s390/qeth: fix race in used-buffer accounting

BugLink: https://bugs.launchpad.net/bugs/1798587

[ Upstream commit a702349a ]

By updating q->used_buffers only _after_ do_QDIO() has completed, there
is a potential race against the buffer's TX completion. In the unlikely
case that the TX completion path wins, qeth_qdio_output_handler() would
decrement the counter before qeth_flush_buffers() even incremented it.
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/s390/net/qeth_core_main.c

@@ -3513,13 +3513,14 @@ static void qeth_flush_buffers(struct qeth_qdio_out_q *queue, int index,
 	qdio_flags = QDIO_FLAG_SYNC_OUTPUT;
 	if (atomic_read(&queue->set_pci_flags_count))
 		qdio_flags |= QDIO_FLAG_PCI_OUT;
+	atomic_add(count, &queue->used_buffers);
+
 	rc = do_QDIO(CARD_DDEV(queue->card), qdio_flags,
 		     queue->queue_no, index, count);
 	if (queue->card->options.performance_stats)
 		queue->card->perf_stats.outbound_do_qdio_time +=
 			qeth_get_micros() -
 			queue->card->perf_stats.outbound_do_qdio_start_time;
-	atomic_add(count, &queue->used_buffers);
 	if (rc) {
 		queue->card->stats.tx_errors += count;
 		/* ignore temporary SIGA errors without busy condition */