Arrange for GetSnapshotData to copy live-subtransaction XIDs from the

PGPROC array into snapshots, and use this information to avoid visits
to pg_subtrans in HeapTupleSatisfiesSnapshot.  This appears to solve
the pg_subtrans-related context swap storm problem that's been reported
by several people for 8.1.  While at it, modify GetSnapshotData to not
take an exclusive lock on ProcArrayLock, as closer analysis shows that
shared lock is always sufficient.
Itagaki Takahiro and Tom Lane

1 files changed, 17 insertions, 6 deletions

diff --git a/src/backend/access/transam/varsup.c b/src/backend/access/transam/varsup.c
index 443dab4b0eb..8db94f2a924 100644
--- a/src/backend/access/transam/varsup.c
+++ b/src/backend/access/transam/varsup.c
@@ -6,7 +6,7 @@
  * Copyright (c) 2000-2006, PostgreSQL Global Development Group
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/transam/varsup.c,v 1.72 2006/07/14 14:52:17 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/transam/varsup.c,v 1.73 2006/09/03 15:59:38 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -131,17 +131,28 @@ GetNewTransactionId(bool isSubXact)
 	 */
 	if (MyProc != NULL)
 	{
+		/*
+		 * Use volatile pointer to prevent code rearrangement; other backends
+		 * could be examining my subxids info concurrently, and we don't
+		 * want them to see an invalid intermediate state, such as
+		 * incrementing nxids before filling the array entry.  Note we are
+		 * assuming that TransactionId and int fetch/store are atomic.
+		 */
+		volatile PGPROC *myproc = MyProc;
+
 		if (!isSubXact)
-			MyProc->xid = xid;
+			myproc->xid = xid;
 		else
 		{
-			if (MyProc->subxids.nxids < PGPROC_MAX_CACHED_SUBXIDS)
+			int		nxids = myproc->subxids.nxids;
+
+			if (nxids < PGPROC_MAX_CACHED_SUBXIDS)
 			{
-				MyProc->subxids.xids[MyProc->subxids.nxids] = xid;
-				MyProc->subxids.nxids++;
+				myproc->subxids.xids[nxids] = xid;
+				myproc->subxids.nxids = nxids + 1;
 			}
 			else
-				MyProc->subxids.overflowed = true;
+				myproc->subxids.overflowed = true;
 		}
 	}