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apache/cloudberry/HEAD/./src/backend/storage/lmgr/predicate.c
blob: f5668bdb4ff02c0e3565cf7a45c5eb2625514157 [file]
/*-------------------------------------------------------------------------
*
* predicate.c
* POSTGRES predicate locking
* to support full serializable transaction isolation
*
*
* The approach taken is to implement Serializable Snapshot Isolation (SSI)
* as initially described in this paper:
*
* Michael J. Cahill, Uwe Röhm, and Alan D. Fekete. 2008.
* Serializable isolation for snapshot databases.
* In SIGMOD '08: Proceedings of the 2008 ACM SIGMOD
* international conference on Management of data,
* pages 729-738, New York, NY, USA. ACM.
* http://doi.acm.org/10.1145/1376616.1376690
*
* and further elaborated in Cahill's doctoral thesis:
*
* Michael James Cahill. 2009.
* Serializable Isolation for Snapshot Databases.
* Sydney Digital Theses.
* University of Sydney, School of Information Technologies.
* http://hdl.handle.net/2123/5353
*
*
* Predicate locks for Serializable Snapshot Isolation (SSI) are SIREAD
* locks, which are so different from normal locks that a distinct set of
* structures is required to handle them. They are needed to detect
* rw-conflicts when the read happens before the write. (When the write
* occurs first, the reading transaction can check for a conflict by
* examining the MVCC data.)
*
* (1) Besides tuples actually read, they must cover ranges of tuples
* which would have been read based on the predicate. This will
* require modelling the predicates through locks against database
* objects such as pages, index ranges, or entire tables.
*
* (2) They must be kept in RAM for quick access. Because of this, it
* isn't possible to always maintain tuple-level granularity -- when
* the space allocated to store these approaches exhaustion, a
* request for a lock may need to scan for situations where a single
* transaction holds many fine-grained locks which can be coalesced
* into a single coarser-grained lock.
*
* (3) They never block anything; they are more like flags than locks
* in that regard; although they refer to database objects and are
* used to identify rw-conflicts with normal write locks.
*
* (4) While they are associated with a transaction, they must survive
* a successful COMMIT of that transaction, and remain until all
* overlapping transactions complete. This even means that they
* must survive termination of the transaction's process. If a
* top level transaction is rolled back, however, it is immediately
* flagged so that it can be ignored, and its SIREAD locks can be
* released any time after that.
*
* (5) The only transactions which create SIREAD locks or check for
* conflicts with them are serializable transactions.
*
* (6) When a write lock for a top level transaction is found to cover
* an existing SIREAD lock for the same transaction, the SIREAD lock
* can be deleted.
*
* (7) A write from a serializable transaction must ensure that an xact
* record exists for the transaction, with the same lifespan (until
* all concurrent transaction complete or the transaction is rolled
* back) so that rw-dependencies to that transaction can be
* detected.
*
* We use an optimization for read-only transactions. Under certain
* circumstances, a read-only transaction's snapshot can be shown to
* never have conflicts with other transactions. This is referred to
* as a "safe" snapshot (and one known not to be is "unsafe").
* However, it can't be determined whether a snapshot is safe until
* all concurrent read/write transactions complete.
*
* Once a read-only transaction is known to have a safe snapshot, it
* can release its predicate locks and exempt itself from further
* predicate lock tracking. READ ONLY DEFERRABLE transactions run only
* on safe snapshots, waiting as necessary for one to be available.
*
*
* Lightweight locks to manage access to the predicate locking shared
* memory objects must be taken in this order, and should be released in
* reverse order:
*
* SerializableFinishedListLock
* - Protects the list of transactions which have completed but which
* may yet matter because they overlap still-active transactions.
*
* SerializablePredicateListLock
* - Protects the linked list of locks held by a transaction. Note
* that the locks themselves are also covered by the partition
* locks of their respective lock targets; this lock only affects
* the linked list connecting the locks related to a transaction.
* - All transactions share this single lock (with no partitioning).
* - There is never a need for a process other than the one running
* an active transaction to walk the list of locks held by that
* transaction, except parallel query workers sharing the leader's
* transaction. In the parallel case, an extra per-sxact lock is
* taken; see below.
* - It is relatively infrequent that another process needs to
* modify the list for a transaction, but it does happen for such
* things as index page splits for pages with predicate locks and
* freeing of predicate locked pages by a vacuum process. When
* removing a lock in such cases, the lock itself contains the
* pointers needed to remove it from the list. When adding a
* lock in such cases, the lock can be added using the anchor in
* the transaction structure. Neither requires walking the list.
* - Cleaning up the list for a terminated transaction is sometimes
* not done on a retail basis, in which case no lock is required.
* - Due to the above, a process accessing its active transaction's
* list always uses a shared lock, regardless of whether it is
* walking or maintaining the list. This improves concurrency
* for the common access patterns.
* - A process which needs to alter the list of a transaction other
* than its own active transaction must acquire an exclusive
* lock.
*
* SERIALIZABLEXACT's member 'perXactPredicateListLock'
* - Protects the linked list of predicate locks held by a transaction.
* Only needed for parallel mode, where multiple backends share the
* same SERIALIZABLEXACT object. Not needed if
* SerializablePredicateListLock is held exclusively.
*
* PredicateLockHashPartitionLock(hashcode)
* - The same lock protects a target, all locks on that target, and
* the linked list of locks on the target.
* - When more than one is needed, acquire in ascending address order.
* - When all are needed (rare), acquire in ascending index order with
* PredicateLockHashPartitionLockByIndex(index).
*
* SerializableXactHashLock
* - Protects both PredXact and SerializableXidHash.
*
*
* Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
*
* IDENTIFICATION
* src/backend/storage/lmgr/predicate.c
*
*-------------------------------------------------------------------------
*/
/*
* INTERFACE ROUTINES
*
* housekeeping for setting up shared memory predicate lock structures
* InitPredicateLocks(void)
* PredicateLockShmemSize(void)
*
* predicate lock reporting
* GetPredicateLockStatusData(void)
* PageIsPredicateLocked(Relation relation, BlockNumber blkno)
*
* predicate lock maintenance
* GetSerializableTransactionSnapshot(Snapshot snapshot)
* SetSerializableTransactionSnapshot(Snapshot snapshot,
* VirtualTransactionId *sourcevxid)
* RegisterPredicateLockingXid(void)
* PredicateLockRelation(Relation relation, Snapshot snapshot)
* PredicateLockPage(Relation relation, BlockNumber blkno,
* Snapshot snapshot)
* PredicateLockTID(Relation relation, ItemPointer tid, Snapshot snapshot,
* TransactionId insert_xid)
* PredicateLockPageSplit(Relation relation, BlockNumber oldblkno,
* BlockNumber newblkno)
* PredicateLockPageCombine(Relation relation, BlockNumber oldblkno,
* BlockNumber newblkno)
* TransferPredicateLocksToHeapRelation(Relation relation)
* ReleasePredicateLocks(bool isCommit, bool isReadOnlySafe)
*
* conflict detection (may also trigger rollback)
* CheckForSerializableConflictOut(Relation relation, TransactionId xid,
* Snapshot snapshot)
* CheckForSerializableConflictIn(Relation relation, ItemPointer tid,
* BlockNumber blkno)
* CheckTableForSerializableConflictIn(Relation relation)
*
* final rollback checking
* PreCommit_CheckForSerializationFailure(void)
*
* two-phase commit support
* AtPrepare_PredicateLocks(void);
* PostPrepare_PredicateLocks(TransactionId xid);
* PredicateLockTwoPhaseFinish(TransactionId xid, bool isCommit);
* predicatelock_twophase_recover(TransactionId xid, uint16 info,
* void *recdata, uint32 len);
*/
#include "postgres.h"
#include "access/parallel.h"
#include "access/slru.h"
#include "access/subtrans.h"
#include "access/transam.h"
#include "access/twophase.h"
#include "access/twophase_rmgr.h"
#include "access/xact.h"
#include "access/xlog.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "storage/bufmgr.h"
#include "storage/predicate.h"
#include "storage/predicate_internals.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "utils/rel.h"
#include "utils/snapmgr.h"
/* Uncomment the next line to test the graceful degradation code. */
/* #define TEST_SUMMARIZE_SERIAL */
/*
* Test the most selective fields first, for performance.
*
* a is covered by b if all of the following hold:
* 1) a.database = b.database
* 2) a.relation = b.relation
* 3) b.offset is invalid (b is page-granularity or higher)
* 4) either of the following:
* 4a) a.offset is valid (a is tuple-granularity) and a.page = b.page
* or 4b) a.offset is invalid and b.page is invalid (a is
* page-granularity and b is relation-granularity
*/
#define TargetTagIsCoveredBy(covered_target, covering_target) \
((GET_PREDICATELOCKTARGETTAG_RELATION(covered_target) == /* (2) */ \
GET_PREDICATELOCKTARGETTAG_RELATION(covering_target)) \
&& (GET_PREDICATELOCKTARGETTAG_OFFSET(covering_target) == \
InvalidOffsetNumber) /* (3) */ \
&& (((GET_PREDICATELOCKTARGETTAG_OFFSET(covered_target) != \
InvalidOffsetNumber) /* (4a) */ \
&& (GET_PREDICATELOCKTARGETTAG_PAGE(covering_target) == \
GET_PREDICATELOCKTARGETTAG_PAGE(covered_target))) \
|| ((GET_PREDICATELOCKTARGETTAG_PAGE(covering_target) == \
InvalidBlockNumber) /* (4b) */ \
&& (GET_PREDICATELOCKTARGETTAG_PAGE(covered_target) \
!= InvalidBlockNumber))) \
&& (GET_PREDICATELOCKTARGETTAG_DB(covered_target) == /* (1) */ \
GET_PREDICATELOCKTARGETTAG_DB(covering_target)))
/*
* The predicate locking target and lock shared hash tables are partitioned to
* reduce contention. To determine which partition a given target belongs to,
* compute the tag's hash code with PredicateLockTargetTagHashCode(), then
* apply one of these macros.
* NB: NUM_PREDICATELOCK_PARTITIONS must be a power of 2!
*/
#define PredicateLockHashPartition(hashcode) \
((hashcode) % NUM_PREDICATELOCK_PARTITIONS)
#define PredicateLockHashPartitionLock(hashcode) \
(&MainLWLockArray[PREDICATELOCK_MANAGER_LWLOCK_OFFSET + \
PredicateLockHashPartition(hashcode)].lock)
#define PredicateLockHashPartitionLockByIndex(i) \
(&MainLWLockArray[PREDICATELOCK_MANAGER_LWLOCK_OFFSET + (i)].lock)
#define NPREDICATELOCKTARGETENTS() \
mul_size(max_predicate_locks_per_xact, add_size(MaxBackends, max_prepared_xacts))
#define SxactIsOnFinishedList(sxact) (!SHMQueueIsDetached(&((sxact)->finishedLink)))
/*
* Note that a sxact is marked "prepared" once it has passed
* PreCommit_CheckForSerializationFailure, even if it isn't using
* 2PC. This is the point at which it can no longer be aborted.
*
* The PREPARED flag remains set after commit, so SxactIsCommitted
* implies SxactIsPrepared.
*/
#define SxactIsCommitted(sxact) (((sxact)->flags & SXACT_FLAG_COMMITTED) != 0)
#define SxactIsPrepared(sxact) (((sxact)->flags & SXACT_FLAG_PREPARED) != 0)
#define SxactIsRolledBack(sxact) (((sxact)->flags & SXACT_FLAG_ROLLED_BACK) != 0)
#define SxactIsDoomed(sxact) (((sxact)->flags & SXACT_FLAG_DOOMED) != 0)
#define SxactIsReadOnly(sxact) (((sxact)->flags & SXACT_FLAG_READ_ONLY) != 0)
#define SxactHasSummaryConflictIn(sxact) (((sxact)->flags & SXACT_FLAG_SUMMARY_CONFLICT_IN) != 0)
#define SxactHasSummaryConflictOut(sxact) (((sxact)->flags & SXACT_FLAG_SUMMARY_CONFLICT_OUT) != 0)
/*
* The following macro actually means that the specified transaction has a
* conflict out *to a transaction which committed ahead of it*. It's hard
* to get that into a name of a reasonable length.
*/
#define SxactHasConflictOut(sxact) (((sxact)->flags & SXACT_FLAG_CONFLICT_OUT) != 0)
#define SxactIsDeferrableWaiting(sxact) (((sxact)->flags & SXACT_FLAG_DEFERRABLE_WAITING) != 0)
#define SxactIsROSafe(sxact) (((sxact)->flags & SXACT_FLAG_RO_SAFE) != 0)
#define SxactIsROUnsafe(sxact) (((sxact)->flags & SXACT_FLAG_RO_UNSAFE) != 0)
#define SxactIsPartiallyReleased(sxact) (((sxact)->flags & SXACT_FLAG_PARTIALLY_RELEASED) != 0)
/*
* Compute the hash code associated with a PREDICATELOCKTARGETTAG.
*
* To avoid unnecessary recomputations of the hash code, we try to do this
* just once per function, and then pass it around as needed. Aside from
* passing the hashcode to hash_search_with_hash_value(), we can extract
* the lock partition number from the hashcode.
*/
#define PredicateLockTargetTagHashCode(predicatelocktargettag) \
get_hash_value(PredicateLockTargetHash, predicatelocktargettag)
/*
* Given a predicate lock tag, and the hash for its target,
* compute the lock hash.
*
* To make the hash code also depend on the transaction, we xor the sxid
* struct's address into the hash code, left-shifted so that the
* partition-number bits don't change. Since this is only a hash, we
* don't care if we lose high-order bits of the address; use an
* intermediate variable to suppress cast-pointer-to-int warnings.
*/
#define PredicateLockHashCodeFromTargetHashCode(predicatelocktag, targethash) \
((targethash) ^ ((uint32) PointerGetDatum((predicatelocktag)->myXact)) \
<< LOG2_NUM_PREDICATELOCK_PARTITIONS)
/*
* The SLRU buffer area through which we access the old xids.
*/
static SlruCtlData SerialSlruCtlData;
#define SerialSlruCtl (&SerialSlruCtlData)
#define SERIAL_PAGESIZE BLCKSZ
#define SERIAL_ENTRYSIZE sizeof(SerCommitSeqNo)
#define SERIAL_ENTRIESPERPAGE (SERIAL_PAGESIZE / SERIAL_ENTRYSIZE)
/*
* Set maximum pages based on the number needed to track all transactions.
*/
#define SERIAL_MAX_PAGE (MaxTransactionId / SERIAL_ENTRIESPERPAGE)
#define SerialNextPage(page) (((page) >= SERIAL_MAX_PAGE) ? 0 : (page) + 1)
#define SerialValue(slotno, xid) (*((SerCommitSeqNo *) \
(SerialSlruCtl->shared->page_buffer[slotno] + \
((((uint32) (xid)) % SERIAL_ENTRIESPERPAGE) * SERIAL_ENTRYSIZE))))
#define SerialPage(xid) (((uint32) (xid)) / SERIAL_ENTRIESPERPAGE)
typedef struct SerialControlData
{
int headPage; /* newest initialized page */
TransactionId headXid; /* newest valid Xid in the SLRU */
TransactionId tailXid; /* oldest xmin we might be interested in */
} SerialControlData;
typedef struct SerialControlData *SerialControl;
static SerialControl serialControl;
/*
* When the oldest committed transaction on the "finished" list is moved to
* SLRU, its predicate locks will be moved to this "dummy" transaction,
* collapsing duplicate targets. When a duplicate is found, the later
* commitSeqNo is used.
*/
static SERIALIZABLEXACT *OldCommittedSxact;
/*
* These configuration variables are used to set the predicate lock table size
* and to control promotion of predicate locks to coarser granularity in an
* attempt to degrade performance (mostly as false positive serialization
* failure) gracefully in the face of memory pressure.
*/
int max_predicate_locks_per_xact; /* set by guc.c */
int max_predicate_locks_per_relation; /* set by guc.c */
int max_predicate_locks_per_page; /* set by guc.c */
/*
* This provides a list of objects in order to track transactions
* participating in predicate locking. Entries in the list are fixed size,
* and reside in shared memory. The memory address of an entry must remain
* fixed during its lifetime. The list will be protected from concurrent
* update externally; no provision is made in this code to manage that. The
* number of entries in the list, and the size allowed for each entry is
* fixed upon creation.
*/
static PredXactList PredXact;
/*
* This provides a pool of RWConflict data elements to use in conflict lists
* between transactions.
*/
static RWConflictPoolHeader RWConflictPool;
/*
* The predicate locking hash tables are in shared memory.
* Each backend keeps pointers to them.
*/
static HTAB *SerializableXidHash;
static HTAB *PredicateLockTargetHash;
static HTAB *PredicateLockHash;
static SHM_QUEUE *FinishedSerializableTransactions;
/*
* Tag for a dummy entry in PredicateLockTargetHash. By temporarily removing
* this entry, you can ensure that there's enough scratch space available for
* inserting one entry in the hash table. This is an otherwise-invalid tag.
*/
static const PREDICATELOCKTARGETTAG ScratchTargetTag = {0, 0, 0, 0};
static uint32 ScratchTargetTagHash;
static LWLock *ScratchPartitionLock;
/*
* The local hash table used to determine when to combine multiple fine-
* grained locks into a single courser-grained lock.
*/
static HTAB *LocalPredicateLockHash = NULL;
/*
* Keep a pointer to the currently-running serializable transaction (if any)
* for quick reference. Also, remember if we have written anything that could
* cause a rw-conflict.
*/
static SERIALIZABLEXACT *MySerializableXact = InvalidSerializableXact;
static bool MyXactDidWrite = false;
/*
* The SXACT_FLAG_RO_UNSAFE optimization might lead us to release
* MySerializableXact early. If that happens in a parallel query, the leader
* needs to defer the destruction of the SERIALIZABLEXACT until end of
* transaction, because the workers still have a reference to it. In that
* case, the leader stores it here.
*/
static SERIALIZABLEXACT *SavedSerializableXact = InvalidSerializableXact;
/* local functions */
static SERIALIZABLEXACT *CreatePredXact(void);
static void ReleasePredXact(SERIALIZABLEXACT *sxact);
static SERIALIZABLEXACT *FirstPredXact(void);
static SERIALIZABLEXACT *NextPredXact(SERIALIZABLEXACT *sxact);
static bool RWConflictExists(const SERIALIZABLEXACT *reader, const SERIALIZABLEXACT *writer);
static void SetRWConflict(SERIALIZABLEXACT *reader, SERIALIZABLEXACT *writer);
static void SetPossibleUnsafeConflict(SERIALIZABLEXACT *roXact, SERIALIZABLEXACT *activeXact);
static void ReleaseRWConflict(RWConflict conflict);
static void FlagSxactUnsafe(SERIALIZABLEXACT *sxact);
static bool SerialPagePrecedesLogically(int page1, int page2);
static void SerialInit(void);
static void SerialAdd(TransactionId xid, SerCommitSeqNo minConflictCommitSeqNo);
static SerCommitSeqNo SerialGetMinConflictCommitSeqNo(TransactionId xid);
static void SerialSetActiveSerXmin(TransactionId xid);
static uint32 predicatelock_hash(const void *key, Size keysize);
static void SummarizeOldestCommittedSxact(void);
static Snapshot GetSafeSnapshot(Snapshot snapshot);
static Snapshot GetSerializableTransactionSnapshotInt(Snapshot snapshot,
VirtualTransactionId *sourcevxid,
int sourcepid);
static bool PredicateLockExists(const PREDICATELOCKTARGETTAG *targettag);
static bool GetParentPredicateLockTag(const PREDICATELOCKTARGETTAG *tag,
PREDICATELOCKTARGETTAG *parent);
static bool CoarserLockCovers(const PREDICATELOCKTARGETTAG *newtargettag);
static void RemoveScratchTarget(bool lockheld);
static void RestoreScratchTarget(bool lockheld);
static void RemoveTargetIfNoLongerUsed(PREDICATELOCKTARGET *target,
uint32 targettaghash);
static void DeleteChildTargetLocks(const PREDICATELOCKTARGETTAG *newtargettag);
static int MaxPredicateChildLocks(const PREDICATELOCKTARGETTAG *tag);
static bool CheckAndPromotePredicateLockRequest(const PREDICATELOCKTARGETTAG *reqtag);
static void DecrementParentLocks(const PREDICATELOCKTARGETTAG *targettag);
static void CreatePredicateLock(const PREDICATELOCKTARGETTAG *targettag,
uint32 targettaghash,
SERIALIZABLEXACT *sxact);
static void DeleteLockTarget(PREDICATELOCKTARGET *target, uint32 targettaghash);
static bool TransferPredicateLocksToNewTarget(PREDICATELOCKTARGETTAG oldtargettag,
PREDICATELOCKTARGETTAG newtargettag,
bool removeOld);
static void PredicateLockAcquire(const PREDICATELOCKTARGETTAG *targettag);
static void DropAllPredicateLocksFromTable(Relation relation,
bool transfer);
static void SetNewSxactGlobalXmin(void);
static void ClearOldPredicateLocks(void);
static void ReleaseOneSerializableXact(SERIALIZABLEXACT *sxact, bool partial,
bool summarize);
static bool XidIsConcurrent(TransactionId xid);
static void CheckTargetForConflictsIn(PREDICATELOCKTARGETTAG *targettag);
static void FlagRWConflict(SERIALIZABLEXACT *reader, SERIALIZABLEXACT *writer);
static void OnConflict_CheckForSerializationFailure(const SERIALIZABLEXACT *reader,
SERIALIZABLEXACT *writer);
static void CreateLocalPredicateLockHash(void);
static void ReleasePredicateLocksLocal(void);
/*------------------------------------------------------------------------*/
/*
* Does this relation participate in predicate locking? Temporary and system
* relations are exempt, as are materialized views.
