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;
15 typedef map_key_t version_t;
20 map_val_t next; // an earlier update
23 typedef struct write_rec {
39 static txn_state_e txn_validate (txn_t *txn);
41 static version_t version_ = 1;
43 static skiplist_t *active_ = NULL;
45 void txn_init (void) {
46 active_ = sl_alloc(NULL);
49 // Validate the updates for <key>. Validation fails if there is a write-write conflict. That is if after our
50 // read version another transaction committed a change to an entry we are also trying to change.
52 // If we encounter a potential conflict with a transaction that is in the process of validating, we help it
53 // complete validating. It must be finished before we can decide to rollback or commit.
55 static txn_state_e validate_key (txn_t *txn, map_key_t key) {
56 assert(txn->state != TXN_RUNNING);
58 map_val_t val = map_get(txn->map, key);
59 update_t *update = NULL;
60 for (; val != DOES_NOT_EXIST; val = update->next) {
62 // If the update or its version is not tagged it means the update is committed.
64 // We can stop at the first committed record we find that is at least as old as our read version. All
65 // the other committed records following it will be older. And all the uncommitted records following it
66 // will eventually conflict with it and abort.
67 if (!IS_TAGGED(val, TAG2))
69 update = (update_t *)STRIP_TAG(val, TAG2);
70 if (!IS_TAGGED(update->version, TAG1))
71 return (update->version <= txn->rv) ? TXN_VALIDATED : TXN_ABORTED;
73 // If the update's version is tagged then either the update was aborted or the the version number is
74 // actually a pointer to a running transaction's txn_t.
76 // Skip aborted transactions.
77 if (EXPECT_FALSE(update->version == ABORTED_VERSION))
80 // The update's transaction is still in progress. Access its txn_t.
81 txn_t *writer = (txn_t *)STRIP_TAG(update->version, TAG1);
83 continue; // Skip our own updates.
84 txn_state_e writer_state = writer->state;
86 // Any running transaction will only be able to acquire a wv greater than ours. A transaction changes its
87 // state to validating before aquiring a wv. We can ignore an unvalidated transaction if its version is
88 // greater than ours. See next comment below for why.
89 if (writer_state == TXN_RUNNING)
92 // If <writer> has a later version than us we can safely ignore its updates. It will not commit until
93 // we have completed validation (in order to remain non-blocking it will help us validate if necessary).
94 // This protocol ensures a deterministic resolution to every conflict and avoids infinite ping-ponging
95 // between validating two conflicting transactions.
96 if (writer_state == TXN_VALIDATING) {
97 if (writer->wv > txn->wv)
99 // Help <writer> commit. We need to know if <writer> aborts or commits before we can decide what to
100 // do. But we don't want to block, so we assist.
101 writer_state = txn_validate(writer);
104 // Skip updates from aborted transactions.
105 if (writer_state == TXN_ABORTED)
108 assert(writer_state == TXN_VALIDATED);
109 return (writer->wv <= txn->rv) ? TXN_VALIDATED : TXN_ABORTED;
112 return TXN_VALIDATED;
115 static txn_state_e txn_validate (txn_t *txn) {
116 assert(txn->state != TXN_RUNNING);
118 switch (txn->state) {
121 if (txn->wv == UNDETERMINED_VERSION) {
122 version_t wv = SYNC_ADD(&version_, 1);
123 SYNC_CAS(&txn->wv, UNDETERMINED_VERSION, wv);
126 for (i = 0; i < txn->writes_count; ++i) {
127 txn_state_e s = validate_key(txn, txn->writes[i].key);
128 if (s == TXN_ABORTED) {
129 txn->state = TXN_ABORTED;
132 assert(s == TXN_VALIDATED);
134 if (txn->state == TXN_VALIDATING) {
135 txn->state = TXN_VALIDATED;
150 static update_t *alloc_update_rec (void) {
151 update_t *u = (update_t *)nbd_malloc(sizeof(update_t));
152 memset(u, 0, sizeof(update_t));
156 txn_t *txn_begin (map_t *map) {
157 txn_t *txn = (txn_t *)nbd_malloc(sizeof(txn_t));
158 memset(txn, 0, sizeof(txn_t));
159 txn->wv = UNDETERMINED_VERSION;
160 txn->state = TXN_RUNNING;
162 txn->writes = nbd_malloc(sizeof(*txn->writes) * INITIAL_WRITES_SIZE);
163 txn->writes_size = INITIAL_WRITES_SIZE;
165 // acquire the read version for txn. must be careful to avoid a race
173 temp = sl_cas(active_, txn->rv, old_count, old_count + 1);
174 } while (temp != old_count);
176 if (txn->rv == version_)
182 temp = sl_cas(active_, (map_key_t)txn->rv, old_count, old_count - 1);
183 } while (temp != old_count);
189 void txn_abort (txn_t *txn) {
190 if (txn->state != TXN_RUNNING)
191 return; // TODO: return some sort of error code
194 for (i = 0; i < txn->writes_count; ++i) {
195 update_t *update = (update_t *)txn->writes[i].rec;
196 update->version = ABORTED_VERSION;
199 nbd_defer_free(txn->writes);
203 txn_state_e txn_commit (txn_t *txn) {
204 if (txn->state != TXN_RUNNING)
205 return txn->state; // TODO: return some sort of error code
207 assert(txn->state == TXN_RUNNING);
208 txn->state = TXN_VALIDATING;
209 txn_state_e state = txn_validate(txn);
211 // Detach <txn> from its updates.
