net: dcb: disable softirqs in dcbnl_flush_dev()

Ido Schimmel points out that since commit 52cff74 ("dcbnl : Disable
software interrupts before taking dcb_lock"), the DCB API can be called
by drivers from softirq context.

One such in-tree example is the chelsio cxgb4 driver:
dcb_rpl
-> cxgb4_dcb_handle_fw_update
   -> dcb_ieee_setapp

If the firmware for this driver happened to send an event which resulted
in a call to dcb_ieee_setapp() at the exact same time as another
DCB-enabled interface was unregistering on the same CPU, the softirq
would deadlock, because the interrupted process was already holding the
dcb_lock in dcbnl_flush_dev().

Fix this unlikely event by using spin_lock_bh() in dcbnl_flush_dev() as
in the rest of the dcbnl code.

Fixes: 91b0383 ("net: dcb: flush lingering app table entries for unregistered devices")
Reported-by: Ido Schimmel <[email protected]>
Signed-off-by: Vladimir Oltean <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

net/dcb/dcbnl.c

@@ -2077,7 +2077,7 @@ static void dcbnl_flush_dev(struct net_device *dev)
 {
 	struct dcb_app_type *itr, *tmp;
 
-	spin_lock(&dcb_lock);
+	spin_lock_bh(&dcb_lock);
 
 	list_for_each_entry_safe(itr, tmp, &dcb_app_list, list) {
 		if (itr->ifindex == dev->ifindex) {
@@ -2086,7 +2086,7 @@ static void dcbnl_flush_dev(struct net_device *dev)
 		}
 	}
 
-	spin_unlock(&dcb_lock);
+	spin_unlock_bh(&dcb_lock);
 }
 
 static int dcbnl_netdevice_event(struct notifier_block *nb,