Don't take ProcArrayLock while exiting a transaction that has no XID; there is
no need for serialization against snapshot-taking because the xact doesn't affect anyone else's snapshot anyway. Per discussion. Also, move various info about the interlocking of transactions and snapshots out of code comments and into a hopefully-more-cohesive discussion in access/transam/README. Also, remove a couple of now-obsolete comments about having to force some WAL to be written to persuade RecordTransactionCommit to do its thing.
This commit is contained in:
Tom Lane
parent
85e79a4a83
commit
0a51e7073c
@ -8,7 +8,7 @@
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*
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*
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* IDENTIFICATION
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* $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.238 2007/09/05 18:10:47 tgl Exp $
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* $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.239 2007/09/07 20:59:26 tgl Exp $
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*
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*
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* INTERFACE ROUTINES
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@ -1546,9 +1546,8 @@ UpdateXmaxHintBits(HeapTupleHeader tuple, Buffer buffer, TransactionId xid)
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* If use_wal is false, the new tuple is not logged in WAL, even for a
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* non-temp relation. Safe usage of this behavior requires that we arrange
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* that all new tuples go into new pages not containing any tuples from other
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* transactions, that the relation gets fsync'd before commit, and that the
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* transaction emits at least one WAL record to ensure RecordTransactionCommit
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* will decide to WAL-log the commit. (See also heap_sync() comments)
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* transactions, and that the relation gets fsync'd before commit.
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* (See also heap_sync() comments)
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*
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* use_fsm is passed directly to RelationGetBufferForTuple, which see for
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* more info.
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@ -1,4 +1,4 @@
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$PostgreSQL: pgsql/src/backend/access/transam/README,v 1.7 2007/09/05 18:10:47 tgl Exp $
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$PostgreSQL: pgsql/src/backend/access/transam/README,v 1.8 2007/09/07 20:59:26 tgl Exp $
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The Transaction System
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----------------------
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@ -221,6 +221,110 @@ InvalidSubTransactionId.) Note that subtransactions do not have their
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own VXIDs; they use the parent top transaction's VXID.
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Interlocking transaction begin, transaction end, and snapshots
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--------------------------------------------------------------
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We try hard to minimize the amount of overhead and lock contention involved
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in the frequent activities of beginning/ending a transaction and taking a
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snapshot. Unfortunately, we must have some interlocking for this, because
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we must ensure consistency about the commit order of transactions.
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For example, suppose an UPDATE in xact A is blocked by xact B's prior
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update of the same row, and xact B is doing commit while xact C gets a
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snapshot. Xact A can complete and commit as soon as B releases its locks.
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If xact C's GetSnapshotData sees xact B as still running, then it had
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better see xact A as still running as well, or it will be able to see two
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tuple versions - one deleted by xact B and one inserted by xact A. Another
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reason why this would be bad is that C would see (in the row inserted by A)
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earlier changes by B, and it would be inconsistent for C not to see any
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of B's changes elsewhere in the database.
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Formally, the correctness requirement is "if A sees B as committed,
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and B sees C as committed, then A must see C as committed".
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What we actually enforce is strict serialization of commits and rollbacks
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with snapshot-taking: we do not allow any transaction to exit the set of
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running transactions while a snapshot is being taken. (This rule is
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stronger than necessary for consistency, but is relatively simple to
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enforce, and it assists with some other issues as explained below.) The
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implementation of this is that GetSnapshotData takes the ProcArrayLock in
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shared mode (so that multiple backends can take snapshots in parallel),
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but xact.c must take the ProcArrayLock in exclusive mode while clearing
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MyProc->xid at transaction end (either commit or abort).
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GetSnapshotData must in fact acquire ProcArrayLock before it calls
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ReadNewTransactionId. Otherwise it would be possible for a transaction A
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postdating the xmax to commit, and then an existing transaction B that saw
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A as committed to commit, before GetSnapshotData is able to acquire
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ProcArrayLock and finish taking its snapshot. This would violate the
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consistency requirement, because A would be still running and B not
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according to this snapshot.
