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Acquire ProcArrayLock exclusively in ProcArrayClearTransaction.

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This corrects an oversight by me in 2072932407, which made
ProcArrayClearTransaction() increment xactCompletionCount. That requires an
exclusive lock, obviously.

There's other approaches that avoid the exclusive acquisition, but given that a
2PC commit is fairly heavyweight, it doesn't seem worth doing so. I've not been
able to measure a performance difference, unsurprisingly.  I did add a
comment documenting that we could do so, should it ever become a bottleneck.

Reported-By: Tom Lane <tgl@sss.pgh.pa.us>
Author: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/1355915.1597794204@sss.pgh.pa.us
This commit is contained in:
Andres Freund
2020-08-19 18:19:52 -07:00
parent 2072932407
commit 1fe1f42e3e
1 changed files with 13 additions and 6 deletions
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19
src/backend/storage/ipc/procarray.c
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@@ -840,13 +840,20 @@ ProcArrayClearTransaction(PGPROC *proc)
size_t pgxactoff;
/*
* We can skip locking ProcArrayLock exclusively here, because this action
* does not actually change anyone's view of the set of running XIDs: our
* entry is duplicate with the gxact that has already been inserted into
* the ProcArray. But need it in shared mode for pgproc->pgxactoff to stay
* the same.
* Currently we need to lock ProcArrayLock exclusively here, as we
* increment xactCompletionCount below. We also need it at least in shared
* mode for pgproc->pgxactoff to stay the same below.
*
* We could however, as this action does not actually change anyone's view
* of the set of running XIDs (our entry is duplicate with the gxact that
* has already been inserted into the ProcArray), lower the lock level to
* shared if we were to make xactCompletionCount an atomic variable. But
* that doesn't seem worth it currently, as a 2PC commit is heavyweight
* enough for this not to be the bottleneck. If it ever becomes a
* bottleneck it may also be worth considering to combine this with the
* subsequent ProcArrayRemove()
*/
LWLockAcquire(ProcArrayLock, LW_SHARED);
LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
pgxactoff = proc->pgxactoff;