-/*
+/*
* Written by Josh Dybnis and released to the public domain, as explained at
* http://creativecommons.org/licenses/publicdomain
*/
#include "common.h"
#include "txn.h"
#include "mem.h"
+#include "rcu.h"
+#include "lwt.h"
+#include "skiplist.h"
#define UNDETERMINED_VERSION 0
+#define ABORTED_VERSION TAG_VALUE(0, TAG1)
#define INITIAL_WRITES_SIZE 4
+#define PTR_TO_VAL(x) ((size_t)(x) >> 2)
+#define VAL_TO_PTR(x) ((update_t *)((x) << 2))
-typedef enum { TXN_RUNNING, TXN_VALIDATING, TXN_VALIDATED, TXN_ABORTED } txn_state_t;
-typedef enum { UPDATE_TYPE_PUT, UPDATE_TYPE_DELETE } update_type_t;
-
-typedef struct update_rec update_rec_t;
+typedef struct update_rec update_t;
+typedef map_key_t version_t;
struct update_rec {
- update_type_t type;
- uint64_t value;
- uint64_t version;
- update_rec_t *prev; // a previous update
+ version_t version; // tagged versions are txn_t pointers, untagged are actual version numbers
+ map_val_t value;
+ map_val_t next; // an earlier update
};
typedef struct write_rec {
- const char *key;
- update_rec_t *rec;
+ map_key_t key;
+ update_t *rec;
} write_rec_t;
struct txn {
- uint64_t rv;
- uint64_t wv;
- hashtable_t *ht;
+ version_t rv;
+ version_t wv;
+ map_t *map;
write_rec_t *writes;
- uint32_t writes_size;
- uint32_t writes_count;
- uint32_t writes_scan;
- txn_access_t access;
- txn_isolation_t isolation;
- txn_state_t state;
+ size_t writes_size;
+ size_t writes_count;
+ size_t writes_scan;
+ txn_state_e state;
};
-uint64_t GlobalVersion = 1;
-uint64_t MinActiveTxnVersion = 0;
-
-static txn_state_t txn_validate (txn_t *txn);
+static txn_state_e txn_validate (txn_t *txn);
-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));
- return u;
-}
+static skiplist_t *active_ = NULL;
-txn_t *txn_begin (txn_access_t access, txn_isolation_t isolation, hashtable_t *ht) {
- txn_t *txn = (txn_t *)nbd_malloc(sizeof(txn_t));
- memset(txn, 0, sizeof(txn_t));
- txn->access = access;
- txn->isolation = isolation;
- txn->rv = GlobalVersion;
- txn->wv = UNDETERMINED_VERSION;
- txn->state = TXN_RUNNING;
- txn->ht = ht;
- if (isolation != TXN_READ_ONLY) {
- txn->writes = nbd_malloc(sizeof(*txn->writes) * INITIAL_WRITES_SIZE);
- txn->writes_size = INITIAL_WRITES_SIZE;
- }
- return txn;
-}
-
-// Get most recent committed version prior to our read version.
-int64_t txn_ht_get (txn_t *txn, const char *key, uint32_t key_len) {
-
- // 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.
- update_rec_t *update = (update_rec_t *) ht_get(txn->ht, key, key_len);
- 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_t writer_state = writer->state;
- if (EXPECT_FALSE(writer_state == TXN_ABORTED))
- continue;
+static version_t version_ = 1;
- if (writer_state == TXN_VALIDATING) {
- writer_state = txn_validate(writer);
- }
+static inline skiplist_t *get_active (void) {
- if (writer_state == TXN_VALIDATED && writer->wv <= txn->rv && writer->wv != UNDETERMINED_VERSION)
- return update->type == UPDATE_TYPE_DELETE ? DOES_NOT_EXIST : update->value;
- }
- }
- return DOES_NOT_EXIST;
+ return active_;
}
-// 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.
