On Mon, 2010-04-19 at 10:55 -0400, Tom Lane wrote: > Simon Riggs <si...@2ndquadrant.com> writes: > > On Mon, 2010-04-19 at 17:44 +0300, Heikki Linnakangas wrote: > >>> Choices are > >>> 1. Check RecoveryInProgress() once outside of lock, plus wild rumour of > >>> Murphy > >>> 2. Check RecoveryInProgress() before and after holding lock > >>> 3. Check RecoveryInProgress() while holding lock > >> > >> 4. Check RecoveryInProgress() once outside of lock, and scan the > >> ProcArray anyway, just in case. That's what we did before this patch. > >> Document that takenDuringRecovery == true means that the snapshot was > >> most likely taken during recovery, but there is some race conditions > >> where takenDuringRecovery is true even though the snapshot was taken > >> just after recovery finished. AFAICS all of the other current uses of > >> takenDuringRecovery work fine with that. > > > Checking RecoveryInProgress() is much cheaper than scanning the whole > > ProcArray, so (4) is definitely worse than 1-3. > > If the lock we're talking about is an LWLock, #3 is okay. If it's a > spinlock, not so much.
Just committed the following patch to implement option #3. We test RecoveryInProgress() after the LWLockAcquire rather than before. -- Simon Riggs www.2ndQuadrant.com
*** a/src/backend/storage/ipc/procarray.c --- b/src/backend/storage/ipc/procarray.c *************** *** 1074,1081 **** GetSnapshotData(Snapshot snapshot) errmsg("out of memory"))); } - snapshot->takenDuringRecovery = RecoveryInProgress(); - /* * It is sufficient to get shared lock on ProcArrayLock, even if we are * going to set MyProc->xmin. --- 1074,1079 ---- *************** *** 1091,1098 **** GetSnapshotData(Snapshot snapshot) globalxmin = xmin = xmax; /* ! * If in recovery get any known assigned xids. */ if (!snapshot->takenDuringRecovery) { /* --- 1089,1103 ---- globalxmin = xmin = xmax; /* ! * If we're in recovery then snapshot data comes from a different place, ! * so decide which route we take before grab the lock. It is possible ! * for recovery to end before we finish taking snapshot, and for newly ! * assigned transaction ids to be added to the procarray. Xmax cannot ! * change while we hold ProcArrayLock, so those newly added transaction ! * ids would be filtered away, so we need not be concerned about them. */ + snapshot->takenDuringRecovery = RecoveryInProgress(); + if (!snapshot->takenDuringRecovery) { /*
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