From: Ben Hutchings <benh@debian.org>
Date: Wed, 26 Apr 2023 22:24:48 +0200
Subject: netpoll: Fix netif_local_xmit_active() for 4.19-rt

Commit be71c3c75a488ca1594a98df0754094179ec8146 "net: don't let
netpoll invoke NAPI if in xmit context", which was commit
275b471e3d2daf1472ae8fa70dc1b50c9e0b9e75 upstream, added a comparison
of net_device::xmit_lock_owner.  In 4.19-rt (but not newer RT
branches) this field holds a task_struct pointer, not a CPU ID.
Change the comparison accordingly.

---
--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c
@@ -143,7 +143,11 @@ static int netif_local_xmit_active(struc
 	for (i = 0; i < dev->num_tx_queues; i++) {
 		struct netdev_queue *txq = netdev_get_tx_queue(dev, i);
 
+#ifdef CONFIG_PREEMPT_RT_FULL
+		if (READ_ONCE(txq->xmit_lock_owner) == current)
+#else
 		if (READ_ONCE(txq->xmit_lock_owner) == smp_processor_id())
+#endif
 			return 1;
 	}