*/
static inline bool
PredicateLockingNeededForRelation(Relation relation)
{
return !(relation->rd_id < FirstBootstrapObjectId ||
RelationUsesLocalBuffers(relation) ||
relation->rd_rel->relkind == RELKIND_MATVIEW);
}
/*
* When a public interface method is called for a read, this is the test to
* see if we should do a quick return.
*
* Note: this function has side-effects! If this transaction has been flagged
* as RO-safe since the last call, we release all predicate locks and reset
* MySerializableXact. That makes subsequent calls to return quickly.
*
* This is marked as 'inline' to eliminate the function call overhead in the
* common case that serialization is not needed.
*/
static inline bool
SerializationNeededForRead(Relation relation, Snapshot snapshot)
{
/* Nothing to do if this is not a serializable transaction */
if (MySerializableXact == InvalidSerializableXact)
return false;
/*
* Don't acquire locks or conflict when scanning with a special snapshot.
* This excludes things like CLUSTER and REINDEX. They use the wholesale
* functions TransferPredicateLocksToHeapRelation() and
* CheckTableForSerializableConflictIn() to participate in serialization,
* but the scans involved don't need serialization.
*/
if (!IsMVCCSnapshot(snapshot))
return false;
/*
* Check if we have just become "RO-safe". If we have, immediately release
* all locks as they're not needed anymore. This also resets
* MySerializableXact, so that subsequent calls to this function can exit
* quickly.
*
* A transaction is flagged as RO_SAFE if all concurrent R/W transactions
* commit without having conflicts out to an earlier snapshot, thus
* ensuring that no conflicts are possible for this transaction.
*/
if (SxactIsROSafe(MySerializableXact))
{
ReleasePredicateLocks(false, true);
return false;
}
/* Check if the relation doesn't participate in predicate locking */
if (!PredicateLockingNeededForRelation(relation))
return false;
return true; /* no excuse to skip predicate locking */
}
/*
* Like SerializationNeededForRead(), but called on writes.
* The logic is the same, but there is no snapshot and we can't be RO-safe.
*/
static inline bool
SerializationNeededForWrite(Relation relation)
{
/* Nothing to do if this is not a serializable transaction */
if (MySerializableXact == InvalidSerializableXact)
return false;
/* Check if the relation doesn't participate in predicate locking */
if (!PredicateLockingNeededForRelation(relation))
return false;
return true; /* no excuse to skip predicate locking */
}
/*------------------------------------------------------------------------*/
/*
* These functions are a simple implementation of a list for this specific
* type of struct. If there is ever a generalized shared memory list, we
* should probably switch to that.
*/
static SERIALIZABLEXACT *
CreatePredXact(void)
{
PredXactListElement ptle;
ptle = (PredXactListElement)
SHMQueueNext(&PredXact->availableList,
&PredXact->availableList,
offsetof(PredXactListElementData, link));
if (!ptle)
return NULL;
SHMQueueDelete(&ptle->link);
SHMQueueInsertBefore(&PredXact->activeList, &ptle->link);
return &ptle->sxact;
}
static void
ReleasePredXact(SERIALIZABLEXACT *sxact)
{
PredXactListElement ptle;
Assert(ShmemAddrIsValid(sxact));
ptle = (PredXactListElement)
(((char *) sxact)
- offsetof(PredXactListElementData, sxact)
+ offsetof(PredXactListElementData, link));
SHMQueueDelete(&ptle->link);
SHMQueueInsertBefore(&PredXact->availableList, &ptle->link);
}
static SERIALIZABLEXACT *
FirstPredXact(void)
{
PredXactListElement ptle;
ptle = (PredXactListElement)
SHMQueueNext(&PredXact->activeList,
&PredXact->activeList,
offsetof(PredXactListElementData, link));
if (!ptle)
return NULL;
return &ptle->sxact;
}
static SERIALIZABLEXACT *
NextPredXact(SERIALIZABLEXACT *sxact)
{
PredXactListElement ptle;
Assert(ShmemAddrIsValid(sxact));
ptle = (PredXactListElement)
(((char *) sxact)
- offsetof(PredXactListElementData, sxact)
+ offsetof(PredXactListElementData, link));
ptle = (PredXactListElement)
SHMQueueNext(&PredXact->activeList,
&ptle->link,
offsetof(PredXactListElementData, link));
if (!ptle)
return NULL;
return &ptle->sxact;
}
/*------------------------------------------------------------------------*/
/*
* These functions manage primitive access to the RWConflict pool and lists.
*/
static bool
RWConflictExists(const SERIALIZABLEXACT *reader, const SERIALIZABLEXACT *writer)
{
RWConflict conflict;
Assert(reader != writer);
/* Check the ends of the purported conflict first. */
if (SxactIsDoomed(reader)
|| SxactIsDoomed(writer)
|| SHMQueueEmpty(&reader->outConflicts)
|| SHMQueueEmpty(&writer->inConflicts))
return false;
/* A conflict is possible; walk the list to find out. */
conflict = (RWConflict)
SHMQueueNext(&reader->outConflicts,
&reader->outConflicts,
offsetof(RWConflictData, outLink));
while (conflict)
{
if (conflict->sxactIn == writer)
return true;
conflict = (RWConflict)
SHMQueueNext(&reader->outConflicts,
&conflict->outLink,
offsetof(RWConflictData, outLink));
}
/* No conflict found. */
return false;
}
static void
SetRWConflict(SERIALIZABLEXACT *reader, SERIALIZABLEXACT *writer)
{
RWConflict conflict;
Assert(reader != writer);
Assert(!RWConflictExists(reader, writer));
conflict = (RWConflict)
SHMQueueNext(&RWConflictPool->availableList,
&RWConflictPool->availableList,
offsetof(RWConflictData, outLink));
if (!conflict)
ereport(ERROR,
(errcode(ERRCODE_OUT_OF_MEMORY),
errmsg("not enough elements in RWConflictPool to record a read/write conflict"),
errhint("You might need to run fewer transactions at a time or increase max_connections.")));
SHMQueueDelete(&conflict->outLink);
conflict->sxactOut = reader;
conflict->sxactIn = writer;
SHMQueueInsertBefore(&reader->outConflicts, &conflict->outLink);
SHMQueueInsertBefore(&writer->inConflicts, &conflict->inLink);
}
static void
SetPossibleUnsafeConflict(SERIALIZABLEXACT *roXact,
SERIALIZABLEXACT *activeXact)
{
RWConflict conflict;
Assert(roXact != activeXact);
Assert(SxactIsReadOnly(roXact));
Assert(!SxactIsReadOnly(activeXact));
conflict = (RWConflict)
SHMQueueNext(&RWConflictPool->availableList,
&RWConflictPool->availableList,
offsetof(RWConflictData, outLink));
if (!conflict)
ereport(ERROR,
(errcode(ERRCODE_OUT_OF_MEMORY),
errmsg("not enough elements in RWConflictPool to record a potential read/write conflict"),
errhint("You might need to run fewer transactions at a time or increase max_connections.")));
SHMQueueDelete(&conflict->outLink);
conflict->sxactOut = activeXact;
conflict->sxactIn = roXact;
SHMQueueInsertBefore(&activeXact->possibleUnsafeConflicts,
&conflict->outLink);
SHMQueueInsertBefore(&roXact->possibleUnsafeConflicts,
&conflict->inLink);
}
static void
ReleaseRWConflict(RWConflict conflict)
{
SHMQueueDelete(&conflict->inLink);
SHMQueueDelete(&conflict->outLink);
SHMQueueInsertBefore(&RWConflictPool->availableList, &conflict->outLink);
}
static void
FlagSxactUnsafe(SERIALIZABLEXACT *sxact)
{
RWConflict conflict,
nextConflict;
Assert(SxactIsReadOnly(sxact));
Assert(!SxactIsROSafe(sxact));
sxact->flags |= SXACT_FLAG_RO_UNSAFE;
/*
* We know this isn't a safe snapshot, so we can stop looking for other
* potential conflicts.
*/
conflict = (RWConflict)
SHMQueueNext(&sxact->possibleUnsafeConflicts,
&sxact->possibleUnsafeConflicts,
offsetof(RWConflictData, inLink));
while (conflict)
{
nextConflict = (RWConflict)
SHMQueueNext(&sxact->possibleUnsafeConflicts,
&conflict->inLink,
offsetof(RWConflictData, inLink));
Assert(!SxactIsReadOnly(conflict->sxactOut));
Assert(sxact == conflict->sxactIn);
ReleaseRWConflict(conflict);
conflict = nextConflict;
}
}
/*------------------------------------------------------------------------*/
/*
* Decide whether a Serial page number is "older" for truncation purposes.
* Analogous to CLOGPagePrecedes().
*/
static bool
SerialPagePrecedesLogically(int page1, int page2)
{
TransactionId xid1;
TransactionId xid2;
xid1 = ((TransactionId) page1) * SERIAL_ENTRIESPERPAGE;
xid1 += FirstNormalTransactionId + 1;
xid2 = ((TransactionId) page2) * SERIAL_ENTRIESPERPAGE;
xid2 += FirstNormalTransactionId + 1;
return (TransactionIdPrecedes(xid1, xid2) &&
TransactionIdPrecedes(xid1, xid2 + SERIAL_ENTRIESPERPAGE - 1));
}
#ifdef USE_ASSERT_CHECKING
static void
SerialPagePrecedesLogicallyUnitTests(void)
{
int per_page = SERIAL_ENTRIESPERPAGE,
offset = per_page / 2;
int newestPage,
oldestPage,
headPage,
targetPage;
TransactionId newestXact,
oldestXact;
/* GetNewTransactionId() has assigned the last XID it can safely use. */
newestPage = 2 * SLRU_PAGES_PER_SEGMENT - 1; /* nothing special */
newestXact = newestPage * per_page + offset;
Assert(newestXact / per_page == newestPage);
oldestXact = newestXact + 1;
oldestXact -= 1U << 31;
oldestPage = oldestXact / per_page;
/*
* In this scenario, the SLRU headPage pertains to the last ~1000 XIDs
* assigned. oldestXact finishes, ~2B XIDs having elapsed since it
* started. Further transactions cause us to summarize oldestXact to
* tailPage. Function must return false so SerialAdd() doesn't zero
* tailPage (which may contain entries for other old, recently-finished
* XIDs) and half the SLRU. Reaching this requires burning ~2B XIDs in
* single-user mode, a negligible possibility.
*/
headPage = newestPage;
targetPage = oldestPage;
Assert(!SerialPagePrecedesLogically(headPage, targetPage));
/*
* In this scenario, the SLRU headPage pertains to oldestXact. We're
* summarizing an XID near newestXact. (Assume few other XIDs used
* SERIALIZABLE, hence the minimal headPage advancement. Assume
* oldestXact was long-running and only recently reached the SLRU.)
* Function must return true to make SerialAdd() create targetPage.
*
* Today's implementation mishandles this case, but it doesn't matter
* enough to fix. Verify that the defect affects just one page by
* asserting correct treatment of its prior page. Reaching this case
* requires burning ~2B XIDs in single-user mode, a negligible
* possibility. Moreover, if it does happen, the consequence would be
* mild, namely a new transaction failing in SimpleLruReadPage().
*/
headPage = oldestPage;
targetPage = newestPage;
Assert(SerialPagePrecedesLogically(headPage, targetPage - 1));
#if 0
Assert(SerialPagePrecedesLogically(headPage, targetPage));
#endif
}
#endif
/*
* Initialize for the tracking of old serializable committed xids.
*/
static void
SerialInit(void)
{
bool found;
/*
* Set up SLRU management of the pg_serial data.
*/
SerialSlruCtl->PagePrecedes = SerialPagePrecedesLogically;
SimpleLruInit(SerialSlruCtl, "Serial",
NUM_SERIAL_BUFFERS, 0, SerialSLRULock, "pg_serial",
LWTRANCHE_SERIAL_BUFFER, SYNC_HANDLER_NONE);
#ifdef USE_ASSERT_CHECKING
SerialPagePrecedesLogicallyUnitTests();
#endif
SlruPagePrecedesUnitTests(SerialSlruCtl, SERIAL_ENTRIESPERPAGE);
/*
* Create or attach to the SerialControl structure.
*/
serialControl = (SerialControl)
ShmemInitStruct("SerialControlData", sizeof(SerialControlData), &found);
Assert(found == IsUnderPostmaster);
if (!found)
{
/*
* Set control information to reflect empty SLRU.
*/
serialControl->headPage = -1;
serialControl->headXid = InvalidTransactionId;
serialControl->tailXid = InvalidTransactionId;
}
}
/*
* Record a committed read write serializable xid and the minimum
* commitSeqNo of any transactions to which this xid had a rw-conflict out.
* An invalid commitSeqNo means that there were no conflicts out from xid.
*/
static void
SerialAdd(TransactionId xid, SerCommitSeqNo minConflictCommitSeqNo)
{
TransactionId tailXid;
int targetPage;
int slotno;
int firstZeroPage;
bool isNewPage;
Assert(TransactionIdIsValid(xid));
targetPage = SerialPage(xid);
LWLockAcquire(SerialSLRULock, LW_EXCLUSIVE);
/*
* If no serializable transactions are active, there shouldn't be anything
* to push out to the SLRU. Hitting this assert would mean there's
* something wrong with the earlier cleanup logic.
*/
tailXid = serialControl->tailXid;
Assert(TransactionIdIsValid(tailXid));
/*
* If the SLRU is currently unused, zero out the whole active region from
* tailXid to headXid before taking it into use. Otherwise zero out only
* any new pages that enter the tailXid-headXid range as we advance
* headXid.
*/
if (serialControl->headPage < 0)
{
firstZeroPage = SerialPage(tailXid);
isNewPage = true;
}
else
{
firstZeroPage = SerialNextPage(serialControl->headPage);
isNewPage = SerialPagePrecedesLogically(serialControl->headPage,
targetPage);
}
if (!TransactionIdIsValid(serialControl->headXid)
|| TransactionIdFollows(xid, serialControl->headXid))
serialControl->headXid = xid;
if (isNewPage)
serialControl->headPage = targetPage;
if (isNewPage)
{
/* Initialize intervening pages. */
while (firstZeroPage != targetPage)
{
(void) SimpleLruZeroPage(SerialSlruCtl, firstZeroPage);
firstZeroPage = SerialNextPage(firstZeroPage);
}
slotno = SimpleLruZeroPage(SerialSlruCtl, targetPage);
}
else
slotno = SimpleLruReadPage(SerialSlruCtl, targetPage, true, xid);
SerialValue(slotno, xid) = minConflictCommitSeqNo;
SerialSlruCtl->shared->page_dirty[slotno] = true;
LWLockRelease(SerialSLRULock);
}
/*
* Get the minimum commitSeqNo for any conflict out for the given xid. For
* a transaction which exists but has no conflict out, InvalidSerCommitSeqNo
* will be returned.
*/
static SerCommitSeqNo
SerialGetMinConflictCommitSeqNo(TransactionId xid)
{
TransactionId headXid;
TransactionId tailXid;
SerCommitSeqNo val;
int slotno;
Assert(TransactionIdIsValid(xid));
LWLockAcquire(SerialSLRULock, LW_SHARED);
headXid = serialControl->headXid;
tailXid = serialControl->tailXid;
LWLockRelease(SerialSLRULock);
if (!TransactionIdIsValid(headXid))
return 0;
Assert(TransactionIdIsValid(tailXid));
if (TransactionIdPrecedes(xid, tailXid)
|| TransactionIdFollows(xid, headXid))
return 0;
/*
* The following function must be called without holding SerialSLRULock,
* but will return with that lock held, which must then be released.
*/
slotno = SimpleLruReadPage_ReadOnly(SerialSlruCtl,
SerialPage(xid), xid);
val = SerialValue(slotno, xid);
LWLockRelease(SerialSLRULock);
return val;
}
/*
* Call this whenever there is a new xmin for active serializable
* transactions. We don't need to keep information on transactions which
* precede that. InvalidTransactionId means none active, so everything in
* the SLRU can be discarded.
*/
static void
SerialSetActiveSerXmin(TransactionId xid)
{
LWLockAcquire(SerialSLRULock, LW_EXCLUSIVE);
/*
* When no sxacts are active, nothing overlaps, set the xid values to
* invalid to show that there are no valid entries. Don't clear headPage,
* though. A new xmin might still land on that page, and we don't want to
* repeatedly zero out the same page.
*/
if (!TransactionIdIsValid(xid))
{
serialControl->tailXid = InvalidTransactionId;
serialControl->headXid = InvalidTransactionId;
LWLockRelease(SerialSLRULock);
return;
}
/*
* When we're recovering prepared transactions, the global xmin might move
* backwards depending on the order they're recovered. Normally that's not
* OK, but during recovery no serializable transactions will commit, so
* the SLRU is empty and we can get away with it.
*/
if (RecoveryInProgress())
{
Assert(serialControl->headPage < 0);
if (!TransactionIdIsValid(serialControl->tailXid)
|| TransactionIdPrecedes(xid, serialControl->tailXid))
{
serialControl->tailXid = xid;
}
LWLockRelease(SerialSLRULock);
return;
}
Assert(!TransactionIdIsValid(serialControl->tailXid)
|| TransactionIdFollows(xid, serialControl->tailXid));
serialControl->tailXid = xid;
LWLockRelease(SerialSLRULock);
}
/*
* Perform a checkpoint --- either during shutdown, or on-the-fly
*
* We don't have any data that needs to survive a restart, but this is a
* convenient place to truncate the SLRU.
*/
void
CheckPointPredicate(void)
{
int tailPage;
LWLockAcquire(SerialSLRULock, LW_EXCLUSIVE);
/* Exit quickly if the SLRU is currently not in use. */
if (serialControl->headPage < 0)
{
LWLockRelease(SerialSLRULock);
return;
}
if (TransactionIdIsValid(serialControl->tailXid))
{
/* We can truncate the SLRU up to the page containing tailXid */
tailPage = SerialPage(serialControl->tailXid);
}
else
{
/*----------
* The SLRU is no longer needed. Truncate to head before we set head
* invalid.
*
* XXX: It's possible that the SLRU is not needed again until XID
* wrap-around has happened, so that the segment containing headPage
* that we leave behind will appear to be new again. In that case it
* won't be removed until XID horizon advances enough to make it
* current again.
*
* XXX: This should happen in vac_truncate_clog(), not in checkpoints.
* Consider this scenario, starting from a system with no in-progress
* transactions and VACUUM FREEZE having maximized oldestXact:
* - Start a SERIALIZABLE transaction.
* - Start, finish, and summarize a SERIALIZABLE transaction, creating
* one SLRU page.
* - Consume XIDs to reach xidStopLimit.
* - Finish all transactions. Due to the long-running SERIALIZABLE
* transaction, earlier checkpoints did not touch headPage. The
* next checkpoint will change it, but that checkpoint happens after
* the end of the scenario.
* - VACUUM to advance XID limits.
* - Consume ~2M XIDs, crossing the former xidWrapLimit.
* - Start, finish, and summarize a SERIALIZABLE transaction.
* SerialAdd() declines to create the targetPage, because headPage
* is not regarded as in the past relative to that targetPage. The
* transaction instigating the summarize fails in
* SimpleLruReadPage().
*/
tailPage = serialControl->headPage;
serialControl->headPage = -1;
}
LWLockRelease(SerialSLRULock);
/* Truncate away pages that are no longer required */
SimpleLruTruncate(SerialSlruCtl, tailPage);
/*
* Write dirty SLRU pages to disk
*
* This is not actually necessary from a correctness point of view. We do
* it merely as a debugging aid.
*
* We're doing this after the truncation to avoid writing pages right
* before deleting the file in which they sit, which would be completely
* pointless.
*/
SimpleLruWriteAll(SerialSlruCtl, true);
}
/*------------------------------------------------------------------------*/
/*
* InitPredicateLocks -- Initialize the predicate locking data structures.
*
* This is called from CreateSharedMemoryAndSemaphores(), which see for
* more comments. In the normal postmaster case, the shared hash tables
* are created here. Backends inherit the pointers
* to the shared tables via fork(). In the EXEC_BACKEND case, each
* backend re-executes this code to obtain pointers to the already existing
* shared hash tables.
*/
void
InitPredicateLocks(void)
{
HASHCTL info;
long max_table_size;
Size requestSize;
bool found;
#ifndef EXEC_BACKEND
Assert(!IsUnderPostmaster);
#endif
/*
* Compute size of predicate lock target hashtable. Note these
* calculations must agree with PredicateLockShmemSize!
*/
max_table_size = NPREDICATELOCKTARGETENTS();
/*
* Allocate hash table for PREDICATELOCKTARGET structs. This stores
* per-predicate-lock-target information.
*/
info.keysize = sizeof(PREDICATELOCKTARGETTAG);
info.entrysize = sizeof(PREDICATELOCKTARGET);
info.num_partitions = NUM_PREDICATELOCK_PARTITIONS;
PredicateLockTargetHash = ShmemInitHash("PREDICATELOCKTARGET hash",
max_table_size,
max_table_size,
&info,
HASH_ELEM | HASH_BLOBS |
HASH_PARTITION | HASH_FIXED_SIZE);
/*
* Reserve a dummy entry in the hash table; we use it to make sure there's
* always one entry available when we need to split or combine a page,
* because running out of space there could mean aborting a
* non-serializable transaction.