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In short, then, the rule is that no transaction may exit the set of
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currently-running transactions between the time we fetch xmax and the time
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we finish building our snapshot. However, this restriction only applies
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to transactions that have an XID --- read-only transactions can end without
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acquiring ProcArrayLock, since they don't affect anyone else's snapshot.
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Transaction start, per se, doesn't have any interlocking with these
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considerations, since we no longer assign an XID immediately at transaction
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start. But when we do decide to allocate an XID, we must require
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GetNewTransactionId to store the new XID into the shared ProcArray before
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releasing XidGenLock. This ensures that when GetSnapshotData calls
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ReadNewTransactionId (which also takes XidGenLock), all active XIDs before
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the returned value of nextXid are already present in the ProcArray and
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can't be missed by GetSnapshotData. Unfortunately, we can't have
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GetNewTransactionId take ProcArrayLock to do this, else it could deadlock
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against GetSnapshotData. Therefore, we simply let GetNewTransactionId
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store into MyProc->xid without any lock. We are thereby relying on
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fetch/store of an XID to be atomic, else other backends might see a
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partially-set XID. (NOTE: for multiprocessors that need explicit memory
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access fence instructions, this means that acquiring/releasing XidGenLock
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is just as necessary as acquiring/releasing ProcArrayLock for
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GetSnapshotData to ensure it sees up-to-date xid fields.) This also means
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that readers of the ProcArray xid fields must be careful to fetch a value
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only once, rather than assume they can read it multiple times and get the
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same answer each time.
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Another important activity that uses the shared ProcArray is GetOldestXmin,
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which must determine a lower bound for the oldest xmin of any active MVCC
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snapshot, system-wide. Each individual backend advertises the smallest
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xmin of its own snapshots in MyProc->xmin, or zero if it currently has no
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live snapshots (eg, if it's between transactions or hasn't yet set a
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snapshot for a new transaction). GetOldestXmin takes the MIN() of the
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valid xmin fields. It does this with only shared lock on ProcArrayLock,
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which means there is a potential race condition against other backends
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doing GetSnapshotData concurrently: we must be certain that a concurrent
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backend that is about to set its xmin does not compute an xmin less than
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what GetOldestXmin returns. We ensure that by including all the active
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XIDs into the MIN() calculation, along with the valid xmins. The rule that
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transactions can't exit without taking exclusive ProcArrayLock ensures that
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concurrent holders of shared ProcArrayLock will compute the same minimum of
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currently-active XIDs: no xact, in particular not the oldest, can exit
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while we hold shared ProcArrayLock. So GetOldestXmin's view of the minimum
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active XID will be the same as that of any concurrent GetSnapshotData, and
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so it can't produce an overestimate. If there is no active transaction at
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all, GetOldestXmin returns the result of ReadNewTransactionId. Note that
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two concurrent executions of GetOldestXmin might not see the same result
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from ReadNewTransactionId --- but if there is a difference, the intervening
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execution(s) of GetNewTransactionId must have stored their XIDs into the
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ProcArray, so the later execution of GetOldestXmin will see them and
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compute the same global xmin anyway.
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GetSnapshotData also performs an oldest-xmin calculation (which had better
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match GetOldestXmin's) and stores that into RecentGlobalXmin, which is used
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for some tuple age cutoff checks where a fresh call of GetOldestXmin seems
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too expensive. Note that while it is certain that two concurrent
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executions of GetSnapshotData will compute the same xmin for their own
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snapshots, as argued above, it is not certain that they will arrive at the
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same estimate of RecentGlobalXmin. This is because we allow XID-less
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transactions to clear their MyProc->xmin asynchronously (without taking
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ProcArrayLock), so one execution might see what had been the oldest xmin,
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and another not. This is OK since RecentGlobalXmin need only be a valid
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lower bound. As noted above, we are already assuming that fetch/store
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of the xid fields is atomic, so assuming it for xmin as well is no extra
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risk.
