diff options
Diffstat (limited to 'src/backend/storage/ipc/sinval.c')
| -rw-r--r-- | src/backend/storage/ipc/sinval.c | 60 |
1 files changed, 48 insertions, 12 deletions
diff --git a/src/backend/storage/ipc/sinval.c b/src/backend/storage/ipc/sinval.c index a9b9046702c..a93e91237f6 100644 --- a/src/backend/storage/ipc/sinval.c +++ b/src/backend/storage/ipc/sinval.c @@ -8,7 +8,7 @@ * * * IDENTIFICATION - * $Header: /cvsroot/pgsql/src/backend/storage/ipc/sinval.c,v 1.36 2001/07/12 04:11:13 tgl Exp $ + * $Header: /cvsroot/pgsql/src/backend/storage/ipc/sinval.c,v 1.37 2001/07/16 22:43:34 tgl Exp $ * *------------------------------------------------------------------------- */ @@ -193,8 +193,10 @@ TransactionIdIsInProgress(TransactionId xid) if (pOffset != INVALID_OFFSET) { PROC *proc = (PROC *) MAKE_PTR(pOffset); + /* Fetch xid just once - see GetNewTransactionId */ + TransactionId pxid = proc->xid; - if (TransactionIdEquals(proc->xid, xid)) + if (TransactionIdEquals(pxid, xid)) { result = true; break; @@ -236,9 +238,9 @@ GetXmaxRecent(TransactionId *XmaxRecent) if (pOffset != INVALID_OFFSET) { PROC *proc = (PROC *) MAKE_PTR(pOffset); - TransactionId xid; + /* Fetch xid just once - see GetNewTransactionId */ + TransactionId xid = proc->xid; - xid = proc->xid; if (! TransactionIdIsSpecial(xid)) { if (TransactionIdPrecedes(xid, result)) @@ -256,8 +258,19 @@ GetXmaxRecent(TransactionId *XmaxRecent) *XmaxRecent = result; } -/* +/*---------- * GetSnapshotData -- returns information about running transactions. + * + * The returned snapshot includes xmin (lowest still-running xact ID), + * xmax (next xact ID to be assigned), and a list of running xact IDs + * in the range xmin <= xid < xmax. It is used as follows: + * All xact IDs < xmin are considered finished. + * All xact IDs >= xmax are considered still running. + * For an xact ID xmin <= xid < xmax, consult list to see whether + * it is considered running or not. + * This ensures that the set of transactions seen as "running" by the + * current xact will not change after it takes the snapshot. + *---------- */ Snapshot GetSnapshotData(bool serializable) @@ -287,12 +300,33 @@ GetSnapshotData(bool serializable) elog(ERROR, "Memory exhausted in GetSnapshotData"); } - /* - * Unfortunately, we have to call ReadNewTransactionId() after - * acquiring SInvalLock above. It's not good because - * ReadNewTransactionId() does SpinAcquire(XidGenLockId) but - * _necessary_. + /*-------------------- + * Unfortunately, we have to call ReadNewTransactionId() after acquiring + * SInvalLock above. It's not good because ReadNewTransactionId() does + * SpinAcquire(XidGenLockId), but *necessary*. We need to be sure that + * no transactions exit the set of currently-running transactions + * between the time we fetch xmax and the time we finish building our + * snapshot. Otherwise we could have a situation like this: + * + * 1. Tx Old is running (in Read Committed mode). + * 2. Tx S reads new transaction ID into xmax, then + * is swapped out before acquiring SInvalLock. + * 3. Tx New gets new transaction ID (>= S' xmax), + * makes changes and commits. + * 4. Tx Old changes some row R changed by Tx New and commits. + * 5. Tx S finishes getting its snapshot data. It sees Tx Old as + * done, but sees Tx New as still running (since New >= xmax). + * + * Now S will see R changed by both Tx Old and Tx New, *but* does not + * see other changes made by Tx New. If S is supposed to be in + * Serializable mode, this is wrong. + * + * By locking SInvalLock before we read xmax, we ensure that TX Old + * cannot exit the set of running transactions seen by Tx S. Therefore + * both Old and New will be seen as still running => no inconsistency. + *-------------------- */ + ReadNewTransactionId(&(snapshot->xmax)); for (index = 0; index < segP->lastBackend; index++) @@ -302,6 +336,7 @@ GetSnapshotData(bool serializable) if (pOffset != INVALID_OFFSET) { PROC *proc = (PROC *) MAKE_PTR(pOffset); + /* Fetch xid just once - see GetNewTransactionId */ TransactionId xid = proc->xid; /* @@ -325,11 +360,12 @@ GetSnapshotData(bool serializable) if (serializable) MyProc->xmin = snapshot->xmin; - /* Serializable snapshot must be computed before any other... */ - Assert(MyProc->xmin != InvalidTransactionId); SpinRelease(SInvalLock); + /* Serializable snapshot must be computed before any other... */ + Assert(MyProc->xmin != InvalidTransactionId); + snapshot->xcnt = count; return snapshot; } |