate through the update records to find the latest committed version prior to our read version.
242 uint64_t newest_val = map_get(txn->map, key);
243 uint64_t val = newest_val;
244 update_t *update = NULL;
245 for ( ; ; val = update->next) {
247 if (!IS_TAGGED(val, TAG2))
250 update = (update_t *)(size_t)STRIP_TAG(val, TAG2);
251 assert(update != NULL);
253 // If the update's version is not tagged it means the update is committed.
254 if (!IS_TAGGED(update->version, TAG1)) {
255 if (update->version <= txn->rv)
260 // If the update's version is tagged then either the update was aborted or the the version number is
261 // actually a pointer to a running transaction's txn_t.
263 // Skip updates from aborted transactions.
264 if (EXPECT_FALSE(update->version == ABORTED_VERSION))
267 // The update's transaction is still in progress. Access its txn_t.
268 txn_t *writer = (txn_t *)(size_t)STRIP_TAG(update->version, TAG1);
269 if (writer == txn) // found our own update
272 txn_state_e writer_state = writer->state;
273 if (writer_state == TXN_RUNNING)
276 if (writer_state == TXN_VALIDATING) {
277 if (writer->wv > txn->rv)
279 writer_state = txn_validate(writer);
282 // Skip updates from aborted transactions.
283 if (writer_state == TXN_ABORTED)
286 assert(writer_state == TXN_VALIDATED);
287 if (writer->wv > txn->rv)
292 uint64_t value = update->value;
294 // collect some garbage
295 uint64_t min_active_version = UNDETERMINED_VERSION;
296 update_t *next_update = NULL;
297 if (IS_TAGGED(update->next, TAG2)) {
298 next_update = (update_t *)(size_t)STRIP_TAG(update->next, TAG2);
299 min_active_version = (uint64_t)sl_min_key(active_);
300 if (next_update->version < min_active_version) {
301 // <next_update> (and all update records following it [execpt if it is aborted]) is old enough that it is
302 // not visible to any active transaction. We can safely free it.
304 // Skip over aborted versions to look for more recent updates
305 update_t *temp = next_update;
306 while (temp->version == ABORTED_VERSION) {
307 assert(!IS_TAGGED(temp->version, TAG1));
308 uint64_t next = next_update->next;
309 if (!IS_TAGGED(next, TAG2))
312 temp = (update_t *)(size_t)STRIP_TAG(next, TAG2);
313 if (temp->version >= min_active_version)
317 // free <next> and all the update records following it
320 uint64_t next = SYNC_SWAP(&temp->next, DOES_NOT_EXIST);
322 // if we find ourself in a race just back off and let the other thread take care of it
323 if (next == DOES_NOT_EXIST)
326 if (!IS_TAGGED(next, TAG2))
329 temp = (update_t *)(size_t)STRIP_TAG(next, TAG2);
335 // If there is one item left and it is visible by all active transactions we can merge it into the map itself.
336 // There is no need for an update record.
337 if (next_update == NULL && val == newest_val) {
338 if (min_active_version == UNDETERMINED_VERSION) {
339 min_active_version = (uint64_t)sl_min_key(active_);
341 if (update->version <= min_active_version) {
342 if (map_cas(txn->map, key, TAG_VALUE(val, TAG2), value) == TAG_VALUE(val, TAG2)) {
343 nbd_defer_free(update);
351 void txn_map_set (txn_t *txn, map_key_t key, uint64_t value) {
352 if (txn->state != TXN_RUNNING)
353 return; // TODO: return some sort of error code
355 // create a new update record
356 update_t *update = alloc_update_rec();
357 update->value = value;
358 update->version = TAG_VALUE((uint64_t)(size_t)txn, TAG1);
360 // push the new update record onto <key>'s update list
363 old_update = map_get(txn->map, key);
364 update->next = old_update;
365 } while (map_cas(txn->map, key, old_update, TAG_VALUE((uint64_t)(size_t)update, TAG2)) != old_update);
367 // add <key> to the write set for commit-time validation
368 if (txn->writes_count == txn->writes_size) {
369 write_rec_t *w = nbd_malloc(sizeof(write_rec_t) * txn->writes_size * 2);
370 memcpy(w, txn->writes, txn->writes_size * sizeof(write_rec_t));
371 txn->writes_size *= 2;
372 nbd_free(txn->writes);
375 int i = txn->writes_count++;
376 txn->writes[i].key = key;
377 txn->writes[i].rec = update;