*/
if (!IsUnderPostmaster)
{
(void) hash_search(PredicateLockTargetHash, &ScratchTargetTag,
HASH_ENTER, &found);
Assert(!found);
}
/* Pre-calculate the hash and partition lock of the scratch entry */
ScratchTargetTagHash = PredicateLockTargetTagHashCode(&ScratchTargetTag);
ScratchPartitionLock = PredicateLockHashPartitionLock(ScratchTargetTagHash);
/*
* Allocate hash table for PREDICATELOCK structs. This stores per
* xact-lock-of-a-target information.
*/
info.keysize = sizeof(PREDICATELOCKTAG);
info.entrysize = sizeof(PREDICATELOCK);
info.hash = predicatelock_hash;
info.num_partitions = NUM_PREDICATELOCK_PARTITIONS;
/* Assume an average of 2 xacts per target */
max_table_size *= 2;
PredicateLockHash = ShmemInitHash("PREDICATELOCK hash",
max_table_size,
max_table_size,
&info,
HASH_ELEM | HASH_FUNCTION |
HASH_PARTITION | HASH_FIXED_SIZE);
/*
* Compute size for serializable transaction hashtable. Note these
* calculations must agree with PredicateLockShmemSize!
*/
max_table_size = (MaxBackends + max_prepared_xacts);
/*
* Allocate a list to hold information on transactions participating in
* predicate locking.
*
* Assume an average of 10 predicate locking transactions per backend.
* This allows aggressive cleanup while detail is present before data must
* be summarized for storage in SLRU and the "dummy" transaction.
*/
max_table_size *= 10;
PredXact = ShmemInitStruct("PredXactList",
PredXactListDataSize,
&found);
Assert(found == IsUnderPostmaster);
if (!found)
{
int i;
SHMQueueInit(&PredXact->availableList);
SHMQueueInit(&PredXact->activeList);
PredXact->SxactGlobalXmin = InvalidTransactionId;
PredXact->SxactGlobalXminCount = 0;
PredXact->WritableSxactCount = 0;
PredXact->LastSxactCommitSeqNo = FirstNormalSerCommitSeqNo - 1;
PredXact->CanPartialClearThrough = 0;
PredXact->HavePartialClearedThrough = 0;
requestSize = mul_size((Size) max_table_size,
PredXactListElementDataSize);
PredXact->element = ShmemAlloc(requestSize);
/* Add all elements to available list, clean. */
memset(PredXact->element, 0, requestSize);
for (i = 0; i < max_table_size; i++)
{
LWLockInitialize(&PredXact->element[i].sxact.perXactPredicateListLock,
LWTRANCHE_PER_XACT_PREDICATE_LIST);
SHMQueueInsertBefore(&(PredXact->availableList),
&(PredXact->element[i].link));
}
PredXact->OldCommittedSxact = CreatePredXact();
SetInvalidVirtualTransactionId(PredXact->OldCommittedSxact->vxid);
PredXact->OldCommittedSxact->prepareSeqNo = 0;
PredXact->OldCommittedSxact->commitSeqNo = 0;
PredXact->OldCommittedSxact->SeqNo.lastCommitBeforeSnapshot = 0;
SHMQueueInit(&PredXact->OldCommittedSxact->outConflicts);
SHMQueueInit(&PredXact->OldCommittedSxact->inConflicts);
SHMQueueInit(&PredXact->OldCommittedSxact->predicateLocks);
SHMQueueInit(&PredXact->OldCommittedSxact->finishedLink);
SHMQueueInit(&PredXact->OldCommittedSxact->possibleUnsafeConflicts);
PredXact->OldCommittedSxact->topXid = InvalidTransactionId;
PredXact->OldCommittedSxact->finishedBefore = InvalidTransactionId;
PredXact->OldCommittedSxact->xmin = InvalidTransactionId;
PredXact->OldCommittedSxact->flags = SXACT_FLAG_COMMITTED;
PredXact->OldCommittedSxact->pid = 0;
}
/* This never changes, so let's keep a local copy. */
OldCommittedSxact = PredXact->OldCommittedSxact;
/*
* Allocate hash table for SERIALIZABLEXID structs. This stores per-xid
* information for serializable transactions which have accessed data.
*/
info.keysize = sizeof(SERIALIZABLEXIDTAG);
info.entrysize = sizeof(SERIALIZABLEXID);
SerializableXidHash = ShmemInitHash("SERIALIZABLEXID hash",
max_table_size,
max_table_size,
&info,
HASH_ELEM | HASH_BLOBS |
HASH_FIXED_SIZE);
/*
* Allocate space for tracking rw-conflicts in lists attached to the
* transactions.
*
* Assume an average of 5 conflicts per transaction. Calculations suggest
* that this will prevent resource exhaustion in even the most pessimal
* loads up to max_connections = 200 with all 200 connections pounding the
* database with serializable transactions. Beyond that, there may be
* occasional transactions canceled when trying to flag conflicts. That's
* probably OK.
*/
max_table_size *= 5;
RWConflictPool = ShmemInitStruct("RWConflictPool",
RWConflictPoolHeaderDataSize,
&found);
Assert(found == IsUnderPostmaster);
if (!found)
{
int i;
SHMQueueInit(&RWConflictPool->availableList);
requestSize = mul_size((Size) max_table_size,
RWConflictDataSize);
RWConflictPool->element = ShmemAlloc(requestSize);
/* Add all elements to available list, clean. */
memset(RWConflictPool->element, 0, requestSize);
for (i = 0; i < max_table_size; i++)
{
SHMQueueInsertBefore(&(RWConflictPool->availableList),
&(RWConflictPool->element[i].outLink));
}
}
/*
* Create or attach to the header for the list of finished serializable
* transactions.
*/
FinishedSerializableTransactions = (SHM_QUEUE *)
ShmemInitStruct("FinishedSerializableTransactions",
sizeof(SHM_QUEUE),
&found);
Assert(found == IsUnderPostmaster);
if (!found)
SHMQueueInit(FinishedSerializableTransactions);
/*
* Initialize the SLRU storage for old committed serializable
* transactions.
*/
SerialInit();
}
/*
* Estimate shared-memory space used for predicate lock table
*/
Size
PredicateLockShmemSize(void)
{
Size size = 0;
long max_table_size;
/* predicate lock target hash table */
max_table_size = NPREDICATELOCKTARGETENTS();
size = add_size(size, hash_estimate_size(max_table_size,
sizeof(PREDICATELOCKTARGET)));
/* predicate lock hash table */
max_table_size *= 2;
size = add_size(size, hash_estimate_size(max_table_size,
sizeof(PREDICATELOCK)));
/*
* Since NPREDICATELOCKTARGETENTS is only an estimate, add 10% safety
* margin.
*/
size = add_size(size, size / 10);
/* transaction list */
max_table_size = MaxBackends + max_prepared_xacts;
max_table_size *= 10;
size = add_size(size, PredXactListDataSize);
size = add_size(size, mul_size((Size) max_table_size,
PredXactListElementDataSize));
/* transaction xid table */
size = add_size(size, hash_estimate_size(max_table_size,
sizeof(SERIALIZABLEXID)));
/* rw-conflict pool */
max_table_size *= 5;
size = add_size(size, RWConflictPoolHeaderDataSize);
size = add_size(size, mul_size((Size) max_table_size,
RWConflictDataSize));
/* Head for list of finished serializable transactions. */
size = add_size(size, sizeof(SHM_QUEUE));
/* Shared memory structures for SLRU tracking of old committed xids. */
size = add_size(size, sizeof(SerialControlData));
size = add_size(size, SimpleLruShmemSize(NUM_SERIAL_BUFFERS, 0));
return size;
}
/*
* Compute the hash code associated with a PREDICATELOCKTAG.
*
* Because we want to use just one set of partition locks for both the
* PREDICATELOCKTARGET and PREDICATELOCK hash tables, we have to make sure
* that PREDICATELOCKs fall into the same partition number as their
* associated PREDICATELOCKTARGETs. dynahash.c expects the partition number
* to be the low-order bits of the hash code, and therefore a
* PREDICATELOCKTAG's hash code must have the same low-order bits as the
* associated PREDICATELOCKTARGETTAG's hash code. We achieve this with this
* specialized hash function.
*/
static uint32
predicatelock_hash(const void *key, Size keysize)
{
const PREDICATELOCKTAG *predicatelocktag = (const PREDICATELOCKTAG *) key;
uint32 targethash;
Assert(keysize == sizeof(PREDICATELOCKTAG));
/* Look into the associated target object, and compute its hash code */
targethash = PredicateLockTargetTagHashCode(&predicatelocktag->myTarget->tag);
return PredicateLockHashCodeFromTargetHashCode(predicatelocktag, targethash);
}
/*
* GetPredicateLockStatusData
* Return a table containing the internal state of the predicate
* lock manager for use in pg_lock_status.
*
* Like GetLockStatusData, this function tries to hold the partition LWLocks
* for as short a time as possible by returning two arrays that simply
* contain the PREDICATELOCKTARGETTAG and SERIALIZABLEXACT for each lock
* table entry. Multiple copies of the same PREDICATELOCKTARGETTAG and
* SERIALIZABLEXACT will likely appear.
*/
PredicateLockData *
GetPredicateLockStatusData(void)
{
PredicateLockData *data;
int i;
int els,
el;
HASH_SEQ_STATUS seqstat;
PREDICATELOCK *predlock;
data = (PredicateLockData *) palloc(sizeof(PredicateLockData));
/*
* To ensure consistency, take simultaneous locks on all partition locks
* in ascending order, then SerializableXactHashLock.
*/
for (i = 0; i < NUM_PREDICATELOCK_PARTITIONS; i++)
LWLockAcquire(PredicateLockHashPartitionLockByIndex(i), LW_SHARED);
LWLockAcquire(SerializableXactHashLock, LW_SHARED);
/* Get number of locks and allocate appropriately-sized arrays. */
els = hash_get_num_entries(PredicateLockHash);
data->nelements = els;
data->locktags = (PREDICATELOCKTARGETTAG *)
palloc(sizeof(PREDICATELOCKTARGETTAG) * els);
data->xacts = (SERIALIZABLEXACT *)
palloc(sizeof(SERIALIZABLEXACT) * els);
/* Scan through PredicateLockHash and copy contents */
hash_seq_init(&seqstat, PredicateLockHash);
el = 0;
while ((predlock = (PREDICATELOCK *) hash_seq_search(&seqstat)))
{
data->locktags[el] = predlock->tag.myTarget->tag;
data->xacts[el] = *predlock->tag.myXact;
el++;
}
Assert(el == els);
/* Release locks in reverse order */
LWLockRelease(SerializableXactHashLock);
for (i = NUM_PREDICATELOCK_PARTITIONS - 1; i >= 0; i--)
LWLockRelease(PredicateLockHashPartitionLockByIndex(i));
return data;
}
/*
* Free up shared memory structures by pushing the oldest sxact (the one at
* the front of the SummarizeOldestCommittedSxact queue) into summary form.
* Each call will free exactly one SERIALIZABLEXACT structure and may also
* free one or more of these structures: SERIALIZABLEXID, PREDICATELOCK,
* PREDICATELOCKTARGET, RWConflictData.
*/
static void
SummarizeOldestCommittedSxact(void)
{
SERIALIZABLEXACT *sxact;
LWLockAcquire(SerializableFinishedListLock, LW_EXCLUSIVE);
/*
* This function is only called if there are no sxact slots available.
* Some of them must belong to old, already-finished transactions, so
* there should be something in FinishedSerializableTransactions list that
* we can summarize. However, there's a race condition: while we were not
* holding any locks, a transaction might have ended and cleaned up all
* the finished sxact entries already, freeing up their sxact slots. In
* that case, we have nothing to do here. The caller will find one of the
* slots released by the other backend when it retries.
*/
if (SHMQueueEmpty(FinishedSerializableTransactions))
{
LWLockRelease(SerializableFinishedListLock);
return;
}
/*
* Grab the first sxact off the finished list -- this will be the earliest
* commit. Remove it from the list.
*/
sxact = (SERIALIZABLEXACT *)
SHMQueueNext(FinishedSerializableTransactions,
FinishedSerializableTransactions,
offsetof(SERIALIZABLEXACT, finishedLink));
SHMQueueDelete(&(sxact->finishedLink));
/* Add to SLRU summary information. */
if (TransactionIdIsValid(sxact->topXid) && !SxactIsReadOnly(sxact))
SerialAdd(sxact->topXid, SxactHasConflictOut(sxact)
? sxact->SeqNo.earliestOutConflictCommit : InvalidSerCommitSeqNo);
/* Summarize and release the detail. */
ReleaseOneSerializableXact(sxact, false, true);
LWLockRelease(SerializableFinishedListLock);
}
/*
* GetSafeSnapshot
* Obtain and register a snapshot for a READ ONLY DEFERRABLE
* transaction. Ensures that the snapshot is "safe", i.e. a
* read-only transaction running on it can execute serializably
* without further checks. This requires waiting for concurrent
* transactions to complete, and retrying with a new snapshot if
* one of them could possibly create a conflict.
*
* As with GetSerializableTransactionSnapshot (which this is a subroutine
* for), the passed-in Snapshot pointer should reference a static data
* area that can safely be passed to GetSnapshotData.
*/
static Snapshot
GetSafeSnapshot(Snapshot origSnapshot)
{
Snapshot snapshot;
Assert(XactReadOnly && XactDeferrable);
while (true)
{
/*
* GetSerializableTransactionSnapshotInt is going to call
* GetSnapshotData, so we need to provide it the static snapshot area
* our caller passed to us. The pointer returned is actually the same
* one passed to it, but we avoid assuming that here.
*/
snapshot = GetSerializableTransactionSnapshotInt(origSnapshot,
NULL, InvalidPid);
if (MySerializableXact == InvalidSerializableXact)
return snapshot; /* no concurrent r/w xacts; it's safe */
LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
/*
* Wait for concurrent transactions to finish. Stop early if one of
* them marked us as conflicted.
*/
MySerializableXact->flags |= SXACT_FLAG_DEFERRABLE_WAITING;
while (!(SHMQueueEmpty(&MySerializableXact->possibleUnsafeConflicts) ||
SxactIsROUnsafe(MySerializableXact)))
{
LWLockRelease(SerializableXactHashLock);
ProcWaitForSignal(WAIT_EVENT_SAFE_SNAPSHOT);
LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
}
MySerializableXact->flags &= ~SXACT_FLAG_DEFERRABLE_WAITING;
if (!SxactIsROUnsafe(MySerializableXact))
{
LWLockRelease(SerializableXactHashLock);
break; /* success */
}
LWLockRelease(SerializableXactHashLock);
/* else, need to retry... */
ereport(DEBUG2,
(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
errmsg_internal("deferrable snapshot was unsafe; trying a new one")));
ReleasePredicateLocks(false, false);
}
/*
* Now we have a safe snapshot, so we don't need to do any further checks.
*/
Assert(SxactIsROSafe(MySerializableXact));
ReleasePredicateLocks(false, true);
return snapshot;
}
/*
* GetSafeSnapshotBlockingPids
* If the specified process is currently blocked in GetSafeSnapshot,
* write the process IDs of all processes that it is blocked by
* into the caller-supplied buffer output[]. The list is truncated at
* output_size, and the number of PIDs written into the buffer is
* returned. Returns zero if the given PID is not currently blocked
* in GetSafeSnapshot.
*/
int
GetSafeSnapshotBlockingPids(int blocked_pid, int *output, int output_size)
{
int num_written = 0;
SERIALIZABLEXACT *sxact;
LWLockAcquire(SerializableXactHashLock, LW_SHARED);
/* Find blocked_pid's SERIALIZABLEXACT by linear search. */
for (sxact = FirstPredXact(); sxact != NULL; sxact = NextPredXact(sxact))
{
if (sxact->pid == blocked_pid)
break;
}
/* Did we find it, and is it currently waiting in GetSafeSnapshot? */
if (sxact != NULL && SxactIsDeferrableWaiting(sxact))
{
RWConflict possibleUnsafeConflict;
/* Traverse the list of possible unsafe conflicts collecting PIDs. */
possibleUnsafeConflict = (RWConflict)
SHMQueueNext(&sxact->possibleUnsafeConflicts,
&sxact->possibleUnsafeConflicts,
offsetof(RWConflictData, inLink));
while (possibleUnsafeConflict != NULL && num_written < output_size)
{
output[num_written++] = possibleUnsafeConflict->sxactOut->pid;
possibleUnsafeConflict = (RWConflict)
SHMQueueNext(&sxact->possibleUnsafeConflicts,
&possibleUnsafeConflict->inLink,
offsetof(RWConflictData, inLink));
}
}
LWLockRelease(SerializableXactHashLock);
return num_written;
}
/*
* Acquire a snapshot that can be used for the current transaction.
*
* Make sure we have a SERIALIZABLEXACT reference in MySerializableXact.
* It should be current for this process and be contained in PredXact.
*
* The passed-in Snapshot pointer should reference a static data area that
* can safely be passed to GetSnapshotData. The return value is actually
* always this same pointer; no new snapshot data structure is allocated
* within this function.
*/
Snapshot
GetSerializableTransactionSnapshot(Snapshot snapshot)
{
Assert(IsolationIsSerializable());
/*
* Can't use serializable mode while recovery is still active, as it is,
* for example, on a hot standby. We could get here despite the check in
* check_XactIsoLevel() if default_transaction_isolation is set to
* serializable, so phrase the hint accordingly.
*/
if (RecoveryInProgress())
ereport(ERROR,
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
errmsg("cannot use serializable mode in a hot standby"),
errdetail("\"default_transaction_isolation\" is set to \"serializable\"."),
errhint("You can use \"SET default_transaction_isolation = 'repeatable read'\" to change the default.")));
/*
* A special optimization is available for SERIALIZABLE READ ONLY
* DEFERRABLE transactions -- we can wait for a suitable snapshot and
* thereby avoid all SSI overhead once it's running.
*/
if (XactReadOnly && XactDeferrable)
return GetSafeSnapshot(snapshot);
return GetSerializableTransactionSnapshotInt(snapshot,
NULL, InvalidPid);
}
/*
* Import a snapshot to be used for the current transaction.
*
* This is nearly the same as GetSerializableTransactionSnapshot, except that
* we don't take a new snapshot, but rather use the data we're handed.
*
* The caller must have verified that the snapshot came from a serializable
* transaction; and if we're read-write, the source transaction must not be
* read-only.
*/
void
SetSerializableTransactionSnapshot(Snapshot snapshot,
VirtualTransactionId *sourcevxid,
int sourcepid)
{
Assert(IsolationIsSerializable());
/*
* If this is called by parallel.c in a parallel worker, we don't want to
* create a SERIALIZABLEXACT just yet because the leader's
* SERIALIZABLEXACT will be installed with AttachSerializableXact(). We
* also don't want to reject SERIALIZABLE READ ONLY DEFERRABLE in this
* case, because the leader has already determined that the snapshot it
* has passed us is safe. So there is nothing for us to do.
*/
if (IsParallelWorker())
return;
/*
* We do not allow SERIALIZABLE READ ONLY DEFERRABLE transactions to
* import snapshots, since there's no way to wait for a safe snapshot when
* we're using the snap we're told to. (XXX instead of throwing an error,
* we could just ignore the XactDeferrable flag?)
*/
if (XactReadOnly && XactDeferrable)
ereport(ERROR,
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
errmsg("a snapshot-importing transaction must not be READ ONLY DEFERRABLE")));
(void) GetSerializableTransactionSnapshotInt(snapshot, sourcevxid,
sourcepid);
}
/*
* Guts of GetSerializableTransactionSnapshot
*
* If sourcevxid is valid, this is actually an import operation and we should
* skip calling GetSnapshotData, because the snapshot contents are already
* loaded up. HOWEVER: to avoid race conditions, we must check that the
* source xact is still running after we acquire SerializableXactHashLock.
* We do that by calling ProcArrayInstallImportedXmin.
*/
static Snapshot
GetSerializableTransactionSnapshotInt(Snapshot snapshot,
VirtualTransactionId *sourcevxid,
int sourcepid)
{
PGPROC *proc;
VirtualTransactionId vxid;
SERIALIZABLEXACT *sxact,
*othersxact;
/* We only do this for serializable transactions. Once. */
Assert(MySerializableXact == InvalidSerializableXact);
Assert(!RecoveryInProgress());
/*
* Since all parts of a serializable transaction must use the same
* snapshot, it is too late to establish one after a parallel operation
* has begun.
*/
if (IsInParallelMode())
elog(ERROR, "cannot establish serializable snapshot during a parallel operation");
proc = MyProc;
Assert(proc != NULL);
GET_VXID_FROM_PGPROC(vxid, *proc);
/*
* First we get the sxact structure, which may involve looping and access
* to the "finished" list to free a structure for use.
*
* We must hold SerializableXactHashLock when taking/checking the snapshot
* to avoid race conditions, for much the same reasons that
* GetSnapshotData takes the ProcArrayLock. Since we might have to
* release SerializableXactHashLock to call SummarizeOldestCommittedSxact,
* this means we have to create the sxact first, which is a bit annoying
* (in particular, an elog(ERROR) in procarray.c would cause us to leak
* the sxact). Consider refactoring to avoid this.
