-/*
+/*
* Written by Josh Dybnis and released to the public domain, as explained at
* http://creativecommons.org/licenses/publicdomain
*/
#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 struct update_rec update_t;
typedef map_key_t version_t;
struct update_rec {
- version_t version;
+ 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 {
map_key_t key;
- update_t *rec;
+ update_t *rec;
} write_rec_t;
struct txn {
static skiplist_t *active_ = NULL;
-void txn_init (void) {
+__attribute__ ((constructor)) void txn_init (void) {
active_ = sl_alloc(NULL);
}
-// Validate the updates for <key>. Validation fails if there is a write-write conflict. That is if after our
+// 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
+// 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
+ // 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;
- update = (update_t *)STRIP_TAG(val, TAG2);
- if (!IS_TAGGED(update->version, TAG1))
+ 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
+ // 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.
continue;
// The update's transaction is still in progress. Access its txn_t.
- txn_t *writer = (txn_t *)STRIP_TAG(update->version, TAG1);
+ 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
+ // 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.
+ // greater than ours. See the next comment below for the explination why.
if (writer_state == TXN_RUNNING)
- continue;
-
+ 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
+ // 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)
return txn->state;
}
-static update_t *alloc_update_rec (void) {
+static update_t *alloc_update_rec (version_t ver, map_val_t val) {
update_t *u = (update_t *)nbd_malloc(sizeof(update_t));
- memset(u, 0, 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;
} 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; // TODO: return some sort of error code
+ return;
int i;
for (i = 0; i < txn->writes_count; ++i) {
txn_state_e txn_commit (txn_t *txn) {
if (txn->state != TXN_RUNNING)
- return txn->state; // TODO: return some sort of error code
+ return txn->state;
assert(txn->state == TXN_RUNNING);
txn->state = TXN_VALIDATING;
version_t wv = (txn->state == TXN_ABORTED) ? ABORTED_VERSION : txn->wv;
int i;
for (i = 0; i < txn->writes_count; ++i) {
- update_t *update = (update_t *)txn->writes[i].rec;
+ update_t *update = txn->writes[i].rec;
update->version = wv;
}
// Get most recent committed version prior to our read version.
map_val_t txn_map_get (txn_t *txn, map_key_t key) {
- if (txn->state != TXN_RUNNING)
+ 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.
+ // 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 = NULL;
- for ( ; ; val = update->next) {
-
- if (!IS_TAGGED(val, TAG2))
+ 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;
-
- update = (update_t *)STRIP_TAG(val, 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)
+ 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
+ // 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))
+ 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 *)STRIP_TAG(update->version, TAG1);
- if (writer == txn) // found our own update
- break; // success
+ 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)
- continue;
+ 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)
+ 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)
+ 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 = (update_t *)STRIP_TAG(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) {
- // <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.
- // Skip over aborted versions to look for more recent updates
+ // 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 = next_update->next;
+ map_val_t next = temp->next;
if (!IS_TAGGED(next, TAG2))
break;
- temp = (update_t *)STRIP_TAG(next, TAG2);
- if (temp->version >= min_active_version)
+ // 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 <next> and all the update records following it
+ // free the next update record and all the ones following it
temp = next_update;
- while (1) {
- map_val_t next = SYNC_SWAP(&temp->next, DOES_NOT_EXIST);
+ 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)
+ if (next == DOES_NOT_EXIST)
return value;
- if (!IS_TAGGED(next, TAG2))
- break;
+ nbd_free(temp);
- temp = (update_t *)STRIP_TAG(next, TAG2);
- nbd_free(update);
- }
+ temp = VAL_TO_PTR(next);
+
+ } while (IS_TAGGED(next, TAG2));
}
}
}
}
}
-
+
return value;
}
void txn_map_set (txn_t *txn, map_key_t key, map_val_t value) {
- if (txn->state != TXN_RUNNING)
- return; // TODO: return some sort of error code
+ 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_t *update = alloc_update_rec();
- update->value = value;
- update->version = TAG_VALUE((version_t)txn, TAG1);
+ 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
- map_val_t old_update;
+ 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 {
- old_update = map_get(txn->map, key);
update->next = old_update;
- } while (map_cas(txn->map, key, old_update, TAG_VALUE((map_val_t)update, TAG2)) != 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) {
projects / nbds / blobdiff