diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 12b74da41de..5b452c613aa 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -1920,20 +1920,21 @@ GetOldestActiveTransactionId(void)
 
 	Assert(!RecoveryInProgress());
 
-	LWLockAcquire(ProcArrayLock, LW_SHARED);
-
 	/*
-	 * It's okay to read nextXid without acquiring XidGenLock because (1) we
-	 * assume TransactionIds can be read atomically and (2) we don't care if
-	 * we get a slightly stale value.  It can't be very stale anyway, because
-	 * the LWLockAcquire above will have done any necessary memory
-	 * interlocking.
+	 * Read nextXid, as the upper bound of what's still active.
+	 *
+	 * Reading a TransactionId is atomic, but we must grab the lock to make
+	 * sure that all XIDs < nextXid are already present in the proc array (or
+	 * have already completed), when we spin over it.
 	 */
+	LWLockAcquire(XidGenLock, LW_SHARED);
 	oldestRunningXid = ShmemVariableCache->nextXid;
+	LWLockRelease(XidGenLock);
 
 	/*
 	 * Spin over procArray collecting all xids and subxids.
 	 */
+	LWLockAcquire(ProcArrayLock, LW_SHARED);
 	for (index = 0; index < arrayP->numProcs; index++)
 	{
 		int			pgprocno = arrayP->pgprocnos[index];
@@ -1955,7 +1956,6 @@ GetOldestActiveTransactionId(void)
 		 * smaller than oldestRunningXid
 		 */
 	}
-
 	LWLockRelease(ProcArrayLock);
 
 	return oldestRunningXid;