212 version_t wv = (txn->state == TXN_ABORTED) ? ABORTED_VERSION : txn->wv;
214 for (i = 0; i < txn->writes_count; ++i) {
215 update_t *update = (update_t *)txn->writes[i].rec;
216 update->version = wv;
219 // Lower the reference count for <txn>'s read version
224 temp = sl_cas(active_, (map_key_t)txn->rv, old_count, old_count - 1);
225 if (temp == 1 && txn->rv != version_) {
226 sl_remove(active_, (map_key_t)txn->rv);
229 } while (old_count != temp);
231 nbd_defer_free(txn->writes);
237 // Get most recent committed version prior to our read version.
238 map_val_t txn_map_get (txn_t *txn, map_key_t key) {
239 if (txn->state != TXN_RUNNING)
240 return ERROR_TXN_NOT_RUNNING;
242 // Iterate through the update records to find the latest committed version prior to our read version.
243 map_val_t newest_val = map_get(txn->map, key);
244 map_val_t val = newest_val;
245 update_t *update = NULL;
246 for ( ; ; val = update->next) {
248 if (!IS_TAGGED(val, TAG2))
251 update = (update_t *)STRIP_TAG(val, TAG2);
252 assert(update != NULL);
254 // If the update's version is not tagged it means the update is committed.
255 if (!IS_TAGGED(update->version, TAG1)) {
256 if (update->version <= txn->rv)
261 // If the update's version is tagged then either the update was aborted or the the version number is
262 // actually a pointer to a running transaction's txn_t.
264 // Skip updates from aborted transactions.
265 if (EXPECT_FALSE(update->version == ABORTED_VERSION))
268 // The update's transaction is still in progress. Access its txn_t.
269 txn_t *writer = (txn_t *)STRIP_TAG(update->version, TAG1);
270 if (writer == txn) // found our own update
273 txn_state_e writer_state = writer->state;
274 if (writer_state == TXN_RUNNING)
277 if (writer_state == TXN_VALIDATING) {
278 if (writer->wv > txn->rv)
280 writer_state = txn_validate(writer);
283 // Skip updates from aborted transactions.
284 if (writer_state == TXN_ABORTED)
287 assert(writer_state == TXN_VALIDATED);
288 if (writer->wv > txn->rv)
293 map_val_t value = update->value;
295 // collect some garbage
296 version_t min_active_version = UNDETERMINED_VERSION;
297 update_t *next_update = NULL;
298 if (IS_TAGGED(update->next, TAG2)) {
299 next_update = (update_t *)STRIP_TAG(update->next, TAG2);
300 min_active_version = (version_t)sl_min_key(active_);
301 if (next_update->version < min_active_version) {
302 // <next_update> (and all update records following it [execpt if it is aborted]) is old enough that it is
303 // not visible to any active transaction. We can safely free it.
305 // Skip over aborted versions to look for more recent updates
306 update_t *temp = next_update;
307 while (temp->version == ABORTED_VERSION) {
308 assert(!IS_TAGGED(temp->version, TAG1));
309 map_val_t next = next_update->next;
310 if (!IS_TAGGED(next, TAG2))
313 temp = (update_t *)STRIP_TAG(next, TAG2);
314 if (temp->version >= min_active_version)
318 // free <next> and all the update records following it
321 map_val_t next = SYNC_SWAP(&temp->next, DOES_NOT_EXIST);
323 // if we find ourself in a race just back off and let the other thread take care of it
324 if (next == DOES_NOT_EXIST)
327 if (!IS_TAGGED(next, TAG2))
330 temp = (update_t *)STRIP_TAG(next, TAG2);
336 // If there is one item left and it is visible by all active transactions we can merge it into the map itself.
337 // There is no need for an update record.
338 if (next_update == NULL && val == newest_val) {
339 if (min_active_version == UNDETERMINED_VERSION) {
340 min_active_version = (version_t)sl_min_key(active_);
342 if (update->version <= min_active_version) {
343 if (map_cas(txn->map, key, TAG_VALUE(val, TAG2), value) == TAG_VALUE(val, TAG2)) {
344 nbd_defer_free(update);
352 void txn_map_set (txn_t *txn, map_key_t key, map_val_t value) {
353 if (txn->state != TXN_RUNNING)
354 return; // TODO: return some sort of error code
356 // create a new update record
357 update_t *update = alloc_update_rec();
358 update->value = value;
359 update->version = TAG_VALUE((version_t)txn, TAG1);
361 // push the new update record onto <key>'s update list
362 map_val_t old_update;
364 old_update = map_get(txn->map, key);
365 update->next = old_update;
366 } while (map_cas(txn->map, key, old_update, TAG_VALUE((map_val_t)update, TAG2)) != old_update);
368 // add <key> to the write set for commit-time validation
369 if (txn->writes_count == txn->writes_size) {
370 write_rec_t *w = nbd_malloc(sizeof(write_rec_t) * txn->writes_size * 2);
371 memcpy(w, txn->writes, txn->writes_size * sizeof(write_rec_t));
372 txn->writes_size *= 2;
373 nbd_free(txn->writes);
376 int i = txn->writes_count++;
377 txn->writes[i].key = key;
378 txn->writes[i].rec = update;