diff options
Diffstat (limited to 'src/backend/access/transam/varsup.c')
| -rw-r--r-- | src/backend/access/transam/varsup.c | 31 |
1 files changed, 18 insertions, 13 deletions
diff --git a/src/backend/access/transam/varsup.c b/src/backend/access/transam/varsup.c index c9754581ec7..14332c6ab26 100644 --- a/src/backend/access/transam/varsup.c +++ b/src/backend/access/transam/varsup.c @@ -6,7 +6,7 @@ * Copyright (c) 2000-2007, PostgreSQL Global Development Group * * IDENTIFICATION - * $PostgreSQL: pgsql/src/backend/access/transam/varsup.c,v 1.78 2007/02/15 23:23:22 alvherre Exp $ + * $PostgreSQL: pgsql/src/backend/access/transam/varsup.c,v 1.79 2007/09/08 20:31:14 tgl Exp $ * *------------------------------------------------------------------------- */ @@ -31,7 +31,9 @@ VariableCache ShmemVariableCache = NULL; /* - * Allocate the next XID for my new transaction. + * Allocate the next XID for my new transaction or subtransaction. + * + * The new XID is also stored into MyProc before returning. */ TransactionId GetNewTransactionId(bool isSubXact) @@ -43,7 +45,11 @@ GetNewTransactionId(bool isSubXact) * transaction id. */ if (IsBootstrapProcessingMode()) + { + Assert(!isSubXact); + MyProc->xid = BootstrapTransactionId; return BootstrapTransactionId; + } LWLockAcquire(XidGenLock, LW_EXCLUSIVE); @@ -112,19 +118,19 @@ GetNewTransactionId(bool isSubXact) TransactionIdAdvance(ShmemVariableCache->nextXid); /* - * We must store the new XID into the shared PGPROC array before releasing - * XidGenLock. This ensures that when GetSnapshotData calls - * ReadNewTransactionId, all active XIDs before the returned value of - * nextXid are already present in PGPROC. Else we have a race condition. + * We must store the new XID into the shared ProcArray before releasing + * XidGenLock. This ensures that every active XID older than + * latestCompletedXid is present in the ProcArray, which is essential + * for correct OldestXmin tracking; see src/backend/access/transam/README. * * XXX by storing xid into MyProc without acquiring ProcArrayLock, we are * relying on fetch/store of an xid to be atomic, else other backends * might see a partially-set xid here. But holding both locks at once - * would be a nasty concurrency hit (and in fact could cause a deadlock - * against GetSnapshotData). So for now, assume atomicity. Note that - * readers of PGPROC xid field should be careful to fetch the value only - * once, rather than assume they can read it multiple times and get the - * same answer each time. + * would be a nasty concurrency hit. So for now, assume atomicity. + * + * Note that readers of PGPROC xid fields should be careful to fetch the + * value only once, rather than assume they can read a value multiple + * times and get the same answer each time. * * The same comments apply to the subxact xid count and overflow fields. * @@ -138,11 +144,10 @@ GetNewTransactionId(bool isSubXact) * race-condition window, in that the new XID will not appear as running * until its parent link has been placed into pg_subtrans. However, that * will happen before anyone could possibly have a reason to inquire about - * the status of the XID, so it seems OK. (Snapshots taken during this + * the status of the XID, so it seems OK. (Snapshots taken during this * window *will* include the parent XID, so they will deliver the correct * answer later on when someone does have a reason to inquire.) */ - if (MyProc != NULL) { /* * Use volatile pointer to prevent code rearrangement; other backends |