-static txn_state_t txn_ht_validate_key (txn_t *txn, const char *key, uint32_t key_len) {
-
- update_rec_t *update = (update_rec_t *) ht_get(txn->ht, key, key_len);
- for (; update != NULL; update = update->prev) {
- uint64_t writer_version = update->version;
- if (writer_version <= txn->rv)
+// 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 validate_key (txn_t *txn, map_key_t key) {
+ assert(txn->state != TXN_RUNNING);
+
+ map_val_t val = map_get(txn->map, key);
+ update_t *update = NULL;
+ for (; val != DOES_NOT_EXIST; val = 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(val, 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)))
+ update = VAL_TO_PTR(val);
+ 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 *)VAL_TO_PTR(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 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 the next comment below for the explination 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;
-
- // 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_t writer_state = writer->state;
- if (EXPECT_FALSE(writer_state == TXN_ABORTED))
- 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;
}
-static txn_state_t txn_validate (txn_t *txn) {
+static txn_state_e txn_validate (txn_t *txn) {
+ assert(txn->state != TXN_RUNNING);
int i;
switch (txn->state) {
case TXN_VALIDATING:
if (txn->wv == UNDETERMINED_VERSION) {
- uint64_t wv = SYNC_ADD(&GlobalVersion, 1);
- SYNC_CAS(&txn->wv, UNDETERMINED_VERSION, wv);
+ version_t wv = SYNC_ADD(&version_, 1);
+ (void)SYNC_CAS(&txn->wv, UNDETERMINED_VERSION, wv);
}
for (i = 0; i < txn->writes_count; ++i) {
- txn_state_t s = txn_ht_validate_key(txn, txn->writes[i].key, strlen(txn->writes[i].key));
+ txn_state_e s = validate_key(txn, txn->writes[i].key);
if (s == TXN_ABORTED) {
txn->state = TXN_ABORTED;
break;
}
+ assert(s == TXN_VALIDATED);
}
if (txn->state == TXN_VALIDATING) {
txn->state = TXN_VALIDATED;
return txn->state;
}
+static update_t *alloc_update_rec (version_t ver, map_val_t val) {
+ update_t *u = (update_t *)nbd_malloc(sizeof(update_t));
+ u->version = ver;
+ u->value = val;
+ u->next = DOES_NOT_EXIST;
+ return u;
+}
+
+txn_t *txn_begin (map_t *map) {
+ TRACE("x1", "txn_begin: map %p", map, 0);
+ txn_t *txn = (txn_t *)nbd_malloc(sizeof(txn_t));
+ memset(txn, 0, sizeof(txn_t));
+ txn->wv = UNDETERMINED_VERSION;
+ txn->state = TXN_RUNNING;
+ txn->map = map;
+ txn->writes = nbd_malloc(sizeof(*txn->writes) * INITIAL_WRITES_SIZE);
+ txn->writes_size = INITIAL_WRITES_SIZE;
+ if (EXPECT_FALSE(active_ == NULL)) {
+ skiplist_t *a = sl_alloc(NULL);
+ if (SYNC_CAS(&active_, NULL, a) != NULL) {
+ sl_free(a);
+ }
+ }
+
+ // acquire the read version for txn. must be careful to avoid a race
+ do {
+ txn->rv = version_;
+
+ unsigned old_count;
+ unsigned temp = 0;
+ do {
+ old_count = temp;
+ temp = sl_cas(active_, txn->rv, old_count, old_count + 1);
+ } while (temp != old_count);
+
+ if (txn->rv == version_)
+ break;
+
+ temp = 1;
+ do {
+ old_count = temp;
+ temp = sl_cas(active_, (map_key_t)txn->rv, old_count, old_count - 1);
+ } while (temp != old_count);
+ } while (1);
+
+ TRACE("x1", "txn_begin: returning new transaction %p (read version %p)", txn, txn->rv);
+ return txn;
+}
+
void txn_abort (txn_t *txn) {
+ if (txn->state != TXN_RUNNING)
+ return;
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);
- nbd_defer_free(txn);
+ rcu_defer_free(txn->writes);
+ rcu_defer_free(txn);
}
-txn_state_t txn_commit (txn_t *txn) {
+txn_state_e txn_commit (txn_t *txn) {
+ if (txn->state != TXN_RUNNING)
+ return txn->state;
assert(txn->state == TXN_RUNNING);
txn->state = TXN_VALIDATING;
- txn_state_t state = txn_validate(txn);
+ txn_state_e state = txn_validate(txn);
// Detach <txn> from its updates.
