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Clarify what is protected by WaitLSNLock

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Not just WaitLSNState.waitersHeap, but also WaitLSNState.procInfos and
updating of WaitLSNState.minWaitedLSN is protected by WaitLSNLock.  There
is one now documented exclusion on fast-path checking of
WaitLSNProcInfo.inHeap flag.

Discussion: https://postgr.es/m/202404030658.hhj3vfxeyhft%40alvherre.pgsql
This commit is contained in:
Alexander Korotkov
2024-04-07 00:32:35 +03:00
parent 25f42429e2
commit ee79928441
2 changed files with 13 additions and 4 deletions
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10
src/backend/commands/waitlsn.c
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@ -109,13 +109,13 @@ addLSNWaiter(XLogRecPtr lsn)
{
WaitLSNProcInfo *procInfo = &waitLSN->procInfos[MyProcNumber];
LWLockAcquire(WaitLSNLock, LW_EXCLUSIVE);
Assert(!procInfo->inHeap);
procInfo->procnum = MyProcNumber;
procInfo->waitLSN = lsn;
LWLockAcquire(WaitLSNLock, LW_EXCLUSIVE);
pairingheap_add(&waitLSN->waitersHeap, &procInfo->phNode);
procInfo->inHeap = true;
updateMinWaitedLSN();
@ -203,6 +203,12 @@ WaitLSNSetLatches(XLogRecPtr currentLSN)
void
WaitLSNCleanup(void)
{
/*
* We do a fast-path check of the 'inHeap' flag without the lock. This
* flag is set to true only by the process itself. So, it's only possible
* to get a false positive. But that will be eliminated by a recheck
* inside deleteLSNWaiter().
*/
if (waitLSN->procInfos[MyProcNumber].inHeap)
deleteLSNWaiter();
}