From 02f377dbe50074a7016f4a476a8d15d294cd0874 Mon Sep 17 00:00:00 2001 From: Heikki Linnakangas Date: Wed, 18 Jan 2012 17:09:44 +0200 Subject: [PATCH] Fix corner case in cleanup of transactions using SSI. When the only remaining active transactions are READ ONLY, we do a "partial cleanup" of committed transactions because certain types of conflicts aren't possible anymore. For committed r/w transactions, we release the SIREAD locks but keep the SERIALIZABLEXACT. However, for committed r/o transactions, we can go further and release the SERIALIZABLEXACT too. The problem was with the latter case: we were returning the SERIALIZABLEXACT to the free list without removing it from the finished list. The only real change in the patch is the SHMQueueDelete line, but I also reworked some of the surrounding code to make it obvious that r/o and r/w transactions are handled differently -- the existing code felt a bit too clever. Dan Ports --- src/backend/storage/lmgr/predicate.c | 26 +++++++++++++++++++++++--- 1 file changed, 23 insertions(+), 3 deletions(-) diff --git a/src/backend/storage/lmgr/predicate.c b/src/backend/storage/lmgr/predicate.c index 1754180453..27124672da 100644 --- a/src/backend/storage/lmgr/predicate.c +++ b/src/backend/storage/lmgr/predicate.c @@ -3457,10 +3457,29 @@ ClearOldPredicateLocks(void) else if (finishedSxact->commitSeqNo > PredXact->HavePartialClearedThrough && finishedSxact->commitSeqNo <= PredXact->CanPartialClearThrough) { + /* + * Any active transactions that took their snapshot before this + * transaction committed are read-only, so we can clear part of + * its state. + */ LWLockRelease(SerializableXactHashLock); - ReleaseOneSerializableXact(finishedSxact, - !SxactIsReadOnly(finishedSxact), - false); + + if (SxactIsReadOnly(finishedSxact)) + { + /* A read-only transaction can be removed entirely */ + SHMQueueDelete(&(finishedSxact->finishedLink)); + ReleaseOneSerializableXact(finishedSxact, false, false); + } + else + { + /* + * A read-write transaction can only be partially + * cleared. We need to keep the SERIALIZABLEXACT but + * can release the SIREAD locks and conflicts in. + */ + ReleaseOneSerializableXact(finishedSxact, true, false); + } + PredXact->HavePartialClearedThrough = finishedSxact->commitSeqNo; LWLockAcquire(SerializableXactHashLock, LW_SHARED); } @@ -3566,6 +3585,7 @@ ReleaseOneSerializableXact(SERIALIZABLEXACT *sxact, bool partial, Assert(sxact != NULL); Assert(SxactIsRolledBack(sxact) || SxactIsCommitted(sxact)); + Assert(partial || !SxactIsOnFinishedList(sxact)); Assert(LWLockHeldByMe(SerializableFinishedListLock)); /* -- 2.40.0