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Commitcb2d158

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Fix locking while setting flags in MySerializableXact.
Even if a flag is modified only by the backend owning the transaction, it'snot safe to modify it without a lock. Another backend might be setting orclearing a different flag in the flags field concurrently, and thatoperation might be lost because setting or clearing a bit in a word is notatomic.Make did-write flag a simple backend-private boolean variable, because itwas only set or tested in the owning backend (except when committing aprepared transaction, but it's not worthwhile to optimize for the case of aread-only prepared transaction). This also eliminates the need to addlocking where that flag is set.Also, set the did-write flag when doing DDL operations like DROP TABLE orTRUNCATE -- that was missed earlier.
1 parentd69149e commitcb2d158

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2 files changed

+61
-39
lines changed

2 files changed

+61
-39
lines changed

‎src/backend/storage/lmgr/predicate.c

Lines changed: 54 additions & 31 deletions
Original file line numberDiff line numberDiff line change
@@ -392,10 +392,11 @@ static HTAB *LocalPredicateLockHash = NULL;
392392

393393
/*
394394
* Keep a pointer to the currently-running serializable transaction (if any)
395-
* for quick reference.
396-
*TODO SSI: Remove volatile qualifier and the then-unnecessary casts?
395+
* for quick reference. Also, remember if we have written anything that could
396+
*cause a rw-conflict.
397397
*/
398-
staticvolatileSERIALIZABLEXACT*MySerializableXact=InvalidSerializableXact;
398+
staticSERIALIZABLEXACT*MySerializableXact=InvalidSerializableXact;
399+
staticboolMyXactDidWrite= false;
399400

400401
/* local functions */
401402

@@ -1424,20 +1425,30 @@ GetSafeSnapshot(Snapshot origSnapshot)
14241425
if (MySerializableXact==InvalidSerializableXact)
14251426
returnsnapshot;/* no concurrent r/w xacts; it's safe */
14261427

1427-
MySerializableXact->flags |=SXACT_FLAG_DEFERRABLE_WAITING;
1428+
LWLockAcquire(SerializableXactHashLock,LW_EXCLUSIVE);
14281429

14291430
/*
14301431
* Wait for concurrent transactions to finish. Stop early if one of
14311432
* them marked us as conflicted.
14321433
*/
1434+
MySerializableXact->flags |=SXACT_FLAG_DEFERRABLE_WAITING;
14331435
while (!(SHMQueueEmpty((SHM_QUEUE*)
14341436
&MySerializableXact->possibleUnsafeConflicts)||
14351437
SxactIsROUnsafe(MySerializableXact)))
1438+
{
1439+
LWLockRelease(SerializableXactHashLock);
14361440
ProcWaitForSignal();
1437-
1441+
LWLockAcquire(SerializableXactHashLock,LW_EXCLUSIVE);
1442+
}
14381443
MySerializableXact->flags &= ~SXACT_FLAG_DEFERRABLE_WAITING;
1444+
14391445
if (!SxactIsROUnsafe(MySerializableXact))
1446+
{
1447+
LWLockRelease(SerializableXactHashLock);
14401448
break;/* success */
1449+
}
1450+
1451+
LWLockRelease(SerializableXactHashLock);
14411452

14421453
/* else, need to retry... */
14431454
ereport(DEBUG2,
@@ -1600,6 +1611,7 @@ RegisterSerializableTransactionInt(Snapshot snapshot)
16001611
}
16011612

16021613
MySerializableXact=sxact;
1614+
MyXactDidWrite= false;/* haven't written anything yet */
16031615

16041616
LWLockRelease(SerializableXactHashLock);
16051617

@@ -1635,24 +1647,24 @@ RegisterPredicateLockingXid(const TransactionId xid)
16351647
if (MySerializableXact==InvalidSerializableXact)
16361648
return;
16371649

1638-
/* This should only be done once per transaction. */
1639-
Assert(MySerializableXact->topXid==InvalidTransactionId);
1640-
16411650
/* We should have a valid XID and be at the top level. */
16421651
Assert(TransactionIdIsValid(xid));
16431652