*/
#ifdef TEST_SUMMARIZE_SERIAL
SummarizeOldestCommittedSxact();
#endif
LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
do
{
sxact = CreatePredXact();
/* If null, push out committed sxact to SLRU summary & retry. */
if (!sxact)
{
LWLockRelease(SerializableXactHashLock);
SummarizeOldestCommittedSxact();
LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
}
} while (!sxact);
/* Get the snapshot, or check that it's safe to use */
if (!sourcevxid)
snapshot = GetSnapshotData(snapshot, DistributedTransactionContext);
else if (!ProcArrayInstallImportedXmin(snapshot->xmin, sourcevxid))
{
ReleasePredXact(sxact);
LWLockRelease(SerializableXactHashLock);
ereport(ERROR,
(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
errmsg("could not import the requested snapshot"),
errdetail("The source process with PID %d is not running anymore.",
sourcepid)));
}
/*
* If there are no serializable transactions which are not read-only, we
* can "opt out" of predicate locking and conflict checking for a
* read-only transaction.
*
* The reason this is safe is that a read-only transaction can only become
* part of a dangerous structure if it overlaps a writable transaction
* which in turn overlaps a writable transaction which committed before
* the read-only transaction started. A new writable transaction can
* overlap this one, but it can't meet the other condition of overlapping
* a transaction which committed before this one started.
*/
if (XactReadOnly && PredXact->WritableSxactCount == 0)
{
ReleasePredXact(sxact);
LWLockRelease(SerializableXactHashLock);
return snapshot;
}
/* Maintain serializable global xmin info. */
if (!TransactionIdIsValid(PredXact->SxactGlobalXmin))
{
Assert(PredXact->SxactGlobalXminCount == 0);
PredXact->SxactGlobalXmin = snapshot->xmin;
PredXact->SxactGlobalXminCount = 1;
SerialSetActiveSerXmin(snapshot->xmin);
}
else if (TransactionIdEquals(snapshot->xmin, PredXact->SxactGlobalXmin))
{
Assert(PredXact->SxactGlobalXminCount > 0);
PredXact->SxactGlobalXminCount++;
}
else
{
Assert(TransactionIdFollows(snapshot->xmin, PredXact->SxactGlobalXmin));
}
/* Initialize the structure. */
sxact->vxid = vxid;
sxact->SeqNo.lastCommitBeforeSnapshot = PredXact->LastSxactCommitSeqNo;
sxact->prepareSeqNo = InvalidSerCommitSeqNo;
sxact->commitSeqNo = InvalidSerCommitSeqNo;
SHMQueueInit(&(sxact->outConflicts));
SHMQueueInit(&(sxact->inConflicts));
SHMQueueInit(&(sxact->possibleUnsafeConflicts));
sxact->topXid = GetTopTransactionIdIfAny();
sxact->finishedBefore = InvalidTransactionId;
sxact->xmin = snapshot->xmin;
sxact->pid = MyProcPid;
SHMQueueInit(&(sxact->predicateLocks));
SHMQueueElemInit(&(sxact->finishedLink));
sxact->flags = 0;
if (XactReadOnly)
{
sxact->flags |= SXACT_FLAG_READ_ONLY;
/*
* Register all concurrent r/w transactions as possible conflicts; if
* all of them commit without any outgoing conflicts to earlier
* transactions then this snapshot can be deemed safe (and we can run
* without tracking predicate locks).
*/
for (othersxact = FirstPredXact();
othersxact != NULL;
othersxact = NextPredXact(othersxact))
{
if (!SxactIsCommitted(othersxact)
&& !SxactIsDoomed(othersxact)
&& !SxactIsReadOnly(othersxact))
{
SetPossibleUnsafeConflict(sxact, othersxact);
}
}
}
else
{
++(PredXact->WritableSxactCount);
Assert(PredXact->WritableSxactCount <=
(MaxBackends + max_prepared_xacts));
}
MySerializableXact = sxact;
MyXactDidWrite = false; /* haven't written anything yet */
LWLockRelease(SerializableXactHashLock);
CreateLocalPredicateLockHash();
return snapshot;
}
static void
CreateLocalPredicateLockHash(void)
{
HASHCTL hash_ctl;
/* Initialize the backend-local hash table of parent locks */
Assert(LocalPredicateLockHash == NULL);
hash_ctl.keysize = sizeof(PREDICATELOCKTARGETTAG);
hash_ctl.entrysize = sizeof(LOCALPREDICATELOCK);
LocalPredicateLockHash = hash_create("Local predicate lock",
max_predicate_locks_per_xact,
&hash_ctl,
HASH_ELEM | HASH_BLOBS);
}
/*
* Register the top level XID in SerializableXidHash.
* Also store it for easy reference in MySerializableXact.
*/
void
RegisterPredicateLockingXid(TransactionId xid)
{
SERIALIZABLEXIDTAG sxidtag;
SERIALIZABLEXID *sxid;
bool found;
/*
* If we're not tracking predicate lock data for this transaction, we
* should ignore the request and return quickly.
*/
if (MySerializableXact == InvalidSerializableXact)
return;
/* We should have a valid XID and be at the top level. */
Assert(TransactionIdIsValid(xid));
LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
/* This should only be done once per transaction. */
Assert(MySerializableXact->topXid == InvalidTransactionId);
MySerializableXact->topXid = xid;
sxidtag.xid = xid;
sxid = (SERIALIZABLEXID *) hash_search(SerializableXidHash,
&sxidtag,
HASH_ENTER, &found);
Assert(!found);
/* Initialize the structure. */
sxid->myXact = MySerializableXact;
LWLockRelease(SerializableXactHashLock);
}
/*
* Check whether there are any predicate locks held by any transaction
* for the page at the given block number.
*
* Note that the transaction may be completed but not yet subject to
* cleanup due to overlapping serializable transactions. This must
* return valid information regardless of transaction isolation level.
*
* Also note that this doesn't check for a conflicting relation lock,
* just a lock specifically on the given page.
*
* One use is to support proper behavior during GiST index vacuum.
*/
bool
PageIsPredicateLocked(Relation relation, BlockNumber blkno)
{
PREDICATELOCKTARGETTAG targettag;
uint32 targettaghash;
LWLock *partitionLock;
PREDICATELOCKTARGET *target;
SET_PREDICATELOCKTARGETTAG_PAGE(targettag,
relation->rd_node.dbNode,
relation->rd_id,
blkno);
targettaghash = PredicateLockTargetTagHashCode(&targettag);
partitionLock = PredicateLockHashPartitionLock(targettaghash);
LWLockAcquire(partitionLock, LW_SHARED);
target = (PREDICATELOCKTARGET *)
hash_search_with_hash_value(PredicateLockTargetHash,
&targettag, targettaghash,
HASH_FIND, NULL);
LWLockRelease(partitionLock);
return (target != NULL);
}
/*
* Check whether a particular lock is held by this transaction.
*
* Important note: this function may return false even if the lock is
* being held, because it uses the local lock table which is not
* updated if another transaction modifies our lock list (e.g. to
* split an index page). It can also return true when a coarser
* granularity lock that covers this target is being held. Be careful
* to only use this function in circumstances where such errors are
* acceptable!
*/
static bool
PredicateLockExists(const PREDICATELOCKTARGETTAG *targettag)
{
LOCALPREDICATELOCK *lock;
/* check local hash table */
lock = (LOCALPREDICATELOCK *) hash_search(LocalPredicateLockHash,
targettag,
HASH_FIND, NULL);
if (!lock)
return false;
/*
* Found entry in the table, but still need to check whether it's actually
* held -- it could just be a parent of some held lock.
*/
return lock->held;
}
/*
* Return the parent lock tag in the lock hierarchy: the next coarser
* lock that covers the provided tag.
*
* Returns true and sets *parent to the parent tag if one exists,
* returns false if none exists.
*/
static bool
GetParentPredicateLockTag(const PREDICATELOCKTARGETTAG *tag,
PREDICATELOCKTARGETTAG *parent)
{
switch (GET_PREDICATELOCKTARGETTAG_TYPE(*tag))
{
case PREDLOCKTAG_RELATION:
/* relation locks have no parent lock */
return false;
case PREDLOCKTAG_PAGE:
/* parent lock is relation lock */
SET_PREDICATELOCKTARGETTAG_RELATION(*parent,
GET_PREDICATELOCKTARGETTAG_DB(*tag),
GET_PREDICATELOCKTARGETTAG_RELATION(*tag));
return true;
case PREDLOCKTAG_TUPLE:
/* parent lock is page lock */
SET_PREDICATELOCKTARGETTAG_PAGE(*parent,
GET_PREDICATELOCKTARGETTAG_DB(*tag),
GET_PREDICATELOCKTARGETTAG_RELATION(*tag),
GET_PREDICATELOCKTARGETTAG_PAGE(*tag));
return true;
}
/* not reachable */
Assert(false);
return false;
}
/*
* Check whether the lock we are considering is already covered by a
* coarser lock for our transaction.
*
* Like PredicateLockExists, this function might return a false
* negative, but it will never return a false positive.
*/
static bool
CoarserLockCovers(const PREDICATELOCKTARGETTAG *newtargettag)
{
PREDICATELOCKTARGETTAG targettag,
parenttag;
targettag = *newtargettag;
/* check parents iteratively until no more */
while (GetParentPredicateLockTag(&targettag, &parenttag))
{
targettag = parenttag;
if (PredicateLockExists(&targettag))
return true;
}
/* no more parents to check; lock is not covered */
return false;
}
/*
* Remove the dummy entry from the predicate lock target hash, to free up some
* scratch space. The caller must be holding SerializablePredicateListLock,
* and must restore the entry with RestoreScratchTarget() before releasing the
* lock.
*
* If lockheld is true, the caller is already holding the partition lock
* of the partition containing the scratch entry.
*/
static void
RemoveScratchTarget(bool lockheld)
{
bool found;
Assert(LWLockHeldByMe(SerializablePredicateListLock));
if (!lockheld)
LWLockAcquire(ScratchPartitionLock, LW_EXCLUSIVE);
hash_search_with_hash_value(PredicateLockTargetHash,
&ScratchTargetTag,
ScratchTargetTagHash,
HASH_REMOVE, &found);
Assert(found);
if (!lockheld)
LWLockRelease(ScratchPartitionLock);
}
/*
* Re-insert the dummy entry in predicate lock target hash.
*/
static void
RestoreScratchTarget(bool lockheld)
{
bool found;
Assert(LWLockHeldByMe(SerializablePredicateListLock));
if (!lockheld)
LWLockAcquire(ScratchPartitionLock, LW_EXCLUSIVE);
hash_search_with_hash_value(PredicateLockTargetHash,
&ScratchTargetTag,
ScratchTargetTagHash,
HASH_ENTER, &found);
Assert(!found);
if (!lockheld)
LWLockRelease(ScratchPartitionLock);
}
/*
* Check whether the list of related predicate locks is empty for a
* predicate lock target, and remove the target if it is.
*/
static void
RemoveTargetIfNoLongerUsed(PREDICATELOCKTARGET *target, uint32 targettaghash)
{
PREDICATELOCKTARGET *rmtarget PG_USED_FOR_ASSERTS_ONLY;
Assert(LWLockHeldByMe(SerializablePredicateListLock));
/* Can't remove it until no locks at this target. */
if (!SHMQueueEmpty(&target->predicateLocks))
return;
/* Actually remove the target. */
rmtarget = hash_search_with_hash_value(PredicateLockTargetHash,
&target->tag,
targettaghash,
HASH_REMOVE, NULL);
Assert(rmtarget == target);
}
/*
* Delete child target locks owned by this process.
* This implementation is assuming that the usage of each target tag field
* is uniform. No need to make this hard if we don't have to.
*
* We acquire an LWLock in the case of parallel mode, because worker
* backends have access to the leader's SERIALIZABLEXACT. Otherwise,
* we aren't acquiring LWLocks for the predicate lock or lock
* target structures associated with this transaction unless we're going
* to modify them, because no other process is permitted to modify our
* locks.
*/
static void
DeleteChildTargetLocks(const PREDICATELOCKTARGETTAG *newtargettag)
{
SERIALIZABLEXACT *sxact;
PREDICATELOCK *predlock;
LWLockAcquire(SerializablePredicateListLock, LW_SHARED);
sxact = MySerializableXact;
if (IsInParallelMode())
LWLockAcquire(&sxact->perXactPredicateListLock, LW_EXCLUSIVE);
predlock = (PREDICATELOCK *)
SHMQueueNext(&(sxact->predicateLocks),
&(sxact->predicateLocks),
offsetof(PREDICATELOCK, xactLink));
while (predlock)
{
SHM_QUEUE *predlocksxactlink;
PREDICATELOCK *nextpredlock;
PREDICATELOCKTAG oldlocktag;
PREDICATELOCKTARGET *oldtarget;
PREDICATELOCKTARGETTAG oldtargettag;
predlocksxactlink = &(predlock->xactLink);
nextpredlock = (PREDICATELOCK *)
SHMQueueNext(&(sxact->predicateLocks),
predlocksxactlink,
offsetof(PREDICATELOCK, xactLink));
oldlocktag = predlock->tag;
Assert(oldlocktag.myXact == sxact);
oldtarget = oldlocktag.myTarget;
oldtargettag = oldtarget->tag;
if (TargetTagIsCoveredBy(oldtargettag, *newtargettag))
{
uint32 oldtargettaghash;
LWLock *partitionLock;
PREDICATELOCK *rmpredlock PG_USED_FOR_ASSERTS_ONLY;
oldtargettaghash = PredicateLockTargetTagHashCode(&oldtargettag);
partitionLock = PredicateLockHashPartitionLock(oldtargettaghash);
LWLockAcquire(partitionLock, LW_EXCLUSIVE);
SHMQueueDelete(predlocksxactlink);
SHMQueueDelete(&(predlock->targetLink));
rmpredlock = hash_search_with_hash_value
(PredicateLockHash,
&oldlocktag,
PredicateLockHashCodeFromTargetHashCode(&oldlocktag,
oldtargettaghash),
HASH_REMOVE, NULL);
Assert(rmpredlock == predlock);
RemoveTargetIfNoLongerUsed(oldtarget, oldtargettaghash);
LWLockRelease(partitionLock);
DecrementParentLocks(&oldtargettag);
}
predlock = nextpredlock;
}
if (IsInParallelMode())
LWLockRelease(&sxact->perXactPredicateListLock);
LWLockRelease(SerializablePredicateListLock);
}
/*
* Returns the promotion limit for a given predicate lock target. This is the
* max number of descendant locks allowed before promoting to the specified
* tag. Note that the limit includes non-direct descendants (e.g., both tuples
* and pages for a relation lock).
*
* Currently the default limit is 2 for a page lock, and half of the value of
* max_pred_locks_per_transaction - 1 for a relation lock, to match behavior
* of earlier releases when upgrading.
*
* TODO SSI: We should probably add additional GUCs to allow a maximum ratio
* of page and tuple locks based on the pages in a relation, and the maximum
* ratio of tuple locks to tuples in a page. This would provide more
* generally "balanced" allocation of locks to where they are most useful,
* while still allowing the absolute numbers to prevent one relation from
* tying up all predicate lock resources.
*/
static int
MaxPredicateChildLocks(const PREDICATELOCKTARGETTAG *tag)
{
switch (GET_PREDICATELOCKTARGETTAG_TYPE(*tag))
{
case PREDLOCKTAG_RELATION:
return max_predicate_locks_per_relation < 0
? (max_predicate_locks_per_xact
/ (-max_predicate_locks_per_relation)) - 1
: max_predicate_locks_per_relation;
case PREDLOCKTAG_PAGE:
return max_predicate_locks_per_page;
case PREDLOCKTAG_TUPLE:
/*
* not reachable: nothing is finer-granularity than a tuple, so we
* should never try to promote to it.
*/
Assert(false);
return 0;
}
/* not reachable */
Assert(false);
return 0;
}
/*
* For all ancestors of a newly-acquired predicate lock, increment
* their child count in the parent hash table. If any of them have
* more descendants than their promotion threshold, acquire the
* coarsest such lock.
*
* Returns true if a parent lock was acquired and false otherwise.
*/
static bool
CheckAndPromotePredicateLockRequest(const PREDICATELOCKTARGETTAG *reqtag)
{
PREDICATELOCKTARGETTAG targettag,
nexttag,
promotiontag;
LOCALPREDICATELOCK *parentlock;
bool found,
promote;
promote = false;
targettag = *reqtag;
/* check parents iteratively */
while (GetParentPredicateLockTag(&targettag, &nexttag))
{
targettag = nexttag;
parentlock = (LOCALPREDICATELOCK *) hash_search(LocalPredicateLockHash,
&targettag,
HASH_ENTER,
&found);
if (!found)
{
parentlock->held = false;
parentlock->childLocks = 1;
}
else
parentlock->childLocks++;
if (parentlock->childLocks >
MaxPredicateChildLocks(&targettag))
{
/*
* We should promote to this parent lock. Continue to check its
* ancestors, however, both to get their child counts right and to
* check whether we should just go ahead and promote to one of
* them.
*/
promotiontag = targettag;
promote = true;
}
}
if (promote)
{
/* acquire coarsest ancestor eligible for promotion */
PredicateLockAcquire(&promotiontag);
return true;
}
else
return false;
}
/*
* When releasing a lock, decrement the child count on all ancestor
* locks.
*
* This is called only when releasing a lock via
* DeleteChildTargetLocks (i.e. when a lock becomes redundant because
* we've acquired its parent, possibly due to promotion) or when a new
* MVCC write lock makes the predicate lock unnecessary. There's no
* point in calling it when locks are released at transaction end, as
* this information is no longer needed.
*/
static void
DecrementParentLocks(const PREDICATELOCKTARGETTAG *targettag)
{
PREDICATELOCKTARGETTAG parenttag,
nexttag;
parenttag = *targettag;
while (GetParentPredicateLockTag(&parenttag, &nexttag))
{
uint32 targettaghash;
LOCALPREDICATELOCK *parentlock,
*rmlock PG_USED_FOR_ASSERTS_ONLY;
parenttag = nexttag;
targettaghash = PredicateLockTargetTagHashCode(&parenttag);
parentlock = (LOCALPREDICATELOCK *)
hash_search_with_hash_value(LocalPredicateLockHash,
&parenttag, targettaghash,
HASH_FIND, NULL);
/*
* There's a small chance the parent lock doesn't exist in the lock
* table. This can happen if we prematurely removed it because an
* index split caused the child refcount to be off.
*/
if (parentlock == NULL)
continue;
parentlock->childLocks--;
/*
* Under similar circumstances the parent lock's refcount might be
* zero. This only happens if we're holding that lock (otherwise we
* would have removed the entry).
*/
if (parentlock->childLocks < 0)
{
Assert(parentlock->held);
parentlock->childLocks = 0;
}
if ((parentlock->childLocks == 0) && (!parentlock->held))
{
rmlock = (LOCALPREDICATELOCK *)
hash_search_with_hash_value(LocalPredicateLockHash,
&parenttag, targettaghash,
HASH_REMOVE, NULL);
Assert(rmlock == parentlock);
}
}
}
/*
* Indicate that a predicate lock on the given target is held by the
* specified transaction. Has no effect if the lock is already held.
*
* This updates the lock table and the sxact's lock list, and creates
* the lock target if necessary, but does *not* do anything related to
* granularity promotion or the local lock table. See
* PredicateLockAcquire for that.
*/
static void
CreatePredicateLock(const PREDICATELOCKTARGETTAG *targettag,
uint32 targettaghash,
SERIALIZABLEXACT *sxact)
{
PREDICATELOCKTARGET *target;
PREDICATELOCKTAG locktag;
PREDICATELOCK *lock;
LWLock *partitionLock;
bool found;
partitionLock = PredicateLockHashPartitionLock(targettaghash);
LWLockAcquire(SerializablePredicateListLock, LW_SHARED);
if (IsInParallelMode())
LWLockAcquire(&sxact->perXactPredicateListLock, LW_EXCLUSIVE);
LWLockAcquire(partitionLock, LW_EXCLUSIVE);
/* Make sure that the target is represented. */
target = (PREDICATELOCKTARGET *)
hash_search_with_hash_value(PredicateLockTargetHash,
targettag, targettaghash,
HASH_ENTER_NULL, &found);
if (!target)
ereport(ERROR,
(errcode(ERRCODE_OUT_OF_MEMORY),
errmsg("out of shared memory"),
errhint("You might need to increase max_pred_locks_per_transaction.")));
if (!found)
SHMQueueInit(&(target->predicateLocks));
/* We've got the sxact and target, make sure they're joined. */
locktag.myTarget = target;
locktag.myXact = sxact;
lock = (PREDICATELOCK *)
hash_search_with_hash_value(PredicateLockHash, &locktag,
PredicateLockHashCodeFromTargetHashCode(&locktag, targettaghash),
HASH_ENTER_NULL, &found);
if (!lock)
ereport(ERROR,
(errcode(ERRCODE_OUT_OF_MEMORY),
errmsg("out of shared memory"),
errhint("You might need to increase max_pred_locks_per_transaction.")));
if (!found)
{
SHMQueueInsertBefore(&(target->predicateLocks), &(lock->targetLink));
SHMQueueInsertBefore(&(sxact->predicateLocks),
&(lock->xactLink));
lock->commitSeqNo = InvalidSerCommitSeqNo;
}
LWLockRelease(partitionLock);
if (IsInParallelMode())
LWLockRelease(&sxact->perXactPredicateListLock);
LWLockRelease(SerializablePredicateListLock);
}
/*
* Acquire a predicate lock on the specified target for the current
* connection if not already held. This updates the local lock table
* and uses it to implement granularity promotion. It will consolidate
* multiple locks into a coarser lock if warranted, and will release
* any finer-grained locks covered by the new one.