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pg_clog and pg_subtrans
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-----------------------
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@ -10,7 +10,7 @@
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*
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*
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* IDENTIFICATION
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* $PostgreSQL: pgsql/src/backend/access/transam/xact.c,v 1.248 2007/09/05 18:10:47 tgl Exp $
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* $PostgreSQL: pgsql/src/backend/access/transam/xact.c,v 1.249 2007/09/07 20:59:26 tgl Exp $
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*
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*-------------------------------------------------------------------------
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*/
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@ -747,6 +747,8 @@ AtSubStart_ResourceOwner(void)
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/*
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* RecordTransactionCommit
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*
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* This is exported only to support an ugly hack in VACUUM FULL.
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*/
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void
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RecordTransactionCommit(void)
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@ -1552,47 +1554,54 @@ CommitTransaction(void)
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*/
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RecordTransactionCommit();
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/*----------
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PG_TRACE1(transaction__commit, MyProc->lxid);
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/*
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* Let others know about no transaction in progress by me. Note that
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* this must be done _before_ releasing locks we hold and _after_
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* RecordTransactionCommit.
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*
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* LWLockAcquire(ProcArrayLock) is required; consider this example:
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* UPDATE with xid 0 is blocked by xid 1's UPDATE.
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* xid 1 is doing commit while xid 2 gets snapshot.
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* If xid 2's GetSnapshotData sees xid 1 as running then it must see
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* xid 0 as running as well, or it will be able to see two tuple versions
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* - one deleted by xid 1 and one inserted by xid 0. See notes in
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* GetSnapshotData.
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*
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* Note: MyProc may be null during bootstrap.
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*----------
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*/
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if (MyProc != NULL)
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{
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/*
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* Lock ProcArrayLock because that's what GetSnapshotData uses.
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* You might assume that we can skip this step if we had no
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* transaction id assigned, because the failure case outlined
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* in GetSnapshotData cannot happen in that case. This is true,
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* but we *still* need the lock guarantee that two concurrent
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* computations of the *oldest* xmin will get the same result.
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*/
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LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
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MyProc->xid = InvalidTransactionId;
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MyProc->lxid = InvalidLocalTransactionId;
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MyProc->xmin = InvalidTransactionId;
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MyProc->inVacuum = false; /* must be cleared with xid/xmin */
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if (TransactionIdIsValid(MyProc->xid))
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{
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/*
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* We must lock ProcArrayLock while clearing MyProc->xid, so
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* that we do not exit the set of "running" transactions while
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* someone else is taking a snapshot. See discussion in
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* src/backend/access/transam/README.
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*/
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LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
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/* Clear the subtransaction-XID cache too while holding the lock */
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MyProc->subxids.nxids = 0;
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MyProc->subxids.overflowed = false;
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MyProc->xid = InvalidTransactionId;
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MyProc->lxid = InvalidLocalTransactionId;
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MyProc->xmin = InvalidTransactionId;
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MyProc->inVacuum = false; /* must be cleared with xid/xmin */
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LWLockRelease(ProcArrayLock);
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/* Clear the subtransaction-XID cache too while holding the lock */
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MyProc->subxids.nxids = 0;
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MyProc->subxids.overflowed = false;
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LWLockRelease(ProcArrayLock);
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}
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else
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{
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/*
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* If we have no XID, we don't need to lock, since we won't
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* affect anyone else's calculation of a snapshot. We might
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* change their estimate of global xmin, but that's OK.
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*/
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MyProc->lxid = InvalidLocalTransactionId;
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MyProc->xmin = InvalidTransactionId;
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MyProc->inVacuum = false; /* must be cleared with xid/xmin */
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Assert(MyProc->subxids.nxids == 0);
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Assert(MyProc->subxids.overflowed == false);
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}
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}
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PG_TRACE1(transaction__commit, s->transactionId);
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/*
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* This is all post-commit cleanup. Note that if an error is raised here,
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* it's too late to abort the transaction. This should be just
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@ -1815,28 +1824,21 @@ PrepareTransaction(void)
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* Let others know about no transaction in progress by me. This has to be
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* done *after* the prepared transaction has been marked valid, else
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* someone may think it is unlocked and recyclable.
