diff options
Diffstat (limited to 'src/backend/access/transam/varsup.c')
| -rw-r--r-- | src/backend/access/transam/varsup.c | 49 |
1 files changed, 37 insertions, 12 deletions
diff --git a/src/backend/access/transam/varsup.c b/src/backend/access/transam/varsup.c index 9d3b0b323aa..81b60c9fda6 100644 --- a/src/backend/access/transam/varsup.c +++ b/src/backend/access/transam/varsup.c @@ -6,7 +6,7 @@ * Copyright (c) 2000-2003, PostgreSQL Global Development Group * * IDENTIFICATION - * $PostgreSQL: pgsql/src/backend/access/transam/varsup.c,v 1.56 2004/07/01 00:49:42 tgl Exp $ + * $PostgreSQL: pgsql/src/backend/access/transam/varsup.c,v 1.57 2004/08/01 17:32:13 tgl Exp $ * *------------------------------------------------------------------------- */ @@ -68,10 +68,10 @@ GetNewTransactionId(bool isSubXact) TransactionIdAdvance(ShmemVariableCache->nextXid); /* - * Must set MyProc->xid before releasing XidGenLock. This ensures - * that when GetSnapshotData calls ReadNewTransactionId, all active - * XIDs before the returned value of nextXid are already present in - * the shared PGPROC array. Else we have a race condition. + * 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. * * XXX by storing xid into MyProc without acquiring SInvalLock, we are * relying on fetch/store of an xid to be atomic, else other backends @@ -82,16 +82,41 @@ GetNewTransactionId(bool isSubXact) * the value only once, rather than assume they can read it multiple * times and get the same answer each time. * + * The same comments apply to the subxact xid count and overflow fields. + * * A solution to the atomic-store problem would be to give each PGPROC - * its own spinlock used only for fetching/storing that PGPROC's xid. - * (SInvalLock would then mean primarily that PGPROCs couldn't be added/ - * removed while holding the lock.) + * its own spinlock used only for fetching/storing that PGPROC's xid + * and related fields. (SInvalLock would then mean primarily that + * PGPROCs couldn't be added/removed while holding the lock.) * - * We don't want a subtransaction to update the stored Xid; we'll check - * if a transaction Xid is a running subxact by checking pg_subtrans. + * If there's no room to fit a subtransaction XID into PGPROC, set the + * cache-overflowed flag instead. This forces readers to look in + * pg_subtrans to map subtransaction XIDs up to top-level XIDs. + * There is a 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 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 && !isSubXact) - MyProc->xid = xid; + if (MyProc != NULL) + { + if (!isSubXact) + MyProc->xid = xid; + else + { + if (MyProc->subxids.nxids < PGPROC_MAX_CACHED_SUBXIDS) + { + MyProc->subxids.xids[MyProc->subxids.nxids] = xid; + MyProc->subxids.nxids++; + } + else + { + MyProc->subxids.overflowed = true; + } + } + } LWLockRelease(XidGenLock); |