- uint64_t wv = (txn->state == TXN_ABORTED) ? TAG_VALUE(0) : txn->wv;
+ version_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 = txn->writes[i].rec;
update->version = wv;
}
- nbd_defer_free(txn->writes);
- nbd_defer_free(txn);
+ // Lower the reference count for <txn>'s read version
+ unsigned temp = 2;
+ unsigned old_count;
+ do {
+ old_count = temp;
+ temp = sl_cas(active_, (map_key_t)txn->rv, old_count, old_count - 1);
+ if (temp == 1 && txn->rv != version_) {
+ sl_remove(active_, (map_key_t)txn->rv);
+ break;
+ }
+ } while (old_count != temp);
+
+ rcu_defer_free(txn->writes);
+ rcu_defer_free(txn);
return state;
}
-void txn_ht_put (txn_t *txn, const char *key, uint32_t key_len, int64_t value) {
+// Get most recent committed version prior to our read version.
+map_val_t txn_map_get (txn_t *txn, map_key_t key) {
+ TRACE("x1", "txn_map_get: txn %p map %p", txn, txn->map);
+ TRACE("x1", "txn_map_get: key %p", key, 0);
+
+ if (txn->state != TXN_RUNNING) {
+ TRACE("x1", "txn_map_get: error txn not running (state %p)", txn->state, 0);
+ return ERROR_TXN_NOT_RUNNING;
+ }
+
+ // Iterate through the update records to find the latest committed version prior to our read version.
+ map_val_t newest_val = map_get(txn->map, key);
+ map_val_t val = newest_val;
+ update_t *update;
+ for ( ; (update = VAL_TO_PTR(val)) != NULL ; val = update->next) {
+
+ // If TAG2 is set in <val> it indicates that <val> is an update record. Otherwise all the following are
+ // true: <val> is a literal value, it is older than any currently active transaction, and it is the most
+ // recently set value for its key. Therefore it is visible to <txn>.
+ if (!IS_TAGGED(val, TAG2)) {
+ TRACE("x1", "txn_map_get: found untagged value; returning %p", val, 0);
+ return val;
+ }
+
+ // 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) {
+ TRACE("x2", "txn_map_get: found committed update %p (version %p)", update, update->version);
+ break; // success
+ }
+ TRACE("x2", "txn_map_get: skipping update %p (version %p)", update, update->version);
+ 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)) {
+ TRACE("x2", "txn_map_get: skipping aborted update %p", update, 0);
+ continue;
+ }
+
+ // The update's transaction is still in progress. Access its txn_t.
+ txn_t *writer = (txn_t *)VAL_TO_PTR(update->version);
+ if (writer == txn) {
+ TRACE("x2", "txn_map_get: found txn's own update %p", update, 0);
+ break; // success
+ }
+
+ txn_state_e writer_state = writer->state;
+ if (writer_state == TXN_RUNNING) {
+ TRACE("x2", "txn_map_get: skipping update %p of in-progress transaction %p", update, writer);
+ continue;
+ }
+
+ if (writer_state == TXN_VALIDATING) {
+ TRACE("x2", "txn_map_get: update %p transaction %p validating", update, writer);
+ if (writer->wv > txn->rv)
+ continue;
+ writer_state = txn_validate(writer);
+ }
+
+ // Skip updates from aborted transactions.
+ if (writer_state == TXN_ABORTED) {
+ TRACE("x2", "txn_map_get: skipping aborted update %p", update, 0);
+ continue;
+ }
+
+ assert(writer_state == TXN_VALIDATED);
+ if (writer->wv > txn->rv) {
+ TRACE("x2", "txn_map_get: skipping update %p (version %p)", update, update->version);
+ continue;
+ }
+ break; // success
+ }
+
+ if (update == NULL) {
+ TRACE("x1", "txn_map_get: key does not exist in map", key, 0);
+ return DOES_NOT_EXIST;
+ }
+
+ map_val_t value = update->value;
+ TRACE("x1", "txn_map_get: key found returning value %p", value, 0);
+
+ return value;
+ // collect some garbage
+ version_t min_active_version = UNDETERMINED_VERSION;
+ update_t *next_update = NULL;
+ if (IS_TAGGED(update->next, TAG2)) {
+ next_update = VAL_TO_PTR(update->next);
+
+ // If <next_update> (and all update records following it [execpt if it is aborted]) is old enough
+ // that it is not visible to any active transaction we can safely free it.
