diff --git a/src/backend/replication/slot.c b/src/backend/replication/slot.c
index 86ae99a3ca9..682eccd116c 100644
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -1572,63 +1572,65 @@ void
 ReplicationSlotReserveWal(void)
 {
 	ReplicationSlot *slot = MyReplicationSlot;
+	XLogSegNo	segno;
+	XLogRecPtr	restart_lsn;
 
 	Assert(slot != NULL);
 	Assert(!XLogRecPtrIsValid(slot->data.restart_lsn));
 	Assert(!XLogRecPtrIsValid(slot->last_saved_restart_lsn));
 
 	/*
-	 * The replication slot mechanism is used to prevent removal of required
-	 * WAL. As there is no interlock between this routine and checkpoints, WAL
-	 * segments could concurrently be removed when a now stale return value of
-	 * ReplicationSlotsComputeRequiredLSN() is used. In the unlikely case that
-	 * this happens we'll just retry.
+	 * The replication slot mechanism is used to prevent the removal of
+	 * required WAL.
+	 *
+	 * Acquire an exclusive lock to prevent the checkpoint process from
+	 * concurrently computing the minimum slot LSN (see
+	 * CheckPointReplicationSlots). This ensures that the WAL reserved for
+	 * replication cannot be removed during a checkpoint.
+	 *
+	 * The mechanism is reliable because if WAL reservation occurs first, the
+	 * checkpoint must wait for the restart_lsn update before determining the
+	 * minimum non-removable LSN. On the other hand, if the checkpoint happens
+	 * first, subsequent WAL reservations will select positions at or beyond
+	 * the redo pointer of that checkpoint.
 	 */
-	while (true)
-	{
-		XLogSegNo	segno;
-		XLogRecPtr	restart_lsn;
+	LWLockAcquire(ReplicationSlotAllocationLock, LW_EXCLUSIVE);
 
-		/*
-		 * For logical slots log a standby snapshot and start logical decoding
-		 * at exactly that position. That allows the slot to start up more
-		 * quickly. But on a standby we cannot do WAL writes, so just use the
-		 * replay pointer; effectively, an attempt to create a logical slot on
-		 * standby will cause it to wait for an xl_running_xact record to be
-		 * logged independently on the primary, so that a snapshot can be
-		 * built using the record.
-		 *
-		 * None of this is needed (or indeed helpful) for physical slots as
-		 * they'll start replay at the last logged checkpoint anyway. Instead
-		 * return the location of the last redo LSN. While that slightly
-		 * increases the chance that we have to retry, it's where a base
-		 * backup has to start replay at.
-		 */
-		if (SlotIsPhysical(slot))
-			restart_lsn = GetRedoRecPtr();
-		else if (RecoveryInProgress())
-			restart_lsn = GetXLogReplayRecPtr(NULL);
-		else
-			restart_lsn = GetXLogInsertRecPtr();
+	/*
+	 * For logical slots log a standby snapshot and start logical decoding at
+	 * exactly that position. That allows the slot to start up more quickly.
+	 * But on a standby we cannot do WAL writes, so just use the replay
+	 * pointer; effectively, an attempt to create a logical slot on standby
+	 * will cause it to wait for an xl_running_xact record to be logged
+	 * independently on the primary, so that a snapshot can be built using the
+	 * record.
+	 *
+	 * None of this is needed (or indeed helpful) for physical slots as
+	 * they'll start replay at the last logged checkpoint anyway. Instead,
+	 * return the location of the last redo LSN, where a base backup has to
+	 * start replay at.
+	 */
+	if (SlotIsPhysical(slot))
+		restart_lsn = GetRedoRecPtr();
+	else if (RecoveryInProgress())
+		restart_lsn = GetXLogReplayRecPtr(NULL);
+	else
+		restart_lsn = GetXLogInsertRecPtr();
 
-		SpinLockAcquire(&slot->mutex);
-		slot->data.restart_lsn = restart_lsn;
-		SpinLockRelease(&slot->mutex);
+	SpinLockAcquire(&slot->mutex);
+	slot->data.restart_lsn = restart_lsn;
+	SpinLockRelease(&slot->mutex);
 
-		/* prevent WAL removal as fast as possible */
-		ReplicationSlotsComputeRequiredLSN();
+	/* prevent WAL removal as fast as possible */
+	ReplicationSlotsComputeRequiredLSN();
 
-		/*
-		 * If all required WAL is still there, great, otherwise retry. The
-		 * slot should prevent further removal of WAL, unless there's a
-		 * concurrent ReplicationSlotsComputeRequiredLSN() after we've written
-		 * the new restart_lsn above, so normally we should never need to loop
-		 * more than twice.
-		 */
-		XLByteToSeg(slot->data.restart_lsn, segno, wal_segment_size);
-		if (XLogGetLastRemovedSegno() < segno)
-			break;
-	}
+	/* Checkpoint shouldn't remove the required WAL. */
+	XLByteToSeg(slot->data.restart_lsn, segno, wal_segment_size);
+	if (XLogGetLastRemovedSegno() >= segno)
+		elog(ERROR, "WAL required by replication slot %s has been removed concurrently",
+			 NameStr(slot->data.name));
+
+	LWLockRelease(ReplicationSlotAllocationLock);
 
 	if (!RecoveryInProgress() && SlotIsLogical(slot))
 	{
@@ -2136,6 +2138,12 @@ CheckPointReplicationSlots(bool is_shutdown)
 	 * acquiring a slot we cannot take the control lock - but that's OK,
 	 * because holding ReplicationSlotAllocationLock is strictly stronger, and
 	 * enough to guarantee that nobody can change the in_use bits on us.
+	 *
+	 * Additionally, acquiring the Allocation lock is necessary to serialize
+	 * the slot flush process with concurrent slot WAL reservation. This
+	 * ensures that the WAL position being reserved is either flushed to disk
+	 * or is beyond or equal to the redo pointer of the current checkpoint
+	 * (See ReplicationSlotReserveWal for details).
 	 */
 	LWLockAcquire(ReplicationSlotAllocationLock, LW_SHARED);