X-Git-Url: https://pd.if.org/git/?p=nbds;a=blobdiff_plain;f=txn%2Ftxn.c;h=26e0209408c4c6a72ceda7916fcec3634d12197b;hp=e05d332386fe8284727fc0ced57c107d26a59cf2;hb=f3a053a46bbb4ba460bcff0920b93dfc8263e02e;hpb=d26bac75802a324ed98c8d3d88cfb9eb87b3b35a diff --git a/txn/txn.c b/txn/txn.c index e05d332..26e0209 100644 --- a/txn/txn.c +++ b/txn/txn.c @@ -8,22 +8,20 @@ #include "skiplist.h" #define UNDETERMINED_VERSION 0 +#define ABORTED_VERSION TAG_VALUE(0, TAG1) #define INITIAL_WRITES_SIZE 4 -typedef enum { UPDATE_TYPE_PUT, UPDATE_TYPE_DELETE } update_type_t; - -typedef struct update_rec update_rec_t; +typedef struct update_rec update_t; struct update_rec { - update_type_t type; - uint64_t value; + update_t *next; // an earlier update uint64_t version; - update_rec_t *prev; // a previous update + uint64_t value; }; typedef struct write_rec { void *key; - update_rec_t *rec; + update_t *rec; } write_rec_t; struct txn { @@ -42,62 +40,79 @@ static uint64_t version_ = 1; static txn_state_e txn_validate (txn_t *txn); -static map_t *active_ = NULL; +static skiplist_t *active_ = NULL; void txn_init (void) { - active_ = map_alloc(MAP_TYPE_SKIPLIST, NULL); + active_ = sl_alloc(NULL); } -// Validate the updates for . Validation fails for a key we have written to if there is a -// write committed newer than our read version. +// Validate the updates for . Validation fails if there is a write-write conflict. That is if after our +// read version another transaction committed a change to an entry we are also trying to change. +// +// If we encounter a potential conflict with a transaction that is in the process of validating, we help it +// complete validating. It must be finished before we can decide to rollback or commit. +// static txn_state_e tm_validate_key (txn_t *txn, void *key) { + assert(txn->state != TXN_RUNNING); - update_rec_t *update = (update_rec_t *) map_get(txn->map, key); - for (; update != NULL; update = update->prev) { - uint64_t writer_version = update->version; - if (writer_version <= txn->rv) + update_t *update = (update_t *) map_get(txn->map, key); + for (; update != NULL; update = update->next) { + + // If the update or its version is not tagged it means the update is committed. + // + // We can stop at the first committed record we find that is at least as old as our read version. All + // the other committed records following it will be older. And all the uncommitted records following it + // will eventually conflict with it and abort. + if (!IS_TAGGED(update, TAG2)) return TXN_VALIDATED; - - // If the version is tagged, it means it is a pointer to a transaction in progress. - if (IS_TAGGED(writer_version)) { - - // Skip aborted transactions. - if (EXPECT_FALSE(writer_version == TAG_VALUE(0))) - continue; - - // Skip our own updates. - txn_t *writer = (txn_t *)STRIP_TAG(writer_version); - if (writer == txn) - continue; - - writer_version = writer->wv; - if (writer_version <= txn->rv && writer_version != UNDETERMINED_VERSION) - return TXN_VALIDATED; - - txn_state_e writer_state = writer->state; - if (EXPECT_FALSE(writer_state == TXN_ABORTED)) + update = (update_t *)STRIP_TAG(update, TAG2); + if (!IS_TAGGED(update->version, TAG1)) + return (update->version <= txn->rv) ? TXN_VALIDATED : TXN_ABORTED; + + // If the update's version is tagged then either the update was aborted or the the version number is + // actually a pointer to a running transaction's txn_t. + + // Skip aborted transactions. + if (EXPECT_FALSE(update->version == ABORTED_VERSION)) + continue; + + // The update's transaction is still in progress. Access its txn_t. + txn_t *writer = (txn_t *)STRIP_TAG(update->version, TAG1); + if (writer == txn) + continue; // Skip our own updates. + txn_state_e writer_state = writer->state; + + // Any running transaction will only be able to acquire a wv greater than ours. A transaction changes its + // state to validating before aquiring a wv. We can ignore an unvalidated transaction if its version is + // greater than ours. See next comment below for why. + if (writer_state == TXN_RUNNING) + continue; + + // If has a later version than us we can safely ignore its updates. It will not commit until + // we have completed validation (in order to remain non-blocking it will help us validate if necessary). + // This protocol ensures a deterministic resolution to every conflict and avoids infinite ping-ponging + // between validating two conflicting transactions. + if (writer_state == TXN_VALIDATING) { + if (writer->wv > txn->wv) continue; - - // Help validate if it is a committing transaction that might cause us to - // abort. However, if the has a later version than us we can safely ignore its - // updates. This protocol ensures a