*/
static void
PredicateLockAcquire(const PREDICATELOCKTARGETTAG *targettag)
{
uint32 targettaghash;
bool found;
LOCALPREDICATELOCK *locallock;
/* Do we have the lock already, or a covering lock? */
if (PredicateLockExists(targettag))
return;
if (CoarserLockCovers(targettag))
return;
/* the same hash and LW lock apply to the lock target and the local lock. */
targettaghash = PredicateLockTargetTagHashCode(targettag);
/* Acquire lock in local table */
locallock = (LOCALPREDICATELOCK *)
hash_search_with_hash_value(LocalPredicateLockHash,
targettag, targettaghash,
HASH_ENTER, &found);
locallock->held = true;
if (!found)
locallock->childLocks = 0;
/* Actually create the lock */
CreatePredicateLock(targettag, targettaghash, MySerializableXact);
/*
* Lock has been acquired. Check whether it should be promoted to a
* coarser granularity, or whether there are finer-granularity locks to
* clean up.
*/
if (CheckAndPromotePredicateLockRequest(targettag))
{
/*
* Lock request was promoted to a coarser-granularity lock, and that
* lock was acquired. It will delete this lock and any of its
* children, so we're done.
*/
}
else
{
/* Clean up any finer-granularity locks */
if (GET_PREDICATELOCKTARGETTAG_TYPE(*targettag) != PREDLOCKTAG_TUPLE)
DeleteChildTargetLocks(targettag);
}
}
/*
* PredicateLockRelation
*
* Gets a predicate lock at the relation level.
* Skip if not in full serializable transaction isolation level.
* Skip if this is a temporary table.
* Clear any finer-grained predicate locks this session has on the relation.
*/
void
PredicateLockRelation(Relation relation, Snapshot snapshot)
{
PREDICATELOCKTARGETTAG tag;
if (!SerializationNeededForRead(relation, snapshot))
return;
SET_PREDICATELOCKTARGETTAG_RELATION(tag,
relation->rd_node.dbNode,
relation->rd_id);
PredicateLockAcquire(&tag);
}
/*
* PredicateLockPage
*
* Gets a predicate lock at the page level.
* Skip if not in full serializable transaction isolation level.
* Skip if this is a temporary table.
* Skip if a coarser predicate lock already covers this page.
* Clear any finer-grained predicate locks this session has on the relation.
*/
void
PredicateLockPage(Relation relation, BlockNumber blkno, Snapshot snapshot)
{
PREDICATELOCKTARGETTAG tag;
if (!SerializationNeededForRead(relation, snapshot))
return;
SET_PREDICATELOCKTARGETTAG_PAGE(tag,
relation->rd_node.dbNode,
relation->rd_id,
blkno);
PredicateLockAcquire(&tag);
}
/*
* PredicateLockTID
*
* Gets a predicate lock at the tuple level.
* Skip if not in full serializable transaction isolation level.
* Skip if this is a temporary table.
*/
void
PredicateLockTID(Relation relation, ItemPointer tid, Snapshot snapshot,
TransactionId tuple_xid)
{
PREDICATELOCKTARGETTAG tag;
if (!SerializationNeededForRead(relation, snapshot))
return;
/*
* Return if this xact wrote it.
*/
if (relation->rd_index == NULL)
{
/* If we wrote it; we already have a write lock. */
if (TransactionIdIsCurrentTransactionId(tuple_xid))
return;
}
/*
* Do quick-but-not-definitive test for a relation lock first. This will
* never cause a return when the relation is *not* locked, but will
* occasionally let the check continue when there really *is* a relation
* level lock.
*/
SET_PREDICATELOCKTARGETTAG_RELATION(tag,
relation->rd_node.dbNode,
relation->rd_id);
if (PredicateLockExists(&tag))
return;
SET_PREDICATELOCKTARGETTAG_TUPLE(tag,
relation->rd_node.dbNode,
relation->rd_id,
ItemPointerGetBlockNumber(tid),
ItemPointerGetOffsetNumber(tid));
PredicateLockAcquire(&tag);
}
/*
* DeleteLockTarget
*
* Remove a predicate lock target along with any locks held for it.
*
* Caller must hold SerializablePredicateListLock and the
* appropriate hash partition lock for the target.
*/
static void
DeleteLockTarget(PREDICATELOCKTARGET *target, uint32 targettaghash)
{
PREDICATELOCK *predlock;
SHM_QUEUE *predlocktargetlink;
PREDICATELOCK *nextpredlock;
bool found;
Assert(LWLockHeldByMeInMode(SerializablePredicateListLock,
LW_EXCLUSIVE));
Assert(LWLockHeldByMe(PredicateLockHashPartitionLock(targettaghash)));
predlock = (PREDICATELOCK *)
SHMQueueNext(&(target->predicateLocks),
&(target->predicateLocks),
offsetof(PREDICATELOCK, targetLink));
LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
while (predlock)
{
predlocktargetlink = &(predlock->targetLink);
nextpredlock = (PREDICATELOCK *)
SHMQueueNext(&(target->predicateLocks),
predlocktargetlink,
offsetof(PREDICATELOCK, targetLink));
SHMQueueDelete(&(predlock->xactLink));
SHMQueueDelete(&(predlock->targetLink));
hash_search_with_hash_value
(PredicateLockHash,
&predlock->tag,
PredicateLockHashCodeFromTargetHashCode(&predlock->tag,
targettaghash),
HASH_REMOVE, &found);
Assert(found);
predlock = nextpredlock;
}
LWLockRelease(SerializableXactHashLock);
/* Remove the target itself, if possible. */
RemoveTargetIfNoLongerUsed(target, targettaghash);
}
/*
* TransferPredicateLocksToNewTarget
*
* Move or copy all the predicate locks for a lock target, for use by
* index page splits/combines and other things that create or replace
* lock targets. If 'removeOld' is true, the old locks and the target
* will be removed.
*
* Returns true on success, or false if we ran out of shared memory to
* allocate the new target or locks. Guaranteed to always succeed if
* removeOld is set (by using the scratch entry in PredicateLockTargetHash
* for scratch space).
*
* Warning: the "removeOld" option should be used only with care,
* because this function does not (indeed, can not) update other
* backends' LocalPredicateLockHash. If we are only adding new
* entries, this is not a problem: the local lock table is used only
* as a hint, so missing entries for locks that are held are
* OK. Having entries for locks that are no longer held, as can happen
* when using "removeOld", is not in general OK. We can only use it
* safely when replacing a lock with a coarser-granularity lock that
* covers it, or if we are absolutely certain that no one will need to
* refer to that lock in the future.
*
* Caller must hold SerializablePredicateListLock exclusively.
*/
static bool
TransferPredicateLocksToNewTarget(PREDICATELOCKTARGETTAG oldtargettag,
PREDICATELOCKTARGETTAG newtargettag,
bool removeOld)
{
uint32 oldtargettaghash;
LWLock *oldpartitionLock;
PREDICATELOCKTARGET *oldtarget;
uint32 newtargettaghash;
LWLock *newpartitionLock;
bool found;
bool outOfShmem = false;
Assert(LWLockHeldByMeInMode(SerializablePredicateListLock,
LW_EXCLUSIVE));
oldtargettaghash = PredicateLockTargetTagHashCode(&oldtargettag);
newtargettaghash = PredicateLockTargetTagHashCode(&newtargettag);
oldpartitionLock = PredicateLockHashPartitionLock(oldtargettaghash);
newpartitionLock = PredicateLockHashPartitionLock(newtargettaghash);
if (removeOld)
{
/*
* Remove the dummy entry to give us scratch space, so we know we'll
* be able to create the new lock target.
*/
RemoveScratchTarget(false);
}
/*
* We must get the partition locks in ascending sequence to avoid
* deadlocks. If old and new partitions are the same, we must request the
* lock only once.
*/
if (oldpartitionLock < newpartitionLock)
{
LWLockAcquire(oldpartitionLock,
(removeOld ? LW_EXCLUSIVE : LW_SHARED));
LWLockAcquire(newpartitionLock, LW_EXCLUSIVE);
}
else if (oldpartitionLock > newpartitionLock)
{
LWLockAcquire(newpartitionLock, LW_EXCLUSIVE);
LWLockAcquire(oldpartitionLock,
(removeOld ? LW_EXCLUSIVE : LW_SHARED));
}
else
LWLockAcquire(newpartitionLock, LW_EXCLUSIVE);
/*
* Look for the old target. If not found, that's OK; no predicate locks
* are affected, so we can just clean up and return. If it does exist,
* walk its list of predicate locks and move or copy them to the new
* target.
*/
oldtarget = hash_search_with_hash_value(PredicateLockTargetHash,
&oldtargettag,
oldtargettaghash,
HASH_FIND, NULL);
if (oldtarget)
{
PREDICATELOCKTARGET *newtarget;
PREDICATELOCK *oldpredlock;
PREDICATELOCKTAG newpredlocktag;
newtarget = hash_search_with_hash_value(PredicateLockTargetHash,
&newtargettag,
newtargettaghash,
HASH_ENTER_NULL, &found);
if (!newtarget)
{
/* Failed to allocate due to insufficient shmem */
outOfShmem = true;
goto exit;
}
/* If we created a new entry, initialize it */
if (!found)
SHMQueueInit(&(newtarget->predicateLocks));
newpredlocktag.myTarget = newtarget;
/*
* Loop through all the locks on the old target, replacing them with
* locks on the new target.
*/
oldpredlock = (PREDICATELOCK *)
SHMQueueNext(&(oldtarget->predicateLocks),
&(oldtarget->predicateLocks),
offsetof(PREDICATELOCK, targetLink));
LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
while (oldpredlock)
{
SHM_QUEUE *predlocktargetlink;
PREDICATELOCK *nextpredlock;
PREDICATELOCK *newpredlock;
SerCommitSeqNo oldCommitSeqNo = oldpredlock->commitSeqNo;
predlocktargetlink = &(oldpredlock->targetLink);
nextpredlock = (PREDICATELOCK *)
SHMQueueNext(&(oldtarget->predicateLocks),
predlocktargetlink,
offsetof(PREDICATELOCK, targetLink));
newpredlocktag.myXact = oldpredlock->tag.myXact;
if (removeOld)
{
SHMQueueDelete(&(oldpredlock->xactLink));
SHMQueueDelete(&(oldpredlock->targetLink));
hash_search_with_hash_value
(PredicateLockHash,
&oldpredlock->tag,
PredicateLockHashCodeFromTargetHashCode(&oldpredlock->tag,
oldtargettaghash),
HASH_REMOVE, &found);
Assert(found);
}
newpredlock = (PREDICATELOCK *)
hash_search_with_hash_value(PredicateLockHash,
&newpredlocktag,
PredicateLockHashCodeFromTargetHashCode(&newpredlocktag,
newtargettaghash),
HASH_ENTER_NULL,
&found);
if (!newpredlock)
{
/* Out of shared memory. Undo what we've done so far. */
LWLockRelease(SerializableXactHashLock);
DeleteLockTarget(newtarget, newtargettaghash);
outOfShmem = true;
goto exit;
}
if (!found)
{
SHMQueueInsertBefore(&(newtarget->predicateLocks),
&(newpredlock->targetLink));
SHMQueueInsertBefore(&(newpredlocktag.myXact->predicateLocks),
&(newpredlock->xactLink));
newpredlock->commitSeqNo = oldCommitSeqNo;
}
else
{
if (newpredlock->commitSeqNo < oldCommitSeqNo)
newpredlock->commitSeqNo = oldCommitSeqNo;
}
Assert(newpredlock->commitSeqNo != 0);
Assert((newpredlock->commitSeqNo == InvalidSerCommitSeqNo)
|| (newpredlock->tag.myXact == OldCommittedSxact));
oldpredlock = nextpredlock;
}
LWLockRelease(SerializableXactHashLock);
if (removeOld)
{
Assert(SHMQueueEmpty(&oldtarget->predicateLocks));
RemoveTargetIfNoLongerUsed(oldtarget, oldtargettaghash);
}
}
exit:
/* Release partition locks in reverse order of acquisition. */
if (oldpartitionLock < newpartitionLock)
{
LWLockRelease(newpartitionLock);
LWLockRelease(oldpartitionLock);
}
else if (oldpartitionLock > newpartitionLock)
{
LWLockRelease(oldpartitionLock);
LWLockRelease(newpartitionLock);
}
else
LWLockRelease(newpartitionLock);
if (removeOld)
{
/* We shouldn't run out of memory if we're moving locks */
Assert(!outOfShmem);
/* Put the scratch entry back */
RestoreScratchTarget(false);
}
return !outOfShmem;
}
/*
* Drop all predicate locks of any granularity from the specified relation,
* which can be a heap relation or an index relation. If 'transfer' is true,
* acquire a relation lock on the heap for any transactions with any lock(s)
* on the specified relation.
*
* This requires grabbing a lot of LW locks and scanning the entire lock
* target table for matches. That makes this more expensive than most
* predicate lock management functions, but it will only be called for DDL
* type commands that are expensive anyway, and there are fast returns when
* no serializable transactions are active or the relation is temporary.
*
* We don't use the TransferPredicateLocksToNewTarget function because it
* acquires its own locks on the partitions of the two targets involved,
* and we'll already be holding all partition locks.
*
* We can't throw an error from here, because the call could be from a
* transaction which is not serializable.
*
* NOTE: This is currently only called with transfer set to true, but that may
* change. If we decide to clean up the locks from a table on commit of a
* transaction which executed DROP TABLE, the false condition will be useful.
*/
static void
DropAllPredicateLocksFromTable(Relation relation, bool transfer)
{
HASH_SEQ_STATUS seqstat;
PREDICATELOCKTARGET *oldtarget;
PREDICATELOCKTARGET *heaptarget;
Oid dbId;
Oid relId;
Oid heapId;
int i;
bool isIndex;
bool found;
uint32 heaptargettaghash;
/*
* Bail out quickly if there are no serializable transactions running.
* It's safe to check this without taking locks because the caller is
* holding an ACCESS EXCLUSIVE lock on the relation. No new locks which
* would matter here can be acquired while that is held.
*/
if (!TransactionIdIsValid(PredXact->SxactGlobalXmin))
return;
if (!PredicateLockingNeededForRelation(relation))
return;
dbId = relation->rd_node.dbNode;
relId = relation->rd_id;
if (relation->rd_index == NULL)
{
isIndex = false;
heapId = relId;
}
else
{
isIndex = true;
heapId = relation->rd_index->indrelid;
}
Assert(heapId != InvalidOid);
Assert(transfer || !isIndex); /* index OID only makes sense with
* transfer */
/* Retrieve first time needed, then keep. */
heaptargettaghash = 0;
heaptarget = NULL;
/* Acquire locks on all lock partitions */
LWLockAcquire(SerializablePredicateListLock, LW_EXCLUSIVE);
for (i = 0; i < NUM_PREDICATELOCK_PARTITIONS; i++)
LWLockAcquire(PredicateLockHashPartitionLockByIndex(i), LW_EXCLUSIVE);
LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
/*
* Remove the dummy entry to give us scratch space, so we know we'll be
* able to create the new lock target.
*/
if (transfer)
RemoveScratchTarget(true);
/* Scan through target map */
hash_seq_init(&seqstat, PredicateLockTargetHash);
while ((oldtarget = (PREDICATELOCKTARGET *) hash_seq_search(&seqstat)))
{
PREDICATELOCK *oldpredlock;
/*
* Check whether this is a target which needs attention.
*/
if (GET_PREDICATELOCKTARGETTAG_RELATION(oldtarget->tag) != relId)
continue; /* wrong relation id */
if (GET_PREDICATELOCKTARGETTAG_DB(oldtarget->tag) != dbId)
continue; /* wrong database id */
if (transfer && !isIndex
&& GET_PREDICATELOCKTARGETTAG_TYPE(oldtarget->tag) == PREDLOCKTAG_RELATION)
continue; /* already the right lock */
/*
* If we made it here, we have work to do. We make sure the heap
* relation lock exists, then we walk the list of predicate locks for
* the old target we found, moving all locks to the heap relation lock
* -- unless they already hold that.
*/
/*
* First make sure we have the heap relation target. We only need to
* do this once.
*/
if (transfer && heaptarget == NULL)
{
PREDICATELOCKTARGETTAG heaptargettag;
SET_PREDICATELOCKTARGETTAG_RELATION(heaptargettag, dbId, heapId);
heaptargettaghash = PredicateLockTargetTagHashCode(&heaptargettag);
heaptarget = hash_search_with_hash_value(PredicateLockTargetHash,
&heaptargettag,
heaptargettaghash,
HASH_ENTER, &found);
if (!found)
SHMQueueInit(&heaptarget->predicateLocks);
}
/*
* Loop through all the locks on the old target, replacing them with
* locks on the new target.
*/
oldpredlock = (PREDICATELOCK *)
SHMQueueNext(&(oldtarget->predicateLocks),
&(oldtarget->predicateLocks),
offsetof(PREDICATELOCK, targetLink));
while (oldpredlock)
{
PREDICATELOCK *nextpredlock;
PREDICATELOCK *newpredlock;
SerCommitSeqNo oldCommitSeqNo;
SERIALIZABLEXACT *oldXact;
nextpredlock = (PREDICATELOCK *)
SHMQueueNext(&(oldtarget->predicateLocks),
&(oldpredlock->targetLink),
offsetof(PREDICATELOCK, targetLink));
/*
* Remove the old lock first. This avoids the chance of running
* out of lock structure entries for the hash table.
*/
oldCommitSeqNo = oldpredlock->commitSeqNo;
oldXact = oldpredlock->tag.myXact;
SHMQueueDelete(&(oldpredlock->xactLink));
/*
* No need for retail delete from oldtarget list, we're removing
* the whole target anyway.
*/
hash_search(PredicateLockHash,
&oldpredlock->tag,
HASH_REMOVE, &found);
Assert(found);
if (transfer)
{
PREDICATELOCKTAG newpredlocktag;
newpredlocktag.myTarget = heaptarget;
newpredlocktag.myXact = oldXact;
newpredlock = (PREDICATELOCK *)
hash_search_with_hash_value(PredicateLockHash,
&newpredlocktag,
PredicateLockHashCodeFromTargetHashCode(&newpredlocktag,
heaptargettaghash),
HASH_ENTER,
&found);
if (!found)
{
SHMQueueInsertBefore(&(heaptarget->predicateLocks),
&(newpredlock->targetLink));
SHMQueueInsertBefore(&(newpredlocktag.myXact->predicateLocks),
&(newpredlock->xactLink));
newpredlock->commitSeqNo = oldCommitSeqNo;
}
else
{
if (newpredlock->commitSeqNo < oldCommitSeqNo)
newpredlock->commitSeqNo = oldCommitSeqNo;
}
Assert(newpredlock->commitSeqNo != 0);
Assert((newpredlock->commitSeqNo == InvalidSerCommitSeqNo)
|| (newpredlock->tag.myXact == OldCommittedSxact));
}
oldpredlock = nextpredlock;
}
hash_search(PredicateLockTargetHash, &oldtarget->tag, HASH_REMOVE,
&found);
Assert(found);
}
/* Put the scratch entry back */
if (transfer)
RestoreScratchTarget(true);
/* Release locks in reverse order */
LWLockRelease(SerializableXactHashLock);
for (i = NUM_PREDICATELOCK_PARTITIONS - 1; i >= 0; i--)
LWLockRelease(PredicateLockHashPartitionLockByIndex(i));
LWLockRelease(SerializablePredicateListLock);
}
/*
* TransferPredicateLocksToHeapRelation
* For all transactions, transfer all predicate locks for the given
* relation to a single relation lock on the heap.
*/
void
TransferPredicateLocksToHeapRelation(Relation relation)
{
DropAllPredicateLocksFromTable(relation, true);
}
/*
* PredicateLockPageSplit
*
* Copies any predicate locks for the old page to the new page.
* Skip if this is a temporary table or toast table.
*
* NOTE: A page split (or overflow) affects all serializable transactions,
* even if it occurs in the context of another transaction isolation level.
*
* NOTE: This currently leaves the local copy of the locks without
* information on the new lock which is in shared memory. This could cause
* problems if enough page splits occur on locked pages without the processes
* which hold the locks getting in and noticing.
*/
void
PredicateLockPageSplit(Relation relation, BlockNumber oldblkno,
BlockNumber newblkno)
{
PREDICATELOCKTARGETTAG oldtargettag;
PREDICATELOCKTARGETTAG newtargettag;
bool success;
/*
* Bail out quickly if there are no serializable transactions running.
*
* It's safe to do this check without taking any additional locks. Even if
* a serializable transaction starts concurrently, we know it can't take
* any SIREAD locks on the page being split because the caller is holding
* the associated buffer page lock. Memory reordering isn't an issue; the
* memory barrier in the LWLock acquisition guarantees that this read
* occurs while the buffer page lock is held.