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*
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* We can skip locking ProcArrayLock here, because this action does not
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* actually change anyone's view of the set of running XIDs: our entry
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* is duplicate with the gxact that has already been inserted into the
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* ProcArray.
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*/
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/*
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* Lock ProcArrayLock because that's what GetSnapshotData uses.
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* You might assume that we can skip this step if we have no
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* transaction id assigned, because the failure case outlined
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* in GetSnapshotData cannot happen in that case. This is true,
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* but we *still* need the lock guarantee that two concurrent
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* computations of the *oldest* xmin will get the same result.
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*/
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LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
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MyProc->xid = InvalidTransactionId;
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MyProc->lxid = InvalidLocalTransactionId;
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MyProc->xmin = InvalidTransactionId;
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MyProc->inVacuum = false; /* must be cleared with xid/xmin */
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/* Clear the subtransaction-XID cache too while holding the lock */
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/* Clear the subtransaction-XID cache too */
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MyProc->subxids.nxids = 0;
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MyProc->subxids.overflowed = false;
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LWLockRelease(ProcArrayLock);
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/*
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* This is all post-transaction cleanup. Note that if an error is raised
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* here, it's too late to abort the transaction. This should be just
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@ -1987,37 +1989,56 @@ AbortTransaction(void)
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*/
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RecordTransactionAbort(false);
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PG_TRACE1(transaction__abort, MyProc->lxid);
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/*
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* Let others know about no transaction in progress by me. Note that this
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* must be done _before_ releasing locks we hold and _after_
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* RecordTransactionAbort.
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*
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* Note: MyProc may be null during bootstrap.
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*/
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if (MyProc != NULL)
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{
|
||||
/*
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* Lock ProcArrayLock because that's what GetSnapshotData uses.
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||||
* You might assume that we can skip this step if we have no
|
||||
* transaction id assigned, because the failure case outlined
|
||||
* in GetSnapshotData cannot happen in that case. This is true,
|
||||
* but we *still* need the lock guarantee that two concurrent
|
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* computations of the *oldest* xmin will get the same result.
|
||||
*/
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LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
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MyProc->xid = InvalidTransactionId;
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MyProc->lxid = InvalidLocalTransactionId;
|
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MyProc->xmin = InvalidTransactionId;
|
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MyProc->inVacuum = false; /* must be cleared with xid/xmin */
|
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MyProc->inCommit = false; /* be sure this gets cleared */
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if (TransactionIdIsValid(MyProc->xid))
|
||||
{
|
||||
/*
|
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* We must lock ProcArrayLock while clearing MyProc->xid, so
|
||||
* that we do not exit the set of "running" transactions while
|
||||
* someone else is taking a snapshot. See discussion in
|
||||
* src/backend/access/transam/README.
|
||||
*/
|
||||
LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
|
||||
|
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/* Clear the subtransaction-XID cache too while holding the lock */
|
||||
MyProc->subxids.nxids = 0;
|
||||
MyProc->subxids.overflowed = false;
|
||||
MyProc->xid = InvalidTransactionId;
|
||||
MyProc->lxid = InvalidLocalTransactionId;
|
||||
MyProc->xmin = InvalidTransactionId;
|
||||
MyProc->inVacuum = false; /* must be cleared with xid/xmin */
|
||||
MyProc->inCommit = false; /* be sure this gets cleared */
|
||||
|
||||
LWLockRelease(ProcArrayLock);
|
||||
/* Clear the subtransaction-XID cache too while holding the lock */
|
||||
MyProc->subxids.nxids = 0;
|
||||
MyProc->subxids.overflowed = false;
|
||||
|
||||
LWLockRelease(ProcArrayLock);
|
||||
}
|
||||
else
|
||||
{
|
||||
/*
|
||||
* If we have no XID, we don't need to lock, since we won't
|
||||
* affect anyone else's calculation of a snapshot. We might
|
||||
* change their estimate of global xmin, but that's OK.