deterministic resolution to every conflict, and - // avoids infinite ping-ponging between validating two conflicting transactions. - if (writer_state == TXN_VALIDATING && (writer_version < txn->wv || - writer_version == UNDETERMINED_VERSION)) { - writer_state = txn_validate(writer); - } - - if (writer_state == TXN_VALIDATED) - return TXN_ABORTED; + // Help commit. We need to know if aborts or commits before we can decide what to + // do. But we don't want to block, so we assist. + writer_state = txn_validate(writer); } - return TXN_ABORTED; + // Skip updates from aborted transactions. + if (writer_state == TXN_ABORTED) + continue; + + assert(writer_state == TXN_VALIDATED); + return (writer->wv <= txn->rv) ? TXN_VALIDATED : TXN_ABORTED; } return TXN_VALIDATED; } static txn_state_e txn_validate (txn_t *txn) { + assert(txn->state != TXN_RUNNING); int i; switch (txn->state) { @@ -113,6 +128,7 @@ static txn_state_e txn_validate (txn_t *txn) { txn->state = TXN_ABORTED; break; } + assert(s == TXN_VALIDATED); } if (txn->state == TXN_VALIDATING) { txn->state = TXN_VALIDATED; @@ -130,9 +146,9 @@ static txn_state_e txn_validate (txn_t *txn) { return txn->state; } -static update_rec_t *alloc_update_rec (void) { - update_rec_t *u = (update_rec_t *)nbd_malloc(sizeof(update_rec_t)); - memset(u, 0, sizeof(update_rec_t)); +static update_t *alloc_update_rec (void) { + update_t *u = (update_t *)nbd_malloc(sizeof(update_t)); + memset(u, 0, sizeof(update_t)); return u; } @@ -148,7 +164,7 @@ txn_t *txn_begin (txn_type_e type, map_t *map) { txn->writes_size = INITIAL_WRITES_SIZE; } - // aquire the read version for txn. + // acquire the read version for txn. must be careful to avoid a race do { txn->rv = version_; @@ -156,7 +172,7 @@ txn_t *txn_begin (txn_type_e type, map_t *map) { uint64_t temp = 0; do { old_count = temp; - temp = (uint64_t)map_cas(active_, (void *)txn->rv, old_count, old_count + 1); + temp = (uint64_t)sl_cas(active_, (void *)txn->rv, old_count, old_count + 1); } while (temp != old_count); if (txn->rv == version_) @@ -165,7 +181,7 @@ txn_t *txn_begin (txn_type_e type, map_t *map) { temp = 1; do { old_count = temp; - temp = map_cas(active_, (void *)txn->rv, old_count, old_count - 1); + temp = sl_cas(active_, (void *)txn->rv, old_count, old_count - 1); } while (temp != old_count); } while (1); @@ -173,11 +189,13 @@ txn_t *txn_begin (txn_type_e type, map_t *map) { } void txn_abort (txn_t *txn) { + if (txn->state != TXN_RUNNING) + return; // TODO: return some sort of error code int i; for (i = 0; i < txn->writes_count; ++i) { - update_rec_t *update = (update_rec_t *)txn->writes[i].rec; - update->version = TAG_VALUE(0); + update_t *update = (update_t *)txn->writes[i].rec; + update->version = ABORTED_VERSION; } nbd_defer_free(txn->writes); @@ -185,19 +203,33 @@ void txn_abort (txn_t *txn) { } txn_state_e txn_commit (txn_t *txn) { + if (txn->state != TXN_RUNNING) + return txn->state; // TODO: return some sort of error code assert(txn->state == TXN_RUNNING); txn->state = TXN_VALIDATING; txn_state_e state = txn_validate(txn); // Detach from its updates. - uint64_t wv = (txn->state == TXN_ABORTED) ? TAG_VALUE(0) : txn->wv; + uint64_t wv = (txn->state == TXN_ABORTED) ? ABORTED_VERSION : txn->wv; int i; for (i = 0; i < txn->writes_count; ++i) { - update_rec_t *update = (update_rec_t *)txn->writes[i].rec; + update_t *update = (update_t *)txn->writes[i].rec; update->version = wv; } + // Lower the reference count for 's read version + uint64_t temp = 2; + uint64_t old_count; + do { + old_count = temp; + temp = sl_cas(active_, (void *)txn->rv, old_count, old_count - 1); + if (temp == 1 && txn->rv != version_) { + sl_remove(active_, (void *)txn->rv); + break; + } + } while (old_count != temp); + nbd_defer_free(txn->writes); nbd_defer_free(txn); @@ -206,59 +238,140 @@ txn_state_e txn_commit (txn_t *txn) { // Get most recent committed version prior to our read version. uint64_t tm_get (txn_t *txn, void *key) { + if (txn->state != TXN_RUNNING) + return ERROR_TXN_NOT_RUNNING; + + update_t *newest_update = (update_t *) map_get(txn->map, key); + if (!IS_TAGGED(newest_update, TAG2)) + return (uint64_t)newest_update; + + // Iterate through the update records to find the latest committed version prior to our read version. + update_t *update; + for (update = newest_update; ; update = update->next) { - // Iterate through update records associated with to find the latest committed version. - // We can use the first matching version. Older updates always come later in the list. - update_rec_t *update = (update_rec_t *) map_get(txn->map, key); - for (; update != NULL; update = update->prev) { - uint64_t writer_version = update->version; - if (writer_version < txn->rv) - return