*/
if (!TransactionIdIsValid(PredXact->SxactGlobalXmin))
return;
if (!PredicateLockingNeededForRelation(relation))
return;
Assert(oldblkno != newblkno);
Assert(BlockNumberIsValid(oldblkno));
Assert(BlockNumberIsValid(newblkno));
SET_PREDICATELOCKTARGETTAG_PAGE(oldtargettag,
relation->rd_node.dbNode,
relation->rd_id,
oldblkno);
SET_PREDICATELOCKTARGETTAG_PAGE(newtargettag,
relation->rd_node.dbNode,
relation->rd_id,
newblkno);
LWLockAcquire(SerializablePredicateListLock, LW_EXCLUSIVE);
/*
* Try copying the locks over to the new page's tag, creating it if
* necessary.
*/
success = TransferPredicateLocksToNewTarget(oldtargettag,
newtargettag,
false);
if (!success)
{
/*
* No more predicate lock entries are available. Failure isn't an
* option here, so promote the page lock to a relation lock.
*/
/* Get the parent relation lock's lock tag */
success = GetParentPredicateLockTag(&oldtargettag,
&newtargettag);
Assert(success);
/*
* Move the locks to the parent. This shouldn't fail.
*
* Note that here we are removing locks held by other backends,
* leading to a possible inconsistency in their local lock hash table.
* This is OK because we're replacing it with a lock that covers the
* old one.
*/
success = TransferPredicateLocksToNewTarget(oldtargettag,
newtargettag,
true);
Assert(success);
}
LWLockRelease(SerializablePredicateListLock);
}
/*
* PredicateLockPageCombine
*
* Combines predicate locks for two existing pages.
* Skip if this is a temporary table or toast table.
*
* NOTE: A page combine affects all serializable transactions, even if it
* occurs in the context of another transaction isolation level.
*/
void
PredicateLockPageCombine(Relation relation, BlockNumber oldblkno,
BlockNumber newblkno)
{
/*
* Page combines differ from page splits in that we ought to be able to
* remove the locks on the old page after transferring them to the new
* page, instead of duplicating them. However, because we can't edit other
* backends' local lock tables, removing the old lock would leave them
* with an entry in their LocalPredicateLockHash for a lock they're not
* holding, which isn't acceptable. So we wind up having to do the same
* work as a page split, acquiring a lock on the new page and keeping the
* old page locked too. That can lead to some false positives, but should
* be rare in practice.
*/
PredicateLockPageSplit(relation, oldblkno, newblkno);
}
/*
* Walk the list of in-progress serializable transactions and find the new
* xmin.
*/
static void
SetNewSxactGlobalXmin(void)
{
SERIALIZABLEXACT *sxact;
Assert(LWLockHeldByMe(SerializableXactHashLock));
PredXact->SxactGlobalXmin = InvalidTransactionId;
PredXact->SxactGlobalXminCount = 0;
for (sxact = FirstPredXact(); sxact != NULL; sxact = NextPredXact(sxact))
{
if (!SxactIsRolledBack(sxact)
&& !SxactIsCommitted(sxact)
&& sxact != OldCommittedSxact)
{
Assert(sxact->xmin != InvalidTransactionId);
if (!TransactionIdIsValid(PredXact->SxactGlobalXmin)
|| TransactionIdPrecedes(sxact->xmin,
PredXact->SxactGlobalXmin))
{
PredXact->SxactGlobalXmin = sxact->xmin;
PredXact->SxactGlobalXminCount = 1;
}
else if (TransactionIdEquals(sxact->xmin,
PredXact->SxactGlobalXmin))
PredXact->SxactGlobalXminCount++;
}
}
SerialSetActiveSerXmin(PredXact->SxactGlobalXmin);
}
/*
* ReleasePredicateLocks
*
* Releases predicate locks based on completion of the current transaction,
* whether committed or rolled back. It can also be called for a read only
* transaction when it becomes impossible for the transaction to become
* part of a dangerous structure.
*
* We do nothing unless this is a serializable transaction.
*
* This method must ensure that shared memory hash tables are cleaned
* up in some relatively timely fashion.
*
* If this transaction is committing and is holding any predicate locks,
* it must be added to a list of completed serializable transactions still
* holding locks.
*
* If isReadOnlySafe is true, then predicate locks are being released before
* the end of the transaction because MySerializableXact has been determined
* to be RO_SAFE. In non-parallel mode we can release it completely, but it
* in parallel mode we partially release the SERIALIZABLEXACT and keep it
* around until the end of the transaction, allowing each backend to clear its
* MySerializableXact variable and benefit from the optimization in its own
* time.
*/
void
ReleasePredicateLocks(bool isCommit, bool isReadOnlySafe)
{
bool needToClear;
RWConflict conflict,
nextConflict,
possibleUnsafeConflict;
SERIALIZABLEXACT *roXact;
/*
* We can't trust XactReadOnly here, because a transaction which started
* as READ WRITE can show as READ ONLY later, e.g., within
* subtransactions. We want to flag a transaction as READ ONLY if it
* commits without writing so that de facto READ ONLY transactions get the
* benefit of some RO optimizations, so we will use this local variable to
* get some cleanup logic right which is based on whether the transaction
* was declared READ ONLY at the top level.
*/
bool topLevelIsDeclaredReadOnly;
/* We can't be both committing and releasing early due to RO_SAFE. */
Assert(!(isCommit && isReadOnlySafe));
/* Are we at the end of a transaction, that is, a commit or abort? */
if (!isReadOnlySafe)
{
/*
* Parallel workers mustn't release predicate locks at the end of
* their transaction. The leader will do that at the end of its
* transaction.
*/
if (IsParallelWorker())
{
ReleasePredicateLocksLocal();
return;
}
/*
* By the time the leader in a parallel query reaches end of
* transaction, it has waited for all workers to exit.
*/
Assert(!ParallelContextActive());
/*
* If the leader in a parallel query earlier stashed a partially
* released SERIALIZABLEXACT for final clean-up at end of transaction
* (because workers might still have been accessing it), then it's
* time to restore it.
*/
if (SavedSerializableXact != InvalidSerializableXact)
{
Assert(MySerializableXact == InvalidSerializableXact);
MySerializableXact = SavedSerializableXact;
SavedSerializableXact = InvalidSerializableXact;
Assert(SxactIsPartiallyReleased(MySerializableXact));
}
}
if (MySerializableXact == InvalidSerializableXact)
{
Assert(LocalPredicateLockHash == NULL);
return;
}
LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
/*
* If the transaction is committing, but it has been partially released
* already, then treat this as a roll back. It was marked as rolled back.
*/
if (isCommit && SxactIsPartiallyReleased(MySerializableXact))
isCommit = false;
/*
* If we're called in the middle of a transaction because we discovered
* that the SXACT_FLAG_RO_SAFE flag was set, then we'll partially release
* it (that is, release the predicate locks and conflicts, but not the
* SERIALIZABLEXACT itself) if we're the first backend to have noticed.
*/
if (isReadOnlySafe && IsInParallelMode())
{
/*
* The leader needs to stash a pointer to it, so that it can
* completely release it at end-of-transaction.
*/
if (!IsParallelWorker())
SavedSerializableXact = MySerializableXact;
/*
* The first backend to reach this condition will partially release
* the SERIALIZABLEXACT. All others will just clear their
* backend-local state so that they stop doing SSI checks for the rest
* of the transaction.
*/
if (SxactIsPartiallyReleased(MySerializableXact))
{
LWLockRelease(SerializableXactHashLock);
ReleasePredicateLocksLocal();
return;
}
else
{
MySerializableXact->flags |= SXACT_FLAG_PARTIALLY_RELEASED;
/* ... and proceed to perform the partial release below. */
}
}
Assert(!isCommit || SxactIsPrepared(MySerializableXact));
Assert(!isCommit || !SxactIsDoomed(MySerializableXact));
Assert(!SxactIsCommitted(MySerializableXact));
Assert(SxactIsPartiallyReleased(MySerializableXact)
|| !SxactIsRolledBack(MySerializableXact));
/* may not be serializable during COMMIT/ROLLBACK PREPARED */
Assert(MySerializableXact->pid == 0 || IsolationIsSerializable());
/* We'd better not already be on the cleanup list. */
Assert(!SxactIsOnFinishedList(MySerializableXact));
topLevelIsDeclaredReadOnly = SxactIsReadOnly(MySerializableXact);
/*
* We don't hold XidGenLock lock here, assuming that TransactionId is
* atomic!
*
* If this value is changing, we don't care that much whether we get the
* old or new value -- it is just used to determine how far
* SxactGlobalXmin must advance before this transaction can be fully
* cleaned up. The worst that could happen is we wait for one more
* transaction to complete before freeing some RAM; correctness of visible
* behavior is not affected.
*/
MySerializableXact->finishedBefore = XidFromFullTransactionId(ShmemVariableCache->nextXid);
/*
* If it's not a commit it's either a rollback or a read-only transaction
* flagged SXACT_FLAG_RO_SAFE, and we can clear our locks immediately.
*/
if (isCommit)
{
MySerializableXact->flags |= SXACT_FLAG_COMMITTED;
MySerializableXact->commitSeqNo = ++(PredXact->LastSxactCommitSeqNo);
/* Recognize implicit read-only transaction (commit without write). */
if (!MyXactDidWrite)
MySerializableXact->flags |= SXACT_FLAG_READ_ONLY;
}
else
{
/*
* The DOOMED flag indicates that we intend to roll back this
* transaction and so it should not cause serialization failures for
* other transactions that conflict with it. Note that this flag might
* already be set, if another backend marked this transaction for
* abort.
*
* The ROLLED_BACK flag further indicates that ReleasePredicateLocks
* has been called, and so the SerializableXact is eligible for
* cleanup. This means it should not be considered when calculating
* SxactGlobalXmin.
*/
MySerializableXact->flags |= SXACT_FLAG_DOOMED;
MySerializableXact->flags |= SXACT_FLAG_ROLLED_BACK;
/*
* If the transaction was previously prepared, but is now failing due
* to a ROLLBACK PREPARED or (hopefully very rare) error after the
* prepare, clear the prepared flag. This simplifies conflict
* checking.
*/
MySerializableXact->flags &= ~SXACT_FLAG_PREPARED;
}
if (!topLevelIsDeclaredReadOnly)
{
Assert(PredXact->WritableSxactCount > 0);
if (--(PredXact->WritableSxactCount) == 0)
{
/*
* Release predicate locks and rw-conflicts in for all committed
* transactions. There are no longer any transactions which might
* conflict with the locks and no chance for new transactions to
* overlap. Similarly, existing conflicts in can't cause pivots,
* and any conflicts in which could have completed a dangerous
* structure would already have caused a rollback, so any
* remaining ones must be benign.
*/
PredXact->CanPartialClearThrough = PredXact->LastSxactCommitSeqNo;
}
}
else
{
/*
* Read-only transactions: clear the list of transactions that might
* make us unsafe. Note that we use 'inLink' for the iteration as
* opposed to 'outLink' for the r/w xacts.
*/
possibleUnsafeConflict = (RWConflict)
SHMQueueNext(&MySerializableXact->possibleUnsafeConflicts,
&MySerializableXact->possibleUnsafeConflicts,
offsetof(RWConflictData, inLink));
while (possibleUnsafeConflict)
{
nextConflict = (RWConflict)
SHMQueueNext(&MySerializableXact->possibleUnsafeConflicts,
&possibleUnsafeConflict->inLink,
offsetof(RWConflictData, inLink));
Assert(!SxactIsReadOnly(possibleUnsafeConflict->sxactOut));
Assert(MySerializableXact == possibleUnsafeConflict->sxactIn);
ReleaseRWConflict(possibleUnsafeConflict);
possibleUnsafeConflict = nextConflict;
}
}
/* Check for conflict out to old committed transactions. */
if (isCommit
&& !SxactIsReadOnly(MySerializableXact)
&& SxactHasSummaryConflictOut(MySerializableXact))
{
/*
* we don't know which old committed transaction we conflicted with,
* so be conservative and use FirstNormalSerCommitSeqNo here
*/
MySerializableXact->SeqNo.earliestOutConflictCommit =
FirstNormalSerCommitSeqNo;
MySerializableXact->flags |= SXACT_FLAG_CONFLICT_OUT;
}
/*
* Release all outConflicts to committed transactions. If we're rolling
* back clear them all. Set SXACT_FLAG_CONFLICT_OUT if any point to
* previously committed transactions.
*/
conflict = (RWConflict)
SHMQueueNext(&MySerializableXact->outConflicts,
&MySerializableXact->outConflicts,
offsetof(RWConflictData, outLink));
while (conflict)
{
nextConflict = (RWConflict)
SHMQueueNext(&MySerializableXact->outConflicts,
&conflict->outLink,
offsetof(RWConflictData, outLink));
if (isCommit
&& !SxactIsReadOnly(MySerializableXact)
&& SxactIsCommitted(conflict->sxactIn))
{
if ((MySerializableXact->flags & SXACT_FLAG_CONFLICT_OUT) == 0
|| conflict->sxactIn->prepareSeqNo < MySerializableXact->SeqNo.earliestOutConflictCommit)
MySerializableXact->SeqNo.earliestOutConflictCommit = conflict->sxactIn->prepareSeqNo;
MySerializableXact->flags |= SXACT_FLAG_CONFLICT_OUT;
}
if (!isCommit
|| SxactIsCommitted(conflict->sxactIn)
|| (conflict->sxactIn->SeqNo.lastCommitBeforeSnapshot >= PredXact->LastSxactCommitSeqNo))
ReleaseRWConflict(conflict);
conflict = nextConflict;
}
/*
* Release all inConflicts from committed and read-only transactions. If
* we're rolling back, clear them all.
*/
conflict = (RWConflict)
SHMQueueNext(&MySerializableXact->inConflicts,
&MySerializableXact->inConflicts,
offsetof(RWConflictData, inLink));
while (conflict)
{
nextConflict = (RWConflict)
SHMQueueNext(&MySerializableXact->inConflicts,
&conflict->inLink,
offsetof(RWConflictData, inLink));
if (!isCommit
|| SxactIsCommitted(conflict->sxactOut)
|| SxactIsReadOnly(conflict->sxactOut))
ReleaseRWConflict(conflict);
conflict = nextConflict;
}
if (!topLevelIsDeclaredReadOnly)
{
/*
* Remove ourselves from the list of possible conflicts for concurrent
* READ ONLY transactions, flagging them as unsafe if we have a
* conflict out. If any are waiting DEFERRABLE transactions, wake them
* up if they are known safe or known unsafe.
*/
possibleUnsafeConflict = (RWConflict)
SHMQueueNext(&MySerializableXact->possibleUnsafeConflicts,
&MySerializableXact->possibleUnsafeConflicts,
offsetof(RWConflictData, outLink));
while (possibleUnsafeConflict)
{
nextConflict = (RWConflict)
SHMQueueNext(&MySerializableXact->possibleUnsafeConflicts,
&possibleUnsafeConflict->outLink,
offsetof(RWConflictData, outLink));
roXact = possibleUnsafeConflict->sxactIn;
Assert(MySerializableXact == possibleUnsafeConflict->sxactOut);
Assert(SxactIsReadOnly(roXact));
/* Mark conflicted if necessary. */
if (isCommit
&& MyXactDidWrite
&& SxactHasConflictOut(MySerializableXact)
&& (MySerializableXact->SeqNo.earliestOutConflictCommit
<= roXact->SeqNo.lastCommitBeforeSnapshot))
{
/*
* This releases possibleUnsafeConflict (as well as all other
* possible conflicts for roXact)
*/
FlagSxactUnsafe(roXact);
}
else
{
ReleaseRWConflict(possibleUnsafeConflict);
/*
* If we were the last possible conflict, flag it safe. The
* transaction can now safely release its predicate locks (but
* that transaction's backend has to do that itself).
*/
if (SHMQueueEmpty(&roXact->possibleUnsafeConflicts))
roXact->flags |= SXACT_FLAG_RO_SAFE;
}
/*
* Wake up the process for a waiting DEFERRABLE transaction if we
* now know it's either safe or conflicted.
*/
if (SxactIsDeferrableWaiting(roXact) &&
(SxactIsROUnsafe(roXact) || SxactIsROSafe(roXact)))
ProcSendSignal(roXact->pid);
possibleUnsafeConflict = nextConflict;
}
}
/*
* Check whether it's time to clean up old transactions. This can only be
* done when the last serializable transaction with the oldest xmin among
* serializable transactions completes. We then find the "new oldest"
* xmin and purge any transactions which finished before this transaction
* was launched.
*/
needToClear = false;
if (TransactionIdEquals(MySerializableXact->xmin, PredXact->SxactGlobalXmin))
{
Assert(PredXact->SxactGlobalXminCount > 0);
if (--(PredXact->SxactGlobalXminCount) == 0)
{
SetNewSxactGlobalXmin();
needToClear = true;
}
}
LWLockRelease(SerializableXactHashLock);
LWLockAcquire(SerializableFinishedListLock, LW_EXCLUSIVE);
/* Add this to the list of transactions to check for later cleanup. */
if (isCommit)
SHMQueueInsertBefore(FinishedSerializableTransactions,
&MySerializableXact->finishedLink);
/*
* If we're releasing a RO_SAFE transaction in parallel mode, we'll only
* partially release it. That's necessary because other backends may have
* a reference to it. The leader will release the SERIALIZABLEXACT itself
* at the end of the transaction after workers have stopped running.
*/
if (!isCommit)
ReleaseOneSerializableXact(MySerializableXact,
isReadOnlySafe && IsInParallelMode(),
false);
LWLockRelease(SerializableFinishedListLock);
if (needToClear)
ClearOldPredicateLocks();
ReleasePredicateLocksLocal();
}
static void
ReleasePredicateLocksLocal(void)
{
MySerializableXact = InvalidSerializableXact;
MyXactDidWrite = false;
/* Delete per-transaction lock table */
if (LocalPredicateLockHash != NULL)
{
hash_destroy(LocalPredicateLockHash);
LocalPredicateLockHash = NULL;
}
}
/*
* Clear old predicate locks, belonging to committed transactions that are no
* longer interesting to any in-progress transaction.
*/
static void
ClearOldPredicateLocks(void)
{
SERIALIZABLEXACT *finishedSxact;
PREDICATELOCK *predlock;
/*
* Loop through finished transactions. They are in commit order, so we can
* stop as soon as we find one that's still interesting.
*/
LWLockAcquire(SerializableFinishedListLock, LW_EXCLUSIVE);
finishedSxact = (SERIALIZABLEXACT *)
SHMQueueNext(FinishedSerializableTransactions,
FinishedSerializableTransactions,
offsetof(SERIALIZABLEXACT, finishedLink));
LWLockAcquire(SerializableXactHashLock, LW_SHARED);
while (finishedSxact)
{
SERIALIZABLEXACT *nextSxact;
nextSxact = (SERIALIZABLEXACT *)
SHMQueueNext(FinishedSerializableTransactions,
&(finishedSxact->finishedLink),
offsetof(SERIALIZABLEXACT, finishedLink));
if (!TransactionIdIsValid(PredXact->SxactGlobalXmin)
|| TransactionIdPrecedesOrEquals(finishedSxact->finishedBefore,
PredXact->SxactGlobalXmin))
{
/*
* This transaction committed before any in-progress transaction
* took its snapshot. It's no longer interesting.
*/
LWLockRelease(SerializableXactHashLock);
SHMQueueDelete(&(finishedSxact->finishedLink));
ReleaseOneSerializableXact(finishedSxact, false, false);
LWLockAcquire(SerializableXactHashLock, LW_SHARED);
}
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);
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);
}
else
{
/* Still interesting. */
break;
}
finishedSxact = nextSxact;
}
LWLockRelease(SerializableXactHashLock);
/*
* Loop through predicate locks on dummy transaction for summarized data.
*/
LWLockAcquire(SerializablePredicateListLock, LW_SHARED);
predlock = (PREDICATELOCK *)
SHMQueueNext(&OldCommittedSxact->predicateLocks,
&OldCommittedSxact->predicateLocks,
offsetof(PREDICATELOCK, xactLink));
while (predlock)
{
PREDICATELOCK *nextpredlock;
bool canDoPartialCleanup;
nextpredlock = (PREDICATELOCK *)
SHMQueueNext(&OldCommittedSxact->predicateLocks,
&predlock->xactLink,
offsetof(PREDICATELOCK, xactLink));
LWLockAcquire(SerializableXactHashLock, LW_SHARED);
Assert(predlock->commitSeqNo != 0);
Assert(predlock->commitSeqNo != InvalidSerCommitSeqNo);
canDoPartialCleanup = (predlock->commitSeqNo <= PredXact->CanPartialClearThrough);
LWLockRelease(SerializableXactHashLock);
/*
* If this lock originally belonged to an old enough transaction, we
* can release it.
*/
if (canDoPartialCleanup)
{
PREDICATELOCKTAG tag;
PREDICATELOCKTARGET *target;
PREDICATELOCKTARGETTAG targettag;
uint32 targettaghash;
LWLock *partitionLock;
tag = predlock->tag;
target = tag.myTarget;
targettag = target->tag;
targettaghash = PredicateLockTargetTagHashCode(&targettag);
partitionLock = PredicateLockHashPartitionLock(targettaghash);
LWLockAcquire(partitionLock, LW_EXCLUSIVE);
SHMQueueDelete(&(predlock->targetLink));
SHMQueueDelete(&(predlock->xactLink));
hash_search_with_hash_value(PredicateLockHash, &tag,
PredicateLockHashCodeFromTargetHashCode(&tag,
targettaghash),
HASH_REMOVE, NULL);
RemoveTargetIfNoLongerUsed(target, targettaghash);
LWLockRelease(partitionLock);
}
predlock = nextpredlock;
}
LWLockRelease(SerializablePredicateListLock);
LWLockRelease(SerializableFinishedListLock);
}
/*
* This is the normal way to delete anything from any of the predicate
* locking hash tables. Given a transaction which we know can be deleted:
* delete all predicate locks held by that transaction and any predicate
* lock targets which are now unreferenced by a lock; delete all conflicts
* for the transaction; delete all xid values for the transaction; then
* delete the transaction.