1653+
LWLockAcquire(SerializableXactHashLock,LW_EXCLUSIVE);
1654+
1655+
/* This should only be done once per transaction. */
1656+
Assert(MySerializableXact->topXid==InvalidTransactionId);
1657+
16441658
MySerializableXact->topXid=xid;
16451659

16461660
sxidtag.xid=xid;
1647-
LWLockAcquire(SerializableXactHashLock,LW_EXCLUSIVE);
16481661
sxid= (SERIALIZABLEXID*)hash_search(SerializableXidHash,
16491662
&sxidtag,
16501663
HASH_ENTER,&found);
1651-
Assert(sxid!=NULL);
16521664
Assert(!found);
16531665

16541666
/* Initialize the structure. */
1655-
sxid->myXact=(SERIALIZABLEXACT*)MySerializableXact;
1667+
sxid->myXact=MySerializableXact;
16561668
LWLockRelease(SerializableXactHashLock);
16571669
}
16581670

@@ -1881,7 +1893,7 @@ DeleteChildTargetLocks(const PREDICATELOCKTARGETTAG *newtargettag)
18811893
PREDICATELOCK*predlock;
18821894

18831895
LWLockAcquire(SerializablePredicateLockListLock,LW_SHARED);
1884-
sxact=(SERIALIZABLEXACT*)MySerializableXact;
1896+
sxact=MySerializableXact;
18851897
predlock= (PREDICATELOCK*)
18861898
SHMQueueNext(&(sxact->predicateLocks),
18871899
&(sxact->predicateLocks),
@@ -2200,8 +2212,7 @@ PredicateLockAcquire(const PREDICATELOCKTARGETTAG *targettag)
22002212
locallock->childLocks=0;
22012213

22022214
/* Actually create the lock */
2203-
CreatePredicateLock(targettag,targettaghash,
2204-
(SERIALIZABLEXACT*)MySerializableXact);
2215+
CreatePredicateLock(targettag,targettaghash,MySerializableXact);
22052216

22062217
/*
22072218
* Lock has been acquired. Check whether it should be promoted to a
@@ -3042,7 +3053,7 @@ ReleasePredicateLocks(const bool isCommit)
30423053
Assert(IsolationIsSerializable());
30433054

30443055
/* We'd better not already be on the cleanup list. */
3045-
Assert(!SxactIsOnFinishedList((SERIALIZABLEXACT*)MySerializableXact));
3056+
Assert(!SxactIsOnFinishedList(MySerializableXact));
30463057

30473058
topLevelIsDeclaredReadOnly=SxactIsReadOnly(MySerializableXact);
30483059

@@ -3070,7 +3081,7 @@ ReleasePredicateLocks(const bool isCommit)
30703081
MySerializableXact->flags |=SXACT_FLAG_COMMITTED;
30713082
MySerializableXact->commitSeqNo=++(PredXact->LastSxactCommitSeqNo);
30723083
/* Recognize implicit read-only transaction (commit without write). */
3073-
if (!(MySerializableXact->flags&SXACT_FLAG_DID_WRITE))
3084+
if (!MyXactDidWrite)
30743085
MySerializableXact->flags |=SXACT_FLAG_READ_ONLY;
30753086
}
30763087
else
@@ -3218,7 +3229,7 @@ ReleasePredicateLocks(const bool isCommit)
32183229

32193230
/* Mark conflicted if necessary. */
32203231
if (isCommit
3221-
&&(MySerializableXact->flags&SXACT_FLAG_DID_WRITE)
3232+
&&MyXactDidWrite
32223233
&&SxactHasConflictOut(MySerializableXact)
32233234
&& (MySerializableXact->SeqNo.earliestOutConflictCommit
32243235
<=roXact->SeqNo.lastCommitBeforeSnapshot))
@@ -3282,15 +3293,15 @@ ReleasePredicateLocks(const bool isCommit)
32823293
(SHM_QUEUE*)&(MySerializableXact->finishedLink));
32833294