update->value; - - // If the version is tagged, it means that it is not a version number, but a pointer to an - // in progress transaction. - if (IS_TAGGED(update->version)) { - txn_t *writer = (txn_t *)STRIP_TAG(writer_version); - - if (writer == txn) - return update->type == UPDATE_TYPE_DELETE ? DOES_NOT_EXIST : update->value; - - // Skip updates from aborted transactions. - txn_state_e writer_state = writer->state; - if (EXPECT_FALSE(writer_state == TXN_ABORTED)) + if (!IS_TAGGED(update, TAG2)) + return (uint64_t)update; + + update = (update_t *)STRIP_TAG(update, TAG2); + assert(update != NULL); + + // If the update's version is not tagged it means the update is committed. + if (!IS_TAGGED(update->version, TAG1)) { + if (update->version <= txn->rv) + break; // success + continue; + } + + // If the update's version is tagged then either the update was aborted or the the version number is + // actually a pointer to a running transaction's txn_t. + + // Skip updates from aborted transactions. + if (EXPECT_FALSE(update->version == ABORTED_VERSION)) + continue; + + // The update's transaction is still in progress. Access its txn_t. + txn_t *writer = (txn_t *)STRIP_TAG(update->version, TAG1); + if (writer == txn) // found our own update + break; // success + + txn_state_e writer_state = writer->state; + if (writer_state == TXN_RUNNING) + continue; + + if (writer_state == TXN_VALIDATING) { + if (writer->wv > txn->rv) continue; + writer_state = txn_validate(writer); + } + + // Skip updates from aborted transactions. + if (writer_state == TXN_ABORTED) + continue; - if (writer_state == TXN_VALIDATING) { - writer_state = txn_validate(writer); + assert(writer_state == TXN_VALIDATED); + if (writer->wv > txn->rv) + continue; + break; // success + } + + uint64_t value = update->value; + + // collect some garbage + update_t *last = update; + update_t *next = update->next; + uint64_t min_active = 0; + if (IS_TAGGED(next, TAG2)) { + next = (update_t *)STRIP_TAG(next, TAG2); + min_active = (uint64_t)sl_min_key(active_); + if (next->version < min_active) { + + // Skip over aborted versions to verify the chain of updates is old enough for collection + update_t *temp = next; + while (temp->version == ABORTED_VERSION) { + assert(!IS_TAGGED(temp->version, TAG1)); + update_t *temp = next->next; + if (!IS_TAGGED(temp, TAG2)) + break; + temp = (update_t *)STRIP_TAG(temp, TAG2); + if (temp->version >= min_active) + return value; + temp = temp->next; } - if (writer_state == TXN_VALIDATED && writer->wv <= txn->rv && writer->wv != UNDETERMINED_VERSION) - return update->type == UPDATE_TYPE_DELETE ? DOES_NOT_EXIST : update->value; + // collect and all the update records following it + do { + next = SYNC_SWAP(&update->next, NULL); + + // if we find ourself in a race just back off and let the other thread take care of it + if (next == NULL) + return value; + + update = next; + next = next->next; + nbd_free(update); + } while (IS_TAGGED(next, TAG2)); } } - return DOES_NOT_EXIST; + + // If there is one item left and it is visible by all active transactions we can merge it into the map itself. + // There is no need for an update record. + if (next == NULL && last == (update_t *)STRIP_TAG(newest_update, TAG2)) { + if (min_active == UNDETERMINED_VERSION) { + min_active = (uint64_t)sl_min_key(active_); + } + if (last->version <= min_active) { + if (map_cas(txn->map, key, TAG_VALUE(last, TAG2), value) == TAG_VALUE(last, TAG2)) { + nbd_defer_free(last); + } + } + } + + return value; } void tm_set (txn_t *txn, void *key, uint64_t value) { + if (txn->state != TXN_RUNNING) + return; // TODO: return some sort of error code // create a new update record - update_rec_t *update = alloc_update_rec(); - update->type = UPDATE_TYPE_PUT; + update_t *update = alloc_update_rec(); update->value = value; - update->version = TAG_VALUE((uint64_t)txn); + update->version = TAG_VALUE(txn, TAG1); // push the new update record onto 's update list - uint64_t update_prev; + uint64_t old_update; do { - update->prev = (update_rec_t *) map_get(txn->map, key); - update_prev = (uint64_t)update->prev; - } while (map_cas(txn->map, key, update_prev, (uint64_t)update) != update_prev); + old_update = map_get(txn->map, key); + update->next = (update_t *)old_update; + } while (map_cas(txn->map, key, old_update, TAG_VALUE(update, TAG2)) != old_update); // add to the write set for commit-time validation if (txn->writes_count == txn->writes_size) { write_rec_t *w = nbd_malloc(sizeof(write_rec_t) * txn->writes_size * 2); memcpy(w, txn->writes, txn->writes_size * sizeof(write_rec_t)); txn->writes_size *= 2; + nbd_free(txn->writes); + txn->writes = w; } int i = txn->writes_count++; txn->writes[i].key = key;