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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
View File

@ -109,13 +109,13 @@ addLSNWaiter(XLogRecPtr lsn)
{ {
WaitLSNProcInfo *procInfo = &waitLSN->procInfos[MyProcNumber]; WaitLSNProcInfo *procInfo = &waitLSN->procInfos[MyProcNumber];
LWLockAcquire(WaitLSNLock, LW_EXCLUSIVE);
Assert(!procInfo->inHeap); Assert(!procInfo->inHeap);
procInfo->procnum = MyProcNumber; procInfo->procnum = MyProcNumber;
procInfo->waitLSN = lsn; procInfo->waitLSN = lsn;
LWLockAcquire(WaitLSNLock, LW_EXCLUSIVE);
pairingheap_add(&waitLSN->waitersHeap, &procInfo->phNode); pairingheap_add(&waitLSN->waitersHeap, &procInfo->phNode);
procInfo->inHeap = true; procInfo->inHeap = true;
updateMinWaitedLSN(); updateMinWaitedLSN();
@ -203,6 +203,12 @@ WaitLSNSetLatches(XLogRecPtr currentLSN)
void void
WaitLSNCleanup(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) if (waitLSN->procInfos[MyProcNumber].inHeap)
deleteLSNWaiter(); deleteLSNWaiter();
} }

7
src/include/commands/waitlsn.h
View File

@ -49,7 +49,7 @@ typedef struct WaitLSNState
/* /*
* The minimum LSN value some process is waiting for. Used for the * The minimum LSN value some process is waiting for. Used for the
* fast-path checking if we need to wake up any waiters after replaying a * fast-path checking if we need to wake up any waiters after replaying a
* WAL record. * WAL record. Could be read lock-less. Update protected by WaitLSNLock.
*/ */
pg_atomic_uint64 minWaitedLSN; pg_atomic_uint64 minWaitedLSN;
@ -59,7 +59,10 @@ typedef struct WaitLSNState
*/ */
pairingheap waitersHeap; pairingheap waitersHeap;
/* An array with per-process information, indexed by the process number */ /*
* An array with per-process information, indexed by the process number.
* Protected by WaitLSNLock.
*/
WaitLSNProcInfo procInfos[FLEXIBLE_ARRAY_MEMBER]; WaitLSNProcInfo procInfos[FLEXIBLE_ARRAY_MEMBER];
} WaitLSNState; } WaitLSNState;