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Clarify what is protected by WaitLSNLock

author Alexander Korotkov <akorotkov@postgresql.org>

Sat, 6 Apr 2024 21:32:35 +0000 (00:32 +0300)

committer Alexander Korotkov <akorotkov@postgresql.org>

Sat, 6 Apr 2024 21:49:53 +0000 (00:49 +0300)

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

src/backend/commands/waitlsn.c patch | blob | blame | history

src/include/commands/waitlsn.h patch | blob | blame | history

diff --git a/src/backend/commands/waitlsn.c b/src/backend/commands/waitlsn.c

index a57b818a2d482dd633deb085c50d394a09fdbb6a..1a83c34e09f7328b05c3db9a34c06fe3af3584be 100644 (file)

--- a/src/backend/commands/waitlsn.c

+++ b/src/backend/commands/waitlsn.c

@@ -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();

}

diff --git a/src/include/commands/waitlsn.h b/src/include/commands/waitlsn.h

index b3d9eed64d816cfd0abc68708ba3317c3c2a9d6e..da17b8be6f9b5223d064dba3f461f01d23813a72 100644 (file)

--- a/src/include/commands/waitlsn.h

+++ b/src/include/commands/waitlsn.h

@@ -49,7 +49,7 @@ typedef struct WaitLSNState

/*

* 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

- * WAL record.

+ * WAL record. Could be read lock-less. Update protected by WaitLSNLock.

*/

pg_atomic_uint64 minWaitedLSN;

@@ -59,7 +59,10 @@ typedef struct WaitLSNState

*/

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];

} WaitLSNState;

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