|
||||
*/
|
||||
MyProc->lxid = InvalidLocalTransactionId;
|
||||
MyProc->xmin = InvalidTransactionId;
|
||||
MyProc->inVacuum = false; /* must be cleared with xid/xmin */
|
||||
MyProc->inCommit = false; /* be sure this gets cleared */
|
||||
|
||||
Assert(MyProc->subxids.nxids == 0);
|
||||
Assert(MyProc->subxids.overflowed == false);
|
||||
}
|
||||
}
|
||||
|
||||
PG_TRACE1(transaction__abort, s->transactionId);
|
||||
|
||||
/*
|
||||
* Post-abort cleanup. See notes in CommitTransaction() concerning
|
||||
* ordering.
|
||||
|
||||
@ -8,7 +8,7 @@
|
||||
*
|
||||
*
|
||||
* IDENTIFICATION
|
||||
* $PostgreSQL: pgsql/src/backend/commands/copy.c,v 1.285 2007/06/20 02:02:49 neilc Exp $
|
||||
* $PostgreSQL: pgsql/src/backend/commands/copy.c,v 1.286 2007/09/07 20:59:26 tgl Exp $
|
||||
*
|
||||
*-------------------------------------------------------------------------
|
||||
*/
|
||||
@ -1678,13 +1678,6 @@ CopyFrom(CopyState cstate)
|
||||
* rd_newRelfilenodeSubid can be cleared before the end of the transaction.
|
||||
* However this is OK since at worst we will fail to make the optimization.
|
||||
*
|
||||
* When skipping WAL it's entirely possible that COPY itself will write no
|
||||
* WAL records at all. This is of concern because RecordTransactionCommit
|
||||
* might decide it doesn't need to log our eventual commit, which we
|
||||
* certainly need it to do. However, we need no special action here for
|
||||
* that, because if we have a new table or new relfilenode then there
|
||||
* must have been a WAL-logged pg_class update earlier in the transaction.
|
||||
*
|
||||
* Also, if the target file is new-in-transaction, we assume that checking
|
||||
* FSM for free space is a waste of time, even if we must use WAL because
|
||||
* of archiving. This could possibly be wrong, but it's unlikely.
|
||||
|
||||
@ -26,7 +26,7 @@
|
||||
*
|
||||
*
|
||||
* IDENTIFICATION
|
||||
* $PostgreSQL: pgsql/src/backend/executor/execMain.c,v 1.296 2007/08/15 21:39:50 tgl Exp $
|
||||
* $PostgreSQL: pgsql/src/backend/executor/execMain.c,v 1.297 2007/09/07 20:59:26 tgl Exp $
|
||||
*
|
||||
*-------------------------------------------------------------------------
|
||||
*/
|
||||
@ -2635,12 +2635,6 @@ OpenIntoRel(QueryDesc *queryDesc)
|
||||
|
||||
/*
|
||||
* We can skip WAL-logging the insertions, unless PITR is in use.
|
||||
*
|
||||
* Note that for a non-temp INTO table, this is safe only because we know
|
||||
* that the catalog changes above will have been WAL-logged, and so
|
||||
* RecordTransactionCommit will think it needs to WAL-log the eventual
|
||||
* transaction commit. Else the commit might be lost, even though all the
|
||||
* data is safely fsync'd ...
|
||||
*/
|
||||
estate->es_into_relation_use_wal = XLogArchivingActive();
|
||||
estate->es_into_relation_descriptor = intoRelationDesc;
|
||||
|
||||
@ -10,7 +10,7 @@
|
||||
*
|
||||
* Because of various subtle race conditions it is critical that a backend
|
||||
* hold the correct locks while setting or clearing its MyProc->xid field.
|
||||
* See notes in GetSnapshotData.
|
||||
* See notes in src/backend/access/transam/README.