+ min_active_version = (version_t)sl_min_key(active_);
+ if (next_update->version < min_active_version) {
+
+ // If the <next_update> is aborted, skip over it to look for more recent ones that may follow
+ update_t *temp = next_update;
+ while (temp->version == ABORTED_VERSION) {
+ assert(!IS_TAGGED(temp->version, TAG1));
+ map_val_t next = temp->next;
+ if (!IS_TAGGED(next, TAG2))
+ break;
+
+ // Bail out of garbage collection if we find a record that might still be accessed by an
+ // ongoing transaction.
+ if (VAL_TO_PTR(next)->version >= min_active_version)
+ return value;
+
+ temp = VAL_TO_PTR(next);
+ }
+
+ // free the next update record and all the ones following it
+ temp = next_update;
+ map_val_t next;
+ do {
+ next = SYNC_SWAP(&temp->next, DOES_NOT_EXIST);
+
+ // if we find ourself in a race just back off and let the other thread take care of it
+ if (next == DOES_NOT_EXIST)
+ return value;
+
+ nbd_free(temp);
+
+ temp = VAL_TO_PTR(next);
+
+ } while (IS_TAGGED(next, TAG2));
+ }
+ }
+
+ // 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_update == NULL && val == newest_val) {
+ if (min_active_version == UNDETERMINED_VERSION) {
+ min_active_version = (version_t)sl_min_key(active_);
+ }
+ if (update->version <= min_active_version) {
+ if (map_cas(txn->map, key, TAG_VALUE(val, TAG2), value) == TAG_VALUE(val, TAG2)) {
+ rcu_defer_free(update);
+ }
+ }
+ }
+
+ return value;
+}
+
+void txn_map_set (txn_t *txn, map_key_t key, map_val_t value) {
+ TRACE("x1", "txn_map_set: txn %p map %p", txn, txn->map);
+ TRACE("x1", "txn_map_set: key %p value %p", key, value);
+ assert(!IS_TAGGED(value, TAG1) && !IS_TAGGED(value, TAG2));
+
+ if (txn->state != TXN_RUNNING) {
+ TRACE("x1", "txn_map_set: error txn not running (state %p)", txn->state, 0);
+ return;
+ }
// create a new update record
- update_rec_t *update = alloc_update_rec();
- update->type = UPDATE_TYPE_PUT;
- update->value = value;
- update->version = TAG_VALUE((uint64_t)txn);
+ version_t ver = TAG_VALUE(PTR_TO_VAL(txn), TAG1); // tagged versions are txn_t pointers
+ update_t *update = alloc_update_rec(ver, value);
// push the new update record onto <key>'s update list
- int64_t update_prev;
+ map_val_t old_update = map_get(txn->map, key);
+ TRACE("x2", "txn_map_set: old update %p new update record %p", old_update, update);
do {
- update->prev = (update_rec_t *) ht_get(txn->ht, key, key_len);
- update_prev = (int64_t)update->prev;
- } while (ht_compare_and_set(txn->ht, key, key_len, update_prev, (int64_t)update) != update_prev);
+ update->next = old_update;
+ map_val_t temp = map_cas(txn->map, key, old_update, TAG_VALUE(PTR_TO_VAL(update), TAG2));
+ if (temp == old_update)
+ break;
+
+ TRACE("x1", "txn_map_set: cas failed; found %p expected %p", temp, old_update);
+ old_update = temp;
+ } while (1);
// add <key> 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;
txn->writes[i].rec = update;
}
-
-#ifdef MAKE_txn_test
-#include "runtime.h"
-int main (void) {
- nbd_init();
- return 0;
-}
-#endif//txn_test
projects / nbds / blobdiff