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Commitee79928

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Clarify what is protected by WaitLSNLock
Not just WaitLSNState.waitersHeap, but also WaitLSNState.procInfos andupdating of WaitLSNState.minWaitedLSN is protected by WaitLSNLock. Thereis one now documented exclusion on fast-path checking ofWaitLSNProcInfo.inHeap flag.Discussion:https://postgr.es/m/202404030658.hhj3vfxeyhft%40alvherre.pgsql
1 parent25f4242 commitee79928

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2 files changed

+13
-4
lines changed

2 files changed

+13
-4
lines changed

‎src/backend/commands/waitlsn.c‎

Lines changed: 8 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -109,13 +109,13 @@ addLSNWaiter(XLogRecPtr lsn)
109109
{
110110
WaitLSNProcInfo*procInfo=&waitLSN->procInfos[MyProcNumber];
111111

112+
LWLockAcquire(WaitLSNLock,LW_EXCLUSIVE);
113+
112114
Assert(!procInfo->inHeap);
113115

114116
procInfo->procnum=MyProcNumber;
115117
procInfo->waitLSN=lsn;
116118

117-
LWLockAcquire(WaitLSNLock,LW_EXCLUSIVE);
118-
119119
pairingheap_add(&waitLSN->waitersHeap,&procInfo->phNode);
120120
procInfo->inHeap= true;
121121
updateMinWaitedLSN();
@@ -203,6 +203,12 @@ WaitLSNSetLatches(XLogRecPtr currentLSN)
203203
void
204204
WaitLSNCleanup(void)
205205
{
206+
/*
207+
* We do a fast-path check of the 'inHeap' flag without the lock. This
208+
* flag is set to true only by the process itself. So, it's only possible
209+
* to get a false positive. But that will be eliminated by a recheck
210+
* inside deleteLSNWaiter().
211+
*/
206212
if (waitLSN->procInfos[MyProcNumber].inHeap)
207213
deleteLSNWaiter();
208214
}

‎src/include/commands/waitlsn.h‎

Lines changed: 5 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -49,7 +49,7 @@ typedef struct WaitLSNState
4949
/*
5050
* The minimum LSN value some process is waiting for. Used for the
5151
* fast-path checking if we need to wake up any waiters after replaying a
52-
* WAL record.
52+
* WAL record. Could be read lock-less. Update protected by WaitLSNLock.
5353
*/
5454
pg_atomic_uint64minWaitedLSN;
5555

@@ -59,7 +59,10 @@ typedef struct WaitLSNState
5959
*/
6060
pairingheapwaitersHeap;
6161

62-
/* An array with per-process information, indexed by the process number */
62+
/*
63+
* An array with per-process information, indexed by the process number.
64+
* Protected by WaitLSNLock.
65+
*/
6366
WaitLSNProcInfoprocInfos[FLEXIBLE_ARRAY_MEMBER];
6467
}WaitLSNState;
6568

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