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Improve documentation about reasoning behind the order of operations

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in GetSnapshotData, GetNewTransactionId, CommitTransaction, AbortTransaction,
etc.  Correct race condition in transaction status testing in
HeapTupleSatisfiesVacuum --- this wasn't important for old VACUUM with
exclusive lock on its table, but it sure is important now.  All per
pghackers discussion 7/11/01 and 7/12/01.
This commit is contained in:
Tom Lane
2001-07-16 22:43:34 +00:00
parent ffbd97c8ac
commit ed5c4e4a14
4 changed files with 110 additions and 49 deletions
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14
src/backend/access/transam/varsup.c
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@@ -6,7 +6,7 @@
* Copyright (c) 2000, PostgreSQL Global Development Group
*
* IDENTIFICATION
* $Header: /cvsroot/pgsql/src/backend/access/transam/varsup.c,v 1.41 2001/07/12 04:11:13 tgl Exp $
* $Header: /cvsroot/pgsql/src/backend/access/transam/varsup.c,v 1.42 2001/07/16 22:43:33 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -60,8 +60,16 @@ GetNewTransactionId(TransactionId *xid)
* XXX by storing xid into MyProc without acquiring SInvalLock, 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 at this writing, could cause a
* deadlock against GetSnapshotData). So for now, assume atomicity.
* would be a nasty concurrency hit (and in fact could cause a deadlock
* against GetSnapshotData). So for now, assume atomicity. Note that
* readers of PROC 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.
*
* A solution to the atomic-store problem would be to give each PROC
* its own spinlock used only for fetching/storing that PROC's xid.
* (SInvalLock would then mean primarily that PROCs couldn't be added/
* removed while holding the lock.)
*/
if (MyProc != (PROC *) NULL)
MyProc->xid = *xid;