32843295
if (!isCommit)
3285-
ReleaseOneSerializableXact((SERIALIZABLEXACT*)MySerializableXact,
3286-
false, false);
3296+
ReleaseOneSerializableXact(MySerializableXact, false, false);
32873297

32883298
LWLockRelease(SerializableFinishedListLock);
32893299

32903300
if (needToClear)
32913301
ClearOldPredicateLocks();
32923302

32933303
MySerializableXact=InvalidSerializableXact;
3304+
MyXactDidWrite= false;
32943305

32953306
/* Delete per-transaction lock table */
32963307
if (LocalPredicateLockHash!=NULL)
@@ -3851,7 +3862,7 @@ CheckForSerializableConflictOut(const bool visible, const Relation relation,
38513862
return;
38523863
}
38533864

3854-
if (RWConflictExists((SERIALIZABLEXACT*)MySerializableXact,sxact))
3865+
if (RWConflictExists(MySerializableXact,sxact))
38553866
{
38563867
/* We don't want duplicate conflict records in the list. */
38573868
LWLockRelease(SerializableXactHashLock);
@@ -3862,7 +3873,7 @@ CheckForSerializableConflictOut(const bool visible, const Relation relation,
38623873
* Flag the conflict. But first, if this conflict creates a dangerous
38633874
* structure, ereport an error.
38643875
*/
3865-
FlagRWConflict((SERIALIZABLEXACT*)MySerializableXact,sxact);
3876+
FlagRWConflict(MySerializableXact,sxact);
38663877
LWLockRelease(SerializableXactHashLock);
38673878
}
38683879

@@ -3944,7 +3955,7 @@ CheckTargetForConflictsIn(PREDICATELOCKTARGETTAG *targettag)
39443955
&& (!SxactIsCommitted(sxact)
39453956
||TransactionIdPrecedes(GetTransactionSnapshot()->xmin,
39463957
sxact->finishedBefore))
3947-
&& !RWConflictExists(sxact, (SERIALIZABLEXACT*)MySerializableXact))
3958+
&& !RWConflictExists(sxact,MySerializableXact))
39483959
{
39493960
LWLockRelease(SerializableXactHashLock);
39503961
LWLockAcquire(SerializableXactHashLock,LW_EXCLUSIVE);
@@ -3957,10 +3968,9 @@ CheckTargetForConflictsIn(PREDICATELOCKTARGETTAG *targettag)
39573968
&& (!SxactIsCommitted(sxact)
39583969
||TransactionIdPrecedes(GetTransactionSnapshot()->xmin,
39593970
sxact->finishedBefore))
3960-
&& !RWConflictExists(sxact,
3961-
(SERIALIZABLEXACT*)MySerializableXact))
3971+
&& !RWConflictExists(sxact,MySerializableXact))
39623972
{
3963-
FlagRWConflict(sxact,(SERIALIZABLEXACT*)MySerializableXact);
3973+
FlagRWConflict(sxact,MySerializableXact);
39643974
}
39653975

39663976
LWLockRelease(SerializableXactHashLock);
@@ -4065,7 +4075,11 @@ CheckForSerializableConflictIn(const Relation relation, const HeapTuple tuple,
40654075
errdetail("Cancelled on identification as a pivot, during conflict in checking."),
40664076
errhint("The transaction might succeed if retried.")));
40674077

4068-
MySerializableXact->flags |=SXACT_FLAG_DID_WRITE;
4078+
/*
4079+
* We're doing a write which might cause rw-conflicts now or later.
4080+
* Memorize that fact.
4081+
*/
4082+
MyXactDidWrite= true;
40694083

40704084
/*
40714085
* It is important that we check for locks from the finest granularity to
@@ -4150,6 +4164,12 @@ CheckTableForSerializableConflictIn(const Relation relation)
41504164
if (SkipSerialization(relation))
41514165
return;
41524166