|
||||
*
|
||||
* The process array now also includes PGPROC structures representing
|
||||
* prepared transactions. The xid and subxids fields of these are valid,
|
||||
@ -23,7 +23,7 @@
|
||||
*
|
||||
*
|
||||
* IDENTIFICATION
|
||||
* $PostgreSQL: pgsql/src/backend/storage/ipc/procarray.c,v 1.31 2007/09/07 00:58:56 tgl Exp $
|
||||
* $PostgreSQL: pgsql/src/backend/storage/ipc/procarray.c,v 1.32 2007/09/07 20:59:26 tgl Exp $
|
||||
*
|
||||
*-------------------------------------------------------------------------
|
||||
*/
|
||||
@ -407,6 +407,7 @@ TransactionIdIsActive(TransactionId xid)
|
||||
* Note: we include all currently running xids in the set of considered xids.
|
||||
* This ensures that if a just-started xact has not yet set its snapshot,
|
||||
* when it does set the snapshot it cannot set xmin less than what we compute.
|
||||
* See notes in src/backend/access/transam/README.
|
||||
*/
|
||||
TransactionId
|
||||
GetOldestXmin(bool allDbs, bool ignoreVacuum)
|
||||
@ -468,7 +469,7 @@ GetOldestXmin(bool allDbs, bool ignoreVacuum)
|
||||
return result;
|
||||
}
|
||||
|
||||
/*----------
|
||||
/*
|
||||
* GetSnapshotData -- returns information about running transactions.
|
||||
*
|
||||
* The returned snapshot includes xmin (lowest still-running xact ID),
|
||||
@ -481,12 +482,13 @@ GetOldestXmin(bool allDbs, bool ignoreVacuum)
|
||||
* This ensures that the set of transactions seen as "running" by the
|
||||
* current xact will not change after it takes the snapshot.
|
||||
*
|
||||
* All running top-level XIDs are included in the snapshot. We also try
|
||||
* to include running subtransaction XIDs, but since PGPROC has only a
|
||||
* limited cache area for subxact XIDs, full information may not be
|
||||
* available. If we find any overflowed subxid arrays, we have to mark
|
||||
* the snapshot's subxid data as overflowed, and extra work will need to
|
||||
* be done to determine what's running (see XidInMVCCSnapshot() in tqual.c).
|
||||
* All running top-level XIDs are included in the snapshot, except for lazy
|
||||
* VACUUM processes. We also try to include running subtransaction XIDs,
|
||||
* but since PGPROC has only a limited cache area for subxact XIDs, full
|
||||
* information may not be available. If we find any overflowed subxid arrays,
|
||||
* we have to mark the snapshot's subxid data as overflowed, and extra work
|
||||
* will need to be done to determine what's running (see XidInMVCCSnapshot()
|
||||
* in tqual.c).
|
||||
*
|
||||
* We also update the following backend-global variables:
|
||||
* TransactionXmin: the oldest xmin of any snapshot in use in the
|
||||
@ -497,7 +499,6 @@ GetOldestXmin(bool allDbs, bool ignoreVacuum)
|
||||
* RecentGlobalXmin: the global xmin (oldest TransactionXmin across all
|
||||
* running transactions, except those running LAZY VACUUM). This is
|
||||
* the same computation done by GetOldestXmin(true, true).
|
||||
*----------
|
||||
*/
|
||||
Snapshot
|
||||
GetSnapshotData(Snapshot snapshot, bool serializable)
|
||||
@ -550,57 +551,16 @@ GetSnapshotData(Snapshot snapshot, bool serializable)
|
||||
|
||||
/*
|
||||
* It is sufficient to get shared lock on ProcArrayLock, even if we are
|
||||
* computing a serializable snapshot and therefore will be setting
|
||||
* MyProc->xmin. This is because any two backends that have overlapping
|
||||
* shared holds on ProcArrayLock will certainly compute the same xmin
|
||||
* (since no xact, in particular not the oldest, can exit the set of
|
||||
* running transactions while we hold ProcArrayLock --- see further
|
||||
* discussion just below). So it doesn't matter whether another backend
|
||||
* concurrently doing GetSnapshotData or GetOldestXmin sees our xmin as
|
||||
* set or not; he'd compute the same xmin for himself either way.
