*
*
* IDENTIFICATION
- * $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.221 2006/11/05 22:42:07 tgl Exp $
+ * $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.222 2006/11/17 18:00:14 tgl Exp $
*
*
* INTERFACE ROUTINES
ItemId lp;
PageHeader dp;
TransactionId xid;
+ TransactionId xmax;
+ uint16 old_infomask;
uint16 new_infomask;
LOCKMODE tuple_lock_type;
bool have_tuple_lock = false;
LockBuffer(*buffer, BUFFER_LOCK_UNLOCK);
+ /*
+ * If we wish to acquire share lock, and the tuple is already
+ * share-locked by a multixact that includes any subtransaction of the
+ * current top transaction, then we effectively hold the desired lock
+ * already. We *must* succeed without trying to take the tuple lock,
+ * else we will deadlock against anyone waiting to acquire exclusive
+ * lock. We don't need to make any state changes in this case.
+ */
+ if (mode == LockTupleShared &&
+ (infomask & HEAP_XMAX_IS_MULTI) &&
+ MultiXactIdIsCurrent((MultiXactId) xwait))
+ {
+ Assert(infomask & HEAP_XMAX_SHARED_LOCK);
+ /* Probably can't hold tuple lock here, but may as well check */
+ if (have_tuple_lock)
+ UnlockTuple(relation, tid, tuple_lock_type);
+ return HeapTupleMayBeUpdated;
+ }
+
/*
* Acquire tuple lock to establish our priority for the tuple.
* LockTuple will release us when we are next-in-line for the tuple.
return result;
}
+ /*
+ * We might already hold the desired lock (or stronger), possibly under
+ * a different subtransaction of the current top transaction. If so,
+ * there is no need to change state or issue a WAL record. We already
+ * handled the case where this is true for xmax being a MultiXactId,
+ * so now check for cases where it is a plain TransactionId.
+ *
+ * Note in particular that this covers the case where we already hold
+ * exclusive lock on the tuple and the caller only wants shared lock.
+ * It would certainly not do to give up the exclusive lock.
+ */
+ xmax = HeapTupleHeaderGetXmax(tuple->t_data);
+ old_infomask = tuple->t_data->t_infomask;
+
+ if (!(old_infomask & (HEAP_XMAX_INVALID |
+ HEAP_XMAX_COMMITTED |
+ HEAP_XMAX_IS_MULTI)) &&
+ (mode == LockTupleShared ?
+ (old_infomask & HEAP_IS_LOCKED) :
+ (old_infomask & HEAP_XMAX_EXCL_LOCK)) &&
+ TransactionIdIsCurrentTransactionId(xmax))
+ {
+ LockBuffer(*buffer, BUFFER_LOCK_UNLOCK);
+ /* Probably can't hold tuple lock here, but may as well check */
+ if (have_tuple_lock)
+ UnlockTuple(relation, tid, tuple_lock_type);
+ return HeapTupleMayBeUpdated;
+ }
+
/*
* Compute the new xmax and infomask to store into the tuple. Note we do
* not modify the tuple just yet, because that would leave it in the wrong
*/
xid = GetCurrentTransactionId();
- new_infomask = tuple->t_data->t_infomask;
-
- new_infomask &= ~(HEAP_XMAX_COMMITTED |
- HEAP_XMAX_INVALID |
- HEAP_XMAX_IS_MULTI |
- HEAP_IS_LOCKED |
- HEAP_MOVED);
+ new_infomask = old_infomask & ~(HEAP_XMAX_COMMITTED |
+ HEAP_XMAX_INVALID |
+ HEAP_XMAX_IS_MULTI |
+ HEAP_IS_LOCKED |
+ HEAP_MOVED);
if (mode == LockTupleShared)
{
- TransactionId xmax = HeapTupleHeaderGetXmax(tuple->t_data);
- uint16 old_infomask = tuple->t_data->t_infomask;
-
/*
* If this is the first acquisition of a shared lock in the current
* transaction, set my per-backend OldestMemberMXactId setting. We can
}
else if (TransactionIdIsInProgress(xmax))
{
- if (TransactionIdEquals(xmax, xid))
- {
- /*
- * If the old locker is ourselves, we'll just mark the
- * tuple again with our own TransactionId. However we
- * have to consider the possibility that we had exclusive
- * rather than shared lock before --- if so, be careful to
- * preserve the exclusivity of the lock.
- */
- if (!(old_infomask & HEAP_XMAX_SHARED_LOCK))
- {
- new_infomask &= ~HEAP_XMAX_SHARED_LOCK;
- new_infomask |= HEAP_XMAX_EXCL_LOCK;
- mode = LockTupleExclusive;
- }
- }
- else
- {
- /*
- * If the Xmax is a valid TransactionId, then we need to
- * create a new MultiXactId that includes both the old
- * locker and our own TransactionId.
- */
- xid = MultiXactIdCreate(xmax, xid);
- new_infomask |= HEAP_XMAX_IS_MULTI;
- }
+ /*
+ * If the XMAX is a valid TransactionId, then we need to
+ * create a new MultiXactId that includes both the old
+ * locker and our own TransactionId.
+ */
+ xid = MultiXactIdCreate(xmax, xid);
+ new_infomask |= HEAP_XMAX_IS_MULTI;
}
else
{
* Portions Copyright (c) 1996-2006, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
- * $PostgreSQL: pgsql/src/backend/access/transam/multixact.c,v 1.21 2006/10/04 00:29:49 momjian Exp $
+ * $PostgreSQL: pgsql/src/backend/access/transam/multixact.c,v 1.22 2006/11/17 18:00:15 tgl Exp $
*
*-------------------------------------------------------------------------
*/
MultiXactIdIsRunning(MultiXactId multi)
{
TransactionId *members;
- TransactionId myXid;
int nmembers;
int i;
return false;
}
- /* checking for myself is cheap */
- myXid = GetTopTransactionId();
-
+ /*
+ * Checking for myself is cheap compared to looking in shared memory,
+ * so first do the equivalent of MultiXactIdIsCurrent(). This is not
+ * needed for correctness, it's just a fast path.
+ */
for (i = 0; i < nmembers; i++)
{
- if (TransactionIdEquals(members[i], myXid))
+ if (TransactionIdIsCurrentTransactionId(members[i]))
{
debug_elog3(DEBUG2, "IsRunning: I (%d) am running!", i);
pfree(members);
return false;
}
+/*
+ * MultiXactIdIsCurrent
+ * Returns true if the current transaction is a member of the MultiXactId.
+ *
+ * We return true if any live subtransaction of the current top-level
+ * transaction is a member. This is appropriate for the same reason that a
+ * lock held by any such subtransaction is globally equivalent to a lock
+ * held by the current subtransaction: no such lock could be released without
+ * aborting this subtransaction, and hence releasing its locks. So it's not
+ * necessary to add the current subxact to the MultiXact separately.
+ */
+bool
+MultiXactIdIsCurrent(MultiXactId multi)
+{
+ bool result = false;
+ TransactionId *members;
+ int nmembers;
+ int i;
+
+ nmembers = GetMultiXactIdMembers(multi, &members);
+
+ if (nmembers < 0)
+ return false;
+
+ for (i = 0; i < nmembers; i++)
+ {
+ if (TransactionIdIsCurrentTransactionId(members[i]))
+ {
+ result = true;
+ break;
+ }
+ }
+
+ pfree(members);
+
+ return result;
+}
+
/*
* MultiXactIdSetOldestMember
* Save the oldest MultiXactId this transaction could be a member of.
projects / postgresql / commitdiff