Revert "net: rtnetlink: remove local list in __linkwatch_run_queue()"

This reverts commit b8dbbbc5 ("net: rtnetlink: remove local list
in __linkwatch_run_queue()"). It's evidently broken when there's a
non-urgent work that gets added back, and then the loop can never
finish.

While reverting, add a note about that.
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Fixes: b8dbbbc5 ("net: rtnetlink: remove local list in __linkwatch_run_queue()")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/link_watch.c

@@ -192,6 +192,11 @@ static void __linkwatch_run_queue(int urgent_only)
 #define MAX_DO_DEV_PER_LOOP	100
 
 	int do_dev = MAX_DO_DEV_PER_LOOP;
+	/* Use a local list here since we add non-urgent
+	 * events back to the global one when called with
+	 * urgent_only=1.
+	 */
+	LIST_HEAD(wrk);
 
 	/* Give urgent case more budget */
 	if (urgent_only)
@@ -213,11 +218,12 @@ static void __linkwatch_run_queue(int urgent_only)
 	clear_bit(LW_URGENT, &linkwatch_flags);
 
 	spin_lock_irq(&lweventlist_lock);
-	while (!list_empty(&lweventlist) && do_dev > 0) {
+	list_splice_init(&lweventlist, &wrk);
+
+	while (!list_empty(&wrk) && do_dev > 0) {
 		struct net_device *dev;
 
-		dev = list_first_entry(&lweventlist, struct net_device,
-				       link_watch_list);
+		dev = list_first_entry(&wrk, struct net_device, link_watch_list);
 		list_del_init(&dev->link_watch_list);
 
 		if (!netif_device_present(dev) ||
@@ -235,6 +241,9 @@ static void __linkwatch_run_queue(int urgent_only)
 		spin_lock_irq(&lweventlist_lock);
 	}
 
+	/* Add the remaining work back to lweventlist */
+	list_splice_init(&wrk, &lweventlist);
+
 	if (!list_empty(&lweventlist))
 		linkwatch_schedule_work(0);
 	spin_unlock_irq(&lweventlist_lock);