*
* When the partial flag is set, we can release all predicate locks and
* in-conflict information -- we've established that there are no longer
* any overlapping read write transactions for which this transaction could
* matter -- but keep the transaction entry itself and any outConflicts.
*
* When the summarize flag is set, we've run short of room for sxact data
* and must summarize to the SLRU. Predicate locks are transferred to a
* dummy "old" transaction, with duplicate locks on a single target
* collapsing to a single lock with the "latest" commitSeqNo from among
* the conflicting locks..
*/
static void
ReleaseOneSerializableXact(SERIALIZABLEXACT *sxact, bool partial,
bool summarize)
{
PREDICATELOCK *predlock;
SERIALIZABLEXIDTAG sxidtag;
RWConflict conflict,
nextConflict;
Assert(sxact != NULL);
Assert(SxactIsRolledBack(sxact) || SxactIsCommitted(sxact));
Assert(partial || !SxactIsOnFinishedList(sxact));
Assert(LWLockHeldByMe(SerializableFinishedListLock));
/*
* First release all the predicate locks held by this xact (or transfer
* them to OldCommittedSxact if summarize is true)
*/
LWLockAcquire(SerializablePredicateListLock, LW_SHARED);
if (IsInParallelMode())
LWLockAcquire(&sxact->perXactPredicateListLock, LW_EXCLUSIVE);
predlock = (PREDICATELOCK *)
SHMQueueNext(&(sxact->predicateLocks),
&(sxact->predicateLocks),
offsetof(PREDICATELOCK, xactLink));
while (predlock)
{
PREDICATELOCK *nextpredlock;
PREDICATELOCKTAG tag;
SHM_QUEUE *targetLink;
PREDICATELOCKTARGET *target;
PREDICATELOCKTARGETTAG targettag;
uint32 targettaghash;
LWLock *partitionLock;
nextpredlock = (PREDICATELOCK *)
SHMQueueNext(&(sxact->predicateLocks),
&(predlock->xactLink),
offsetof(PREDICATELOCK, xactLink));
tag = predlock->tag;
targetLink = &(predlock->targetLink);
target = tag.myTarget;
targettag = target->tag;
targettaghash = PredicateLockTargetTagHashCode(&targettag);
partitionLock = PredicateLockHashPartitionLock(targettaghash);
LWLockAcquire(partitionLock, LW_EXCLUSIVE);
SHMQueueDelete(targetLink);
hash_search_with_hash_value(PredicateLockHash, &tag,
PredicateLockHashCodeFromTargetHashCode(&tag,
targettaghash),
HASH_REMOVE, NULL);
if (summarize)
{
bool found;
/* Fold into dummy transaction list. */
tag.myXact = OldCommittedSxact;
predlock = hash_search_with_hash_value(PredicateLockHash, &tag,
PredicateLockHashCodeFromTargetHashCode(&tag,
targettaghash),
HASH_ENTER_NULL, &found);
if (!predlock)
ereport(ERROR,
(errcode(ERRCODE_OUT_OF_MEMORY),
errmsg("out of shared memory"),
errhint("You might need to increase max_pred_locks_per_transaction.")));
if (found)
{
Assert(predlock->commitSeqNo != 0);
Assert(predlock->commitSeqNo != InvalidSerCommitSeqNo);
if (predlock->commitSeqNo < sxact->commitSeqNo)
predlock->commitSeqNo = sxact->commitSeqNo;
}
else
{
SHMQueueInsertBefore(&(target->predicateLocks),
&(predlock->targetLink));
SHMQueueInsertBefore(&(OldCommittedSxact->predicateLocks),
&(predlock->xactLink));
predlock->commitSeqNo = sxact->commitSeqNo;
}
}
else
RemoveTargetIfNoLongerUsed(target, targettaghash);
LWLockRelease(partitionLock);
predlock = nextpredlock;
}
/*
* Rather than retail removal, just re-init the head after we've run
* through the list.
*/
SHMQueueInit(&sxact->predicateLocks);
if (IsInParallelMode())
LWLockRelease(&sxact->perXactPredicateListLock);
LWLockRelease(SerializablePredicateListLock);
sxidtag.xid = sxact->topXid;
LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
/* Release all outConflicts (unless 'partial' is true) */
if (!partial)
{
conflict = (RWConflict)
SHMQueueNext(&sxact->outConflicts,
&sxact->outConflicts,
offsetof(RWConflictData, outLink));
while (conflict)
{
nextConflict = (RWConflict)
SHMQueueNext(&sxact->outConflicts,
&conflict->outLink,
offsetof(RWConflictData, outLink));
if (summarize)
conflict->sxactIn->flags |= SXACT_FLAG_SUMMARY_CONFLICT_IN;
ReleaseRWConflict(conflict);
conflict = nextConflict;
}
}
/* Release all inConflicts. */
conflict = (RWConflict)
SHMQueueNext(&sxact->inConflicts,
&sxact->inConflicts,
offsetof(RWConflictData, inLink));
while (conflict)
{
nextConflict = (RWConflict)
SHMQueueNext(&sxact->inConflicts,
&conflict->inLink,
offsetof(RWConflictData, inLink));
if (summarize)
conflict->sxactOut->flags |= SXACT_FLAG_SUMMARY_CONFLICT_OUT;
ReleaseRWConflict(conflict);
conflict = nextConflict;
}
/* Finally, get rid of the xid and the record of the transaction itself. */
if (!partial)
{
if (sxidtag.xid != InvalidTransactionId)
hash_search(SerializableXidHash, &sxidtag, HASH_REMOVE, NULL);
ReleasePredXact(sxact);
}
LWLockRelease(SerializableXactHashLock);
}
/*
* Tests whether the given top level transaction is concurrent with
* (overlaps) our current transaction.
*
* We need to identify the top level transaction for SSI, anyway, so pass
* that to this function to save the overhead of checking the snapshot's
* subxip array.
*/
static bool
XidIsConcurrent(TransactionId xid)
{
Snapshot snap;
uint32 i;
Assert(TransactionIdIsValid(xid));
Assert(!TransactionIdEquals(xid, GetTopTransactionIdIfAny()));
snap = GetTransactionSnapshot();
if (TransactionIdPrecedes(xid, snap->xmin))
return false;
if (TransactionIdFollowsOrEquals(xid, snap->xmax))
return true;
for (i = 0; i < snap->xcnt; i++)
{
if (xid == snap->xip[i])
return true;
}
return false;
}
bool
CheckForSerializableConflictOutNeeded(Relation relation, Snapshot snapshot)
{
if (!SerializationNeededForRead(relation, snapshot))
return false;
/* Check if someone else has already decided that we need to die */
if (SxactIsDoomed(MySerializableXact))
{
ereport(ERROR,
(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
errmsg("could not serialize access due to read/write dependencies among transactions"),
errdetail_internal("Reason code: Canceled on identification as a pivot, during conflict out checking."),
errhint("The transaction might succeed if retried.")));
}
return true;
}
/*
* CheckForSerializableConflictOut
* A table AM is reading a tuple that has been modified. If it determines
* that the tuple version it is reading is not visible to us, it should
* pass in the top level xid of the transaction that created it.
* Otherwise, if it determines that it is visible to us but it has been
* deleted or there is a newer version available due to an update, it
* should pass in the top level xid of the modifying transaction.
*
* This function will check for overlap with our own transaction. If the given
* xid is also serializable and the transactions overlap (i.e., they cannot see
* each other's writes), then we have a conflict out.
*/
void
CheckForSerializableConflictOut(Relation relation, TransactionId xid, Snapshot snapshot)
{
SERIALIZABLEXIDTAG sxidtag;
SERIALIZABLEXID *sxid;
SERIALIZABLEXACT *sxact;
if (!SerializationNeededForRead(relation, snapshot))
return;
/* Check if someone else has already decided that we need to die */
if (SxactIsDoomed(MySerializableXact))
{
ereport(ERROR,
(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
errmsg("could not serialize access due to read/write dependencies among transactions"),
errdetail_internal("Reason code: Canceled on identification as a pivot, during conflict out checking."),
errhint("The transaction might succeed if retried.")));
}
Assert(TransactionIdIsValid(xid));
if (TransactionIdEquals(xid, GetTopTransactionIdIfAny()))
return;
/*
* Find sxact or summarized info for the top level xid.
*/
sxidtag.xid = xid;
LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
sxid = (SERIALIZABLEXID *)
hash_search(SerializableXidHash, &sxidtag, HASH_FIND, NULL);
if (!sxid)
{
/*
* Transaction not found in "normal" SSI structures. Check whether it
* got pushed out to SLRU storage for "old committed" transactions.
*/
SerCommitSeqNo conflictCommitSeqNo;
conflictCommitSeqNo = SerialGetMinConflictCommitSeqNo(xid);
if (conflictCommitSeqNo != 0)
{
if (conflictCommitSeqNo != InvalidSerCommitSeqNo
&& (!SxactIsReadOnly(MySerializableXact)
|| conflictCommitSeqNo
<= MySerializableXact->SeqNo.lastCommitBeforeSnapshot))
ereport(ERROR,
(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
errmsg("could not serialize access due to read/write dependencies among transactions"),
errdetail_internal("Reason code: Canceled on conflict out to old pivot %u.", xid),
errhint("The transaction might succeed if retried.")));
if (SxactHasSummaryConflictIn(MySerializableXact)
|| !SHMQueueEmpty(&MySerializableXact->inConflicts))
ereport(ERROR,
(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
errmsg("could not serialize access due to read/write dependencies among transactions"),
errdetail_internal("Reason code: Canceled on identification as a pivot, with conflict out to old committed transaction %u.", xid),
errhint("The transaction might succeed if retried.")));
MySerializableXact->flags |= SXACT_FLAG_SUMMARY_CONFLICT_OUT;
}
/* It's not serializable or otherwise not important. */
LWLockRelease(SerializableXactHashLock);
return;
}
sxact = sxid->myXact;
Assert(TransactionIdEquals(sxact->topXid, xid));
if (sxact == MySerializableXact || SxactIsDoomed(sxact))
{
/* Can't conflict with ourself or a transaction that will roll back. */
LWLockRelease(SerializableXactHashLock);
return;
}
/*
* We have a conflict out to a transaction which has a conflict out to a
* summarized transaction. That summarized transaction must have
* committed first, and we can't tell when it committed in relation to our
* snapshot acquisition, so something needs to be canceled.
*/
if (SxactHasSummaryConflictOut(sxact))
{
if (!SxactIsPrepared(sxact))
{
sxact->flags |= SXACT_FLAG_DOOMED;
LWLockRelease(SerializableXactHashLock);
return;
}
else
{
LWLockRelease(SerializableXactHashLock);
ereport(ERROR,
(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
errmsg("could not serialize access due to read/write dependencies among transactions"),
errdetail_internal("Reason code: Canceled on conflict out to old pivot."),
errhint("The transaction might succeed if retried.")));
}
}
/*
* If this is a read-only transaction and the writing transaction has
* committed, and it doesn't have a rw-conflict to a transaction which
* committed before it, no conflict.
*/
if (SxactIsReadOnly(MySerializableXact)
&& SxactIsCommitted(sxact)
&& !SxactHasSummaryConflictOut(sxact)
&& (!SxactHasConflictOut(sxact)
|| MySerializableXact->SeqNo.lastCommitBeforeSnapshot < sxact->SeqNo.earliestOutConflictCommit))
{
/* Read-only transaction will appear to run first. No conflict. */
LWLockRelease(SerializableXactHashLock);
return;
}
if (!XidIsConcurrent(xid))
{
/* This write was already in our snapshot; no conflict. */
LWLockRelease(SerializableXactHashLock);
return;
}
if (RWConflictExists(MySerializableXact, sxact))
{
/* We don't want duplicate conflict records in the list. */
LWLockRelease(SerializableXactHashLock);
return;
}
/*
* Flag the conflict. But first, if this conflict creates a dangerous
* structure, ereport an error.
*/
FlagRWConflict(MySerializableXact, sxact);
LWLockRelease(SerializableXactHashLock);
}
/*
* Check a particular target for rw-dependency conflict in. A subroutine of
* CheckForSerializableConflictIn().
*/
static void
CheckTargetForConflictsIn(PREDICATELOCKTARGETTAG *targettag)
{
uint32 targettaghash;
LWLock *partitionLock;
PREDICATELOCKTARGET *target;
PREDICATELOCK *predlock;
PREDICATELOCK *mypredlock = NULL;
PREDICATELOCKTAG mypredlocktag;
Assert(MySerializableXact != InvalidSerializableXact);
/*
* The same hash and LW lock apply to the lock target and the lock itself.
*/
targettaghash = PredicateLockTargetTagHashCode(targettag);
partitionLock = PredicateLockHashPartitionLock(targettaghash);
LWLockAcquire(partitionLock, LW_SHARED);
target = (PREDICATELOCKTARGET *)
hash_search_with_hash_value(PredicateLockTargetHash,
targettag, targettaghash,
HASH_FIND, NULL);
if (!target)
{
/* Nothing has this target locked; we're done here. */
LWLockRelease(partitionLock);
return;
}
/*
* Each lock for an overlapping transaction represents a conflict: a
* rw-dependency in to this transaction.
*/
predlock = (PREDICATELOCK *)
SHMQueueNext(&(target->predicateLocks),
&(target->predicateLocks),
offsetof(PREDICATELOCK, targetLink));
LWLockAcquire(SerializableXactHashLock, LW_SHARED);
while (predlock)
{
SHM_QUEUE *predlocktargetlink;
PREDICATELOCK *nextpredlock;
SERIALIZABLEXACT *sxact;
predlocktargetlink = &(predlock->targetLink);
nextpredlock = (PREDICATELOCK *)
SHMQueueNext(&(target->predicateLocks),
predlocktargetlink,
offsetof(PREDICATELOCK, targetLink));
sxact = predlock->tag.myXact;
if (sxact == MySerializableXact)
{
/*
* If we're getting a write lock on a tuple, we don't need a
* predicate (SIREAD) lock on the same tuple. We can safely remove
* our SIREAD lock, but we'll defer doing so until after the loop
* because that requires upgrading to an exclusive partition lock.
*
* We can't use this optimization within a subtransaction because
* the subtransaction could roll back, and we would be left
* without any lock at the top level.
*/
if (!IsSubTransaction()
&& GET_PREDICATELOCKTARGETTAG_OFFSET(*targettag))
{
mypredlock = predlock;
mypredlocktag = predlock->tag;
}
}
else if (!SxactIsDoomed(sxact)
&& (!SxactIsCommitted(sxact)
|| TransactionIdPrecedes(GetTransactionSnapshot()->xmin,
sxact->finishedBefore))
&& !RWConflictExists(sxact, MySerializableXact))
{
LWLockRelease(SerializableXactHashLock);
LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
/*
* Re-check after getting exclusive lock because the other
* transaction may have flagged a conflict.
*/
if (!SxactIsDoomed(sxact)
&& (!SxactIsCommitted(sxact)
|| TransactionIdPrecedes(GetTransactionSnapshot()->xmin,
sxact->finishedBefore))
&& !RWConflictExists(sxact, MySerializableXact))
{
FlagRWConflict(sxact, MySerializableXact);
}
LWLockRelease(SerializableXactHashLock);
LWLockAcquire(SerializableXactHashLock, LW_SHARED);
}
predlock = nextpredlock;
}
LWLockRelease(SerializableXactHashLock);
LWLockRelease(partitionLock);
/*
* If we found one of our own SIREAD locks to remove, remove it now.
*
* At this point our transaction already has a RowExclusiveLock on the
* relation, so we are OK to drop the predicate lock on the tuple, if
* found, without fearing that another write against the tuple will occur
* before the MVCC information makes it to the buffer.
*/
if (mypredlock != NULL)
{
uint32 predlockhashcode;
PREDICATELOCK *rmpredlock;
LWLockAcquire(SerializablePredicateListLock, LW_SHARED);
if (IsInParallelMode())
LWLockAcquire(&MySerializableXact->perXactPredicateListLock, LW_EXCLUSIVE);
LWLockAcquire(partitionLock, LW_EXCLUSIVE);
LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
/*
* Remove the predicate lock from shared memory, if it wasn't removed
* while the locks were released. One way that could happen is from
* autovacuum cleaning up an index.
*/
predlockhashcode = PredicateLockHashCodeFromTargetHashCode
(&mypredlocktag, targettaghash);
rmpredlock = (PREDICATELOCK *)
hash_search_with_hash_value(PredicateLockHash,
&mypredlocktag,
predlockhashcode,
HASH_FIND, NULL);
if (rmpredlock != NULL)
{
Assert(rmpredlock == mypredlock);
SHMQueueDelete(&(mypredlock->targetLink));
SHMQueueDelete(&(mypredlock->xactLink));
rmpredlock = (PREDICATELOCK *)
hash_search_with_hash_value(PredicateLockHash,
&mypredlocktag,
predlockhashcode,
HASH_REMOVE, NULL);
Assert(rmpredlock == mypredlock);
RemoveTargetIfNoLongerUsed(target, targettaghash);
}
LWLockRelease(SerializableXactHashLock);
LWLockRelease(partitionLock);
if (IsInParallelMode())
LWLockRelease(&MySerializableXact->perXactPredicateListLock);
LWLockRelease(SerializablePredicateListLock);
if (rmpredlock != NULL)
{
/*
* Remove entry in local lock table if it exists. It's OK if it
* doesn't exist; that means the lock was transferred to a new
* target by a different backend.
*/
hash_search_with_hash_value(LocalPredicateLockHash,
targettag, targettaghash,
HASH_REMOVE, NULL);
DecrementParentLocks(targettag);
}
}
}
/*
* CheckForSerializableConflictIn
* We are writing the given tuple. If that indicates a rw-conflict
* in from another serializable transaction, take appropriate action.
*
* Skip checking for any granularity for which a parameter is missing.
*
* A tuple update or delete is in conflict if we have a predicate lock
* against the relation or page in which the tuple exists, or against the
* tuple itself.
*/
void
CheckForSerializableConflictIn(Relation relation, ItemPointer tid, BlockNumber blkno)
{
PREDICATELOCKTARGETTAG targettag;
if (!SerializationNeededForWrite(relation))
return;
/* Check if someone else has already decided that we need to die */
if (SxactIsDoomed(MySerializableXact))
ereport(ERROR,
(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
errmsg("could not serialize access due to read/write dependencies among transactions"),
errdetail_internal("Reason code: Canceled on identification as a pivot, during conflict in checking."),
errhint("The transaction might succeed if retried.")));
/*
* We're doing a write which might cause rw-conflicts now or later.
* Memorize that fact.
*/
MyXactDidWrite = true;
/*
* It is important that we check for locks from the finest granularity to
* the coarsest granularity, so that granularity promotion doesn't cause
* us to miss a lock. The new (coarser) lock will be acquired before the
* old (finer) locks are released.
*
* It is not possible to take and hold a lock across the checks for all
* granularities because each target could be in a separate partition.
*/
if (tid != NULL)
{
SET_PREDICATELOCKTARGETTAG_TUPLE(targettag,
relation->rd_node.dbNode,
relation->rd_id,
ItemPointerGetBlockNumber(tid),
ItemPointerGetOffsetNumber(tid));
CheckTargetForConflictsIn(&targettag);
}
if (blkno != InvalidBlockNumber)
{
SET_PREDICATELOCKTARGETTAG_PAGE(targettag,
relation->rd_node.dbNode,
relation->rd_id,
blkno);
CheckTargetForConflictsIn(&targettag);
}
SET_PREDICATELOCKTARGETTAG_RELATION(targettag,
relation->rd_node.dbNode,
relation->rd_id);
CheckTargetForConflictsIn(&targettag);
}
/*
* CheckTableForSerializableConflictIn
* The entire table is going through a DDL-style logical mass delete
* like TRUNCATE or DROP TABLE. If that causes a rw-conflict in from
* another serializable transaction, take appropriate action.
*
* While these operations do not operate entirely within the bounds of
* snapshot isolation, they can occur inside a serializable transaction, and
* will logically occur after any reads which saw rows which were destroyed
* by these operations, so we do what we can to serialize properly under
* SSI.
*
* The relation passed in must be a heap relation. Any predicate lock of any
* granularity on the heap will cause a rw-conflict in to this transaction.
* Predicate locks on indexes do not matter because they only exist to guard
* against conflicting inserts into the index, and this is a mass *delete*.
* When a table is truncated or dropped, the index will also be truncated
* or dropped, and we'll deal with locks on the index when that happens.
*
* Dropping or truncating a table also needs to drop any existing predicate
* locks on heap tuples or pages, because they're about to go away. This
* should be done before altering the predicate locks because the transaction
* could be rolled back because of a conflict, in which case the lock changes
* are not needed. (At the moment, we don't actually bother to drop the
* existing locks on a dropped or truncated table at the moment. That might
* lead to some false positives, but it doesn't seem worth the trouble.)
*/
void
CheckTableForSerializableConflictIn(Relation relation)
{
HASH_SEQ_STATUS seqstat;
PREDICATELOCKTARGET *target;
Oid dbId;
Oid heapId;
int i;
/*
* Bail out quickly if there are no serializable transactions running.