|
||||
* (We are assuming here that xmin can be set and read atomically,
|
||||
* just like xid.)
|
||||
*
|
||||
* There is a corner case in which the above argument doesn't work: if
|
||||
* there isn't any oldest xact, ie, all xids in the array are invalid.
|
||||
* In that case we will compute xmin as the result of ReadNewTransactionId,
|
||||
* and since GetNewTransactionId doesn't take the ProcArrayLock, it's not
|
||||
* so obvious that two backends with overlapping shared locks will get
|
||||
* the same answer. But GetNewTransactionId is required to store the XID
|
||||
* it assigned into the ProcArray before releasing XidGenLock. Therefore
|
||||
* the backend that did ReadNewTransactionId later will see that XID in
|
||||
* the array, and will compute the same xmin as the earlier one that saw
|
||||
* no XIDs in the array.
|
||||
* going to set MyProc->xmin.
|
||||
*/
|
||||
LWLockAcquire(ProcArrayLock, LW_SHARED);
|
||||
|
||||
/*--------------------
|
||||
/*
|
||||
* Unfortunately, we have to call ReadNewTransactionId() after acquiring
|
||||
* ProcArrayLock above. It's not good because ReadNewTransactionId() does
|
||||
* LWLockAcquire(XidGenLock), but *necessary*. We need to be sure that
|
||||
* no transactions exit the set of currently-running transactions
|
||||
* between the time we fetch xmax and the time we finish building our
|
||||
* snapshot. Otherwise we could have a situation like this:
|
||||
*
|
||||
* 1. Tx Old is running (in Read Committed mode).
|
||||
* 2. Tx S reads new transaction ID into xmax, then
|
||||
* is swapped out before acquiring ProcArrayLock.
|
||||
* 3. Tx New gets new transaction ID (>= S' xmax),
|
||||
* makes changes and commits.
|
||||
* 4. Tx Old changes some row R changed by Tx New and commits.
|
||||
* 5. Tx S finishes getting its snapshot data. It sees Tx Old as
|
||||
* done, but sees Tx New as still running (since New >= xmax).
|
||||
*
|
||||
* Now S will see R changed by both Tx Old and Tx New, *but* does not
|
||||
* see other changes made by Tx New. If S is supposed to be in
|
||||
* Serializable mode, this is wrong.
|
||||
*
|
||||
* By locking ProcArrayLock before we read xmax, we ensure that TX Old
|
||||
* cannot exit the set of running transactions seen by Tx S. Therefore
|
||||
* both Old and New will be seen as still running => no inconsistency.
|
||||
*--------------------
|
||||
* ProcArrayLock. It's not good because ReadNewTransactionId() does
|
||||
* LWLockAcquire(XidGenLock), but *necessary*. See discussion in
|
||||
* src/backend/access/transam/README.
|
||||
*/
|
||||
|
||||
xmax = ReadNewTransactionId();
|
||||
|
||||
/* initialize xmin calculation with xmax */
|
||||
@ -1147,9 +1107,10 @@ XidCacheRemoveRunningXids(TransactionId xid, int nxids, TransactionId *xids)
|
||||
|
||||
/*
|
||||
* We must hold ProcArrayLock exclusively in order to remove transactions
|
||||
* from the PGPROC array. (See notes in GetSnapshotData.) It's possible
|
||||
* this could be relaxed since we know this routine is only used to abort
|
||||
* subtransactions, but pending closer analysis we'd best be conservative.
|
||||
* from the PGPROC array. (See src/backend/access/transam/README.) It's
|
||||
* possible this could be relaxed since we know this routine is only used
|
||||
* to abort subtransactions, but pending closer analysis we'd best be
|
||||
* conservative.
|
||||
*/
|
||||
LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
|
||||
|
||||
|
||||
Loading…
x
Reference in New Issue
Block a user