#include "skiplist.h"
#define UNDETERMINED_VERSION 0
+#define ABORTED_VERSION TAG_VALUE(0)
#define INITIAL_WRITES_SIZE 4
typedef enum { UPDATE_TYPE_PUT, UPDATE_TYPE_DELETE } update_type_t;
update_type_t type;
uint64_t value;
uint64_t version;
- update_rec_t *prev; // a previous update
+ update_rec_t *next; // an earlier update
};
typedef struct write_rec {
active_ = map_alloc(&sl_map_impl, NULL);
}
-// Validate the updates for <key>. Validation fails for a key we have written to if there is a
-// write committed newer than our read version.
+// Validate the updates for <key>. 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) {
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 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))
+ for (; update != NULL; update = update->next) {
+
+ // If the update's 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->version))
+ 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);
+ if (writer == txn)
+ continue; // Skip our own updates.
+ txn_state_e writer_state = writer->state;
+
+ // Any running transaction will only be able to aquire 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 <writer> 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 <writer> if it is a committing transaction that might cause us to
- // abort. However, if the <writer> 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 <writer> commit. We need to know if <writer> 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;
txn->writes_size = INITIAL_WRITES_SIZE;
}
- // aquire the read version for txn.
+ // aquire the read version for txn. must be careful to avoid a race
do {
txn->rv = version_;
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->version = ABORTED_VERSION;
}
nbd_defer_free(txn->writes);
txn_state_e state = txn_validate(txn);
// Detach <txn> 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->version = wv;
}
+ /*
+ // Lower the reference count for <txn>'s read version
+ uint64_t temp = 1;
+ uint64_t old_count;
+ do {
+ old_count = temp;
+ temp = map_cas(active_, (void *)txn->rv, old_count, old_count - 1);
+ } while (old_count != temp);
+ if (old_count == 0 && version_ != txn->rv) {
+ map_remove(active_, (void *)txn->rv);
+ }
+ */
+
nbd_defer_free(txn->writes);
nbd_defer_free(txn);
// Get most recent committed version prior to our read version.
uint64_t tm_get (txn_t *txn, void *key) {
- // Iterate through update records associated with <key> to find the latest committed version.
- // We can use the first matching version. Older updates always come later in the list.
+ // Iterate through update records associated with <key> to find the latest committed version prior to our
+ // read version.
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))
- continue;
+ for (; update != NULL; update = update->next) {
- if (writer_state == TXN_VALIDATING) {
- writer_state = txn_validate(writer);
- }
+ // If the update's version is not tagged it means the update is committed.
+ if (!IS_TAGGED(update->version)) {
+ if (update->version <= txn->rv)
+ return update->value;
+ 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.
- if (writer_state == TXN_VALIDATED && writer->wv <= txn->rv && writer->wv != UNDETERMINED_VERSION)
- return update->type == UPDATE_TYPE_DELETE ? DOES_NOT_EXIST : update->value;
+ // 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);
+ if (writer == txn) // found our own update
+ return update->type == UPDATE_TYPE_DELETE ? DOES_NOT_EXIST : update->value;
+
+ 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;
+
+ assert(writer_state == TXN_VALIDATED);
+ if (writer->wv > txn->rv)
+ continue;
+ return update->value;
}
return DOES_NOT_EXIST;
}
// push the new update record onto <key>'s update list
uint64_t update_prev;
do {
- update->prev = (update_rec_t *) map_get(txn->map, key);
- update_prev = (uint64_t)update->prev;
+ update->next = (update_rec_t *) map_get(txn->map, key);
+ update_prev = (uint64_t)update->next;
} while (map_cas(txn->map, key, update_prev, (uint64_t)update) != update_prev);
// add <key> to the write set for commit-time validation
projects / nbds / blobdiff