* It's safe to check this without taking locks because the caller is
* holding an ACCESS EXCLUSIVE lock on the relation. No new locks which
* would matter here can be acquired while that is held.
*/
if (!TransactionIdIsValid(PredXact->SxactGlobalXmin))
return;
if (!SerializationNeededForWrite(relation))
return;
/*
* We're doing a write which might cause rw-conflicts now or later.
* Memorize that fact.
*/
MyXactDidWrite = true;
Assert(relation->rd_index == NULL); /* not an index relation */
dbId = relation->rd_node.dbNode;
heapId = relation->rd_id;
LWLockAcquire(SerializablePredicateListLock, LW_EXCLUSIVE);
for (i = 0; i < NUM_PREDICATELOCK_PARTITIONS; i++)
LWLockAcquire(PredicateLockHashPartitionLockByIndex(i), LW_SHARED);
LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
/* Scan through target list */
hash_seq_init(&seqstat, PredicateLockTargetHash);
while ((target = (PREDICATELOCKTARGET *) hash_seq_search(&seqstat)))
{
PREDICATELOCK *predlock;
/*
* Check whether this is a target which needs attention.
*/
if (GET_PREDICATELOCKTARGETTAG_RELATION(target->tag) != heapId)
continue; /* wrong relation id */
if (GET_PREDICATELOCKTARGETTAG_DB(target->tag) != dbId)
continue; /* wrong database id */
/*
* Loop through locks for this target and flag conflicts.
*/
predlock = (PREDICATELOCK *)
SHMQueueNext(&(target->predicateLocks),
&(target->predicateLocks),
offsetof(PREDICATELOCK, targetLink));
while (predlock)
{
PREDICATELOCK *nextpredlock;
nextpredlock = (PREDICATELOCK *)
SHMQueueNext(&(target->predicateLocks),
&(predlock->targetLink),
offsetof(PREDICATELOCK, targetLink));
if (predlock->tag.myXact != MySerializableXact
&& !RWConflictExists(predlock->tag.myXact, MySerializableXact))
{
FlagRWConflict(predlock->tag.myXact, MySerializableXact);
}
predlock = nextpredlock;
}
}
/* Release locks in reverse order */
LWLockRelease(SerializableXactHashLock);
for (i = NUM_PREDICATELOCK_PARTITIONS - 1; i >= 0; i--)
LWLockRelease(PredicateLockHashPartitionLockByIndex(i));
LWLockRelease(SerializablePredicateListLock);
}
/*
* Flag a rw-dependency between two serializable transactions.
*
* The caller is responsible for ensuring that we have a LW lock on
* the transaction hash table.
*/
static void
FlagRWConflict(SERIALIZABLEXACT *reader, SERIALIZABLEXACT *writer)
{
Assert(reader != writer);
/* First, see if this conflict causes failure. */
OnConflict_CheckForSerializationFailure(reader, writer);
/* Actually do the conflict flagging. */
if (reader == OldCommittedSxact)
writer->flags |= SXACT_FLAG_SUMMARY_CONFLICT_IN;
else if (writer == OldCommittedSxact)
reader->flags |= SXACT_FLAG_SUMMARY_CONFLICT_OUT;
else
SetRWConflict(reader, writer);
}
/*----------------------------------------------------------------------------
* We are about to add a RW-edge to the dependency graph - check that we don't
* introduce a dangerous structure by doing so, and abort one of the
* transactions if so.
*
* A serialization failure can only occur if there is a dangerous structure
* in the dependency graph:
*
* Tin ------> Tpivot ------> Tout
* rw rw
*
* Furthermore, Tout must commit first.
*
* One more optimization is that if Tin is declared READ ONLY (or commits
* without writing), we can only have a problem if Tout committed before Tin
* acquired its snapshot.
*----------------------------------------------------------------------------
*/
static void
OnConflict_CheckForSerializationFailure(const SERIALIZABLEXACT *reader,
SERIALIZABLEXACT *writer)
{
bool failure;
RWConflict conflict;
Assert(LWLockHeldByMe(SerializableXactHashLock));
failure = false;
/*------------------------------------------------------------------------
* Check for already-committed writer with rw-conflict out flagged
* (conflict-flag on W means that T2 committed before W):
*
* R ------> W ------> T2
* rw rw
*
* That is a dangerous structure, so we must abort. (Since the writer
* has already committed, we must be the reader)
*------------------------------------------------------------------------
*/
if (SxactIsCommitted(writer)
&& (SxactHasConflictOut(writer) || SxactHasSummaryConflictOut(writer)))
failure = true;
/*------------------------------------------------------------------------
* Check whether the writer has become a pivot with an out-conflict
* committed transaction (T2), and T2 committed first:
*
* R ------> W ------> T2
* rw rw
*
* Because T2 must've committed first, there is no anomaly if:
* - the reader committed before T2
* - the writer committed before T2
* - the reader is a READ ONLY transaction and the reader was concurrent
* with T2 (= reader acquired its snapshot before T2 committed)
*
* We also handle the case that T2 is prepared but not yet committed
* here. In that case T2 has already checked for conflicts, so if it
* commits first, making the above conflict real, it's too late for it
* to abort.
*------------------------------------------------------------------------
*/
if (!failure)
{
if (SxactHasSummaryConflictOut(writer))
{
failure = true;
conflict = NULL;
}
else
conflict = (RWConflict)
SHMQueueNext(&writer->outConflicts,
&writer->outConflicts,
offsetof(RWConflictData, outLink));
while (conflict)
{
SERIALIZABLEXACT *t2 = conflict->sxactIn;
if (SxactIsPrepared(t2)
&& (!SxactIsCommitted(reader)
|| t2->prepareSeqNo <= reader->commitSeqNo)
&& (!SxactIsCommitted(writer)
|| t2->prepareSeqNo <= writer->commitSeqNo)
&& (!SxactIsReadOnly(reader)
|| t2->prepareSeqNo <= reader->SeqNo.lastCommitBeforeSnapshot))
{
failure = true;
break;
}
conflict = (RWConflict)
SHMQueueNext(&writer->outConflicts,
&conflict->outLink,
offsetof(RWConflictData, outLink));
}
}
/*------------------------------------------------------------------------
* Check whether the reader has become a pivot with a writer
* that's committed (or prepared):
*
* T0 ------> R ------> W
* rw rw
*
* Because W must've committed first for an anomaly to occur, there is no
* anomaly if:
* - T0 committed before the writer
* - T0 is READ ONLY, and overlaps the writer
*------------------------------------------------------------------------
*/
if (!failure && SxactIsPrepared(writer) && !SxactIsReadOnly(reader))
{
if (SxactHasSummaryConflictIn(reader))
{
failure = true;
conflict = NULL;
}
else
conflict = (RWConflict)
SHMQueueNext(&reader->inConflicts,
&reader->inConflicts,
offsetof(RWConflictData, inLink));
while (conflict)
{
SERIALIZABLEXACT *t0 = conflict->sxactOut;
if (!SxactIsDoomed(t0)
&& (!SxactIsCommitted(t0)
|| t0->commitSeqNo >= writer->prepareSeqNo)
&& (!SxactIsReadOnly(t0)
|| t0->SeqNo.lastCommitBeforeSnapshot >= writer->prepareSeqNo))
{
failure = true;
break;
}
conflict = (RWConflict)
SHMQueueNext(&reader->inConflicts,
&conflict->inLink,
offsetof(RWConflictData, inLink));
}
}
if (failure)
{
/*
* We have to kill a transaction to avoid a possible anomaly from
* occurring. If the writer is us, we can just ereport() to cause a
* transaction abort. Otherwise we flag the writer for termination,
* causing it to abort when it tries to commit. However, if the writer
* is a prepared transaction, already prepared, we can't abort it
* anymore, so we have to kill the reader instead.
*/
if (MySerializableXact == writer)
{
LWLockRelease(SerializableXactHashLock);
ereport(ERROR,
(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
errmsg("could not serialize access due to read/write dependencies among transactions"),
errdetail_internal("Reason code: Canceled on identification as a pivot, during write."),
errhint("The transaction might succeed if retried.")));
}
else if (SxactIsPrepared(writer))
{
LWLockRelease(SerializableXactHashLock);
/* if we're not the writer, we have to be the reader */
Assert(MySerializableXact == reader);
ereport(ERROR,
(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
errmsg("could not serialize access due to read/write dependencies among transactions"),
errdetail_internal("Reason code: Canceled on conflict out to pivot %u, during read.", writer->topXid),
errhint("The transaction might succeed if retried.")));
}
writer->flags |= SXACT_FLAG_DOOMED;
}
}
/*
* PreCommit_CheckForSerializationFailure
* Check for dangerous structures in a serializable transaction
* at commit.
*
* We're checking for a dangerous structure as each conflict is recorded.
* The only way we could have a problem at commit is if this is the "out"
* side of a pivot, and neither the "in" side nor the pivot has yet
* committed.
*
* If a dangerous structure is found, the pivot (the near conflict) is
* marked for death, because rolling back another transaction might mean
* that we fail without ever making progress. This transaction is
* committing writes, so letting it commit ensures progress. If we
* canceled the far conflict, it might immediately fail again on retry.
*/
void
PreCommit_CheckForSerializationFailure(void)
{
RWConflict nearConflict;
if (MySerializableXact == InvalidSerializableXact)
return;
Assert(IsolationIsSerializable());
LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
/* Check if someone else has already decided that we need to die */
if (SxactIsDoomed(MySerializableXact))
{
Assert(!SxactIsPartiallyReleased(MySerializableXact));
LWLockRelease(SerializableXactHashLock);
ereport(ERROR,
(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
errmsg("could not serialize access due to read/write dependencies among transactions"),
errdetail_internal("Reason code: Canceled on identification as a pivot, during commit attempt."),
errhint("The transaction might succeed if retried.")));
}
nearConflict = (RWConflict)
SHMQueueNext(&MySerializableXact->inConflicts,
&MySerializableXact->inConflicts,
offsetof(RWConflictData, inLink));
while (nearConflict)
{
if (!SxactIsCommitted(nearConflict->sxactOut)
&& !SxactIsDoomed(nearConflict->sxactOut))
{
RWConflict farConflict;
farConflict = (RWConflict)
SHMQueueNext(&nearConflict->sxactOut->inConflicts,
&nearConflict->sxactOut->inConflicts,
offsetof(RWConflictData, inLink));
while (farConflict)
{
if (farConflict->sxactOut == MySerializableXact
|| (!SxactIsCommitted(farConflict->sxactOut)
&& !SxactIsReadOnly(farConflict->sxactOut)
&& !SxactIsDoomed(farConflict->sxactOut)))
{
/*
* Normally, we kill the pivot transaction to make sure we
* make progress if the failing transaction is retried.
* However, we can't kill it if it's already prepared, so
* in that case we commit suicide instead.
*/
if (SxactIsPrepared(nearConflict->sxactOut))
{
LWLockRelease(SerializableXactHashLock);
ereport(ERROR,
(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
errmsg("could not serialize access due to read/write dependencies among transactions"),
errdetail_internal("Reason code: Canceled on commit attempt with conflict in from prepared pivot."),
errhint("The transaction might succeed if retried.")));
}
nearConflict->sxactOut->flags |= SXACT_FLAG_DOOMED;
break;
}
farConflict = (RWConflict)
SHMQueueNext(&nearConflict->sxactOut->inConflicts,
&farConflict->inLink,
offsetof(RWConflictData, inLink));
}
}
nearConflict = (RWConflict)
SHMQueueNext(&MySerializableXact->inConflicts,
&nearConflict->inLink,
offsetof(RWConflictData, inLink));
}
MySerializableXact->prepareSeqNo = ++(PredXact->LastSxactCommitSeqNo);
MySerializableXact->flags |= SXACT_FLAG_PREPARED;
LWLockRelease(SerializableXactHashLock);
}
/*------------------------------------------------------------------------*/
/*
* Two-phase commit support
*/
/*
* AtPrepare_Locks
* Do the preparatory work for a PREPARE: make 2PC state file
* records for all predicate locks currently held.
*/
void
AtPrepare_PredicateLocks(void)
{
PREDICATELOCK *predlock;
SERIALIZABLEXACT *sxact;
TwoPhasePredicateRecord record;
TwoPhasePredicateXactRecord *xactRecord;
TwoPhasePredicateLockRecord *lockRecord;
sxact = MySerializableXact;
xactRecord = &(record.data.xactRecord);
lockRecord = &(record.data.lockRecord);
if (MySerializableXact == InvalidSerializableXact)
return;
/* Generate an xact record for our SERIALIZABLEXACT */
record.type = TWOPHASEPREDICATERECORD_XACT;
xactRecord->xmin = MySerializableXact->xmin;
xactRecord->flags = MySerializableXact->flags;
/*
* Note that we don't include the list of conflicts in our out in the
* statefile, because new conflicts can be added even after the
* transaction prepares. We'll just make a conservative assumption during
* recovery instead.
*/
RegisterTwoPhaseRecord(TWOPHASE_RM_PREDICATELOCK_ID, 0,
&record, sizeof(record));
/*
* Generate a lock record for each lock.
*
* To do this, we need to walk the predicate lock list in our sxact rather
* than using the local predicate lock table because the latter is not
* guaranteed to be accurate.
*/
LWLockAcquire(SerializablePredicateListLock, LW_SHARED);
/*
* No need to take sxact->perXactPredicateListLock in parallel mode
* because there cannot be any parallel workers running while we are
* preparing a transaction.
*/
Assert(!IsParallelWorker() && !ParallelContextActive());
predlock = (PREDICATELOCK *)
SHMQueueNext(&(sxact->predicateLocks),
&(sxact->predicateLocks),
offsetof(PREDICATELOCK, xactLink));
while (predlock != NULL)
{
record.type = TWOPHASEPREDICATERECORD_LOCK;
lockRecord->target = predlock->tag.myTarget->tag;
RegisterTwoPhaseRecord(TWOPHASE_RM_PREDICATELOCK_ID, 0,
&record, sizeof(record));
predlock = (PREDICATELOCK *)
SHMQueueNext(&(sxact->predicateLocks),
&(predlock->xactLink),
offsetof(PREDICATELOCK, xactLink));
}
LWLockRelease(SerializablePredicateListLock);
}
/*
* PostPrepare_Locks
* Clean up after successful PREPARE. Unlike the non-predicate
* lock manager, we do not need to transfer locks to a dummy
* PGPROC because our SERIALIZABLEXACT will stay around
* anyway. We only need to clean up our local state.
*/
void
PostPrepare_PredicateLocks(TransactionId xid)
{
if (MySerializableXact == InvalidSerializableXact)
return;
Assert(SxactIsPrepared(MySerializableXact));
MySerializableXact->pid = 0;
hash_destroy(LocalPredicateLockHash);
LocalPredicateLockHash = NULL;
MySerializableXact = InvalidSerializableXact;
MyXactDidWrite = false;
}
/*
* PredicateLockTwoPhaseFinish
* Release a prepared transaction's predicate locks once it
* commits or aborts.
*/
void
PredicateLockTwoPhaseFinish(TransactionId xid, bool isCommit)
{
SERIALIZABLEXID *sxid;
SERIALIZABLEXIDTAG sxidtag;
sxidtag.xid = xid;
LWLockAcquire(SerializableXactHashLock, LW_SHARED);
sxid = (SERIALIZABLEXID *)
hash_search(SerializableXidHash, &sxidtag, HASH_FIND, NULL);
LWLockRelease(SerializableXactHashLock);
/* xid will not be found if it wasn't a serializable transaction */
if (sxid == NULL)
return;
/* Release its locks */
MySerializableXact = sxid->myXact;
MyXactDidWrite = true; /* conservatively assume that we wrote
* something */
ReleasePredicateLocks(isCommit, false);
}
/*
* Re-acquire a predicate lock belonging to a transaction that was prepared.
*/
void
predicatelock_twophase_recover(TransactionId xid, uint16 info,
void *recdata, uint32 len)
{
TwoPhasePredicateRecord *record;
Assert(len == sizeof(TwoPhasePredicateRecord));
record = (TwoPhasePredicateRecord *) recdata;
Assert((record->type == TWOPHASEPREDICATERECORD_XACT) ||
(record->type == TWOPHASEPREDICATERECORD_LOCK));
if (record->type == TWOPHASEPREDICATERECORD_XACT)
{
/* Per-transaction record. Set up a SERIALIZABLEXACT. */
TwoPhasePredicateXactRecord *xactRecord;
SERIALIZABLEXACT *sxact;
SERIALIZABLEXID *sxid;
SERIALIZABLEXIDTAG sxidtag;
bool found;
xactRecord = (TwoPhasePredicateXactRecord *) &record->data.xactRecord;
LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
sxact = CreatePredXact();
if (!sxact)
ereport(ERROR,
(errcode(ERRCODE_OUT_OF_MEMORY),
errmsg("out of shared memory")));
/* vxid for a prepared xact is InvalidBackendId/xid; no pid */
sxact->vxid.backendId = InvalidBackendId;
sxact->vxid.localTransactionId = (LocalTransactionId) xid;
sxact->pid = 0;
/* a prepared xact hasn't committed yet */
sxact->prepareSeqNo = RecoverySerCommitSeqNo;
sxact->commitSeqNo = InvalidSerCommitSeqNo;
sxact->finishedBefore = InvalidTransactionId;
sxact->SeqNo.lastCommitBeforeSnapshot = RecoverySerCommitSeqNo;
/*
* Don't need to track this; no transactions running at the time the
* recovered xact started are still active, except possibly other
* prepared xacts and we don't care whether those are RO_SAFE or not.
*/
SHMQueueInit(&(sxact->possibleUnsafeConflicts));
SHMQueueInit(&(sxact->predicateLocks));
SHMQueueElemInit(&(sxact->finishedLink));
sxact->topXid = xid;
sxact->xmin = xactRecord->xmin;
sxact->flags = xactRecord->flags;
Assert(SxactIsPrepared(sxact));
if (!SxactIsReadOnly(sxact))
{
++(PredXact->WritableSxactCount);
Assert(PredXact->WritableSxactCount <=
(MaxBackends + max_prepared_xacts));
}
/*
* We don't know whether the transaction had any conflicts or not, so
* we'll conservatively assume that it had both a conflict in and a
* conflict out, and represent that with the summary conflict flags.
*/
SHMQueueInit(&(sxact->outConflicts));
SHMQueueInit(&(sxact->inConflicts));
sxact->flags |= SXACT_FLAG_SUMMARY_CONFLICT_IN;
sxact->flags |= SXACT_FLAG_SUMMARY_CONFLICT_OUT;
/* Register the transaction's xid */
sxidtag.xid = xid;
sxid = (SERIALIZABLEXID *) hash_search(SerializableXidHash,
&sxidtag,
HASH_ENTER, &found);
Assert(sxid != NULL);
Assert(!found);
sxid->myXact = (SERIALIZABLEXACT *) sxact;
/*
* Update global xmin. Note that this is a special case compared to
* registering a normal transaction, because the global xmin might go
* backwards. That's OK, because until recovery is over we're not
* going to complete any transactions or create any non-prepared
* transactions, so there's no danger of throwing away.
*/
if ((!TransactionIdIsValid(PredXact->SxactGlobalXmin)) ||
(TransactionIdFollows(PredXact->SxactGlobalXmin, sxact->xmin)))
{
PredXact->SxactGlobalXmin = sxact->xmin;
PredXact->SxactGlobalXminCount = 1;
SerialSetActiveSerXmin(sxact->xmin);
}
else if (TransactionIdEquals(sxact->xmin, PredXact->SxactGlobalXmin))
{
Assert(PredXact->SxactGlobalXminCount > 0);
PredXact->SxactGlobalXminCount++;
}
LWLockRelease(SerializableXactHashLock);
}
else if (record->type == TWOPHASEPREDICATERECORD_LOCK)
{
/* Lock record. Recreate the PREDICATELOCK */
TwoPhasePredicateLockRecord *lockRecord;
SERIALIZABLEXID *sxid;
SERIALIZABLEXACT *sxact;
SERIALIZABLEXIDTAG sxidtag;
uint32 targettaghash;
lockRecord = (TwoPhasePredicateLockRecord *) &record->data.lockRecord;
targettaghash = PredicateLockTargetTagHashCode(&lockRecord->target);
LWLockAcquire(SerializableXactHashLock, LW_SHARED);
sxidtag.xid = xid;
sxid = (SERIALIZABLEXID *)
hash_search(SerializableXidHash, &sxidtag, HASH_FIND, NULL);
LWLockRelease(SerializableXactHashLock);
Assert(sxid != NULL);
sxact = sxid->myXact;
Assert(sxact != InvalidSerializableXact);
CreatePredicateLock(&lockRecord->target, targettaghash, sxact);
}
}
/*
* Prepare to share the current SERIALIZABLEXACT with parallel workers.
* Return a handle object that can be used by AttachSerializableXact() in a
* parallel worker.
*/
SerializableXactHandle
ShareSerializableXact(void)
{
return MySerializableXact;
}
/*
* Allow parallel workers to import the leader's SERIALIZABLEXACT.
*/
void
AttachSerializableXact(SerializableXactHandle handle)
{
Assert(MySerializableXact == InvalidSerializableXact);
MySerializableXact = (SERIALIZABLEXACT *) handle;
if (MySerializableXact != InvalidSerializableXact)
CreateLocalPredicateLockHash();
}