[NET]: Add preemption point in qdisc_run

The qdisc_run loop is currently unbounded and runs entirely in a
softirq. This is bad as it may create an unbounded softirq run.

This patch fixes this by calling need_resched and breaking out if
necessary.

It also adds a break out if the jiffies value changes since that would
indicate we've been transmitting for too long which starves other
softirqs.

Signed-off-by: Herbert Xu <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/sched/sch_generic.c

···
 
 void __qdisc_run(struct net_device *dev)
 {
-	do {
-		if (!qdisc_restart(dev))
+	