4167+
/*
4168+
* We're doing a write which might cause rw-conflicts now or later.
4169+
* Memorize that fact.
4170+
*/
4171+
MyXactDidWrite= true;
4172+
41534173
Assert(relation->rd_index==NULL);/* not an index relation */
41544174

41554175
dbId=relation->rd_node.dbNode;
@@ -4192,10 +4212,10 @@ CheckTableForSerializableConflictIn(const Relation relation)
41924212
offsetof(PREDICATELOCK,targetLink));
41934213

41944214
if (predlock->tag.myXact!=MySerializableXact
4195-
&& !RWConflictExists(predlock->tag.myXact,
4196-
(SERIALIZABLEXACT*)MySerializableXact))
4197-
FlagRWConflict(predlock->tag.myXact,
4198-
(SERIALIZABLEXACT*)MySerializableXact);
4215+
&& !RWConflictExists(predlock->tag.myXact,MySerializableXact))
4216+
{
4217+
FlagRWConflict(predlock->tag.myXact,MySerializableXact);
4218+
}
41994219

42004220
predlock=nextpredlock;
42014221
}
@@ -4506,7 +4526,7 @@ AtPrepare_PredicateLocks(void)
45064526
TwoPhasePredicateXactRecord*xactRecord;
45074527
TwoPhasePredicateLockRecord*lockRecord;
45084528

4509-
sxact=(SERIALIZABLEXACT*)MySerializableXact;
4529+
sxact=MySerializableXact;
45104530
xactRecord=&(record.data.xactRecord);
45114531
lockRecord=&(record.data.lockRecord);
45124532

@@ -4583,6 +4603,7 @@ PostPrepare_PredicateLocks(TransactionId xid)
45834603
LocalPredicateLockHash=NULL;
45844604

45854605
MySerializableXact=InvalidSerializableXact;
4606+
MyXactDidWrite= false;
45864607
}
45874608

45884609
/*
@@ -4609,6 +4630,8 @@ PredicateLockTwoPhaseFinish(TransactionId xid, bool isCommit)
46094630

46104631
/* Release its locks */
46114632
MySerializableXact=sxid->myXact;
4633+
MyXactDidWrite= true;/* conservatively assume that we wrote
4634+
* something */
46124635
ReleasePredicateLocks(isCommit);
46134636
}
46144637

‎src/include/storage/predicate_internals.h

Lines changed: 7 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -99,14 +99,13 @@ typedef struct SERIALIZABLEXACT
9999
*/
100100
#defineSXACT_FLAG_CONFLICT_OUT0x00000004
101101
#defineSXACT_FLAG_READ_ONLY0x00000008
102-
#defineSXACT_FLAG_DID_WRITE0x00000010
103-
#defineSXACT_FLAG_MARKED_FOR_DEATH0x00000020
104-
#defineSXACT_FLAG_DEFERRABLE_WAITING0x00000040
105-
#defineSXACT_FLAG_RO_SAFE0x00000080
106-
#defineSXACT_FLAG_RO_UNSAFE0x00000100
107-
#defineSXACT_FLAG_SUMMARY_CONFLICT_IN0x00000200
108-
#defineSXACT_FLAG_SUMMARY_CONFLICT_OUT0x00000400
109-
#defineSXACT_FLAG_PREPARED0x00000800
102+
#defineSXACT_FLAG_MARKED_FOR_DEATH0x00000010
103+
#defineSXACT_FLAG_DEFERRABLE_WAITING0x00000020
104+
#defineSXACT_FLAG_RO_SAFE0x00000040
105+
#defineSXACT_FLAG_RO_UNSAFE0x00000080
106+
#defineSXACT_FLAG_SUMMARY_CONFLICT_IN0x00000100
107+
#defineSXACT_FLAG_SUMMARY_CONFLICT_OUT0x00000200
108+
#defineSXACT_FLAG_PREPARED0x00000400
110109

111110
/*
112111
* The following types are used to provide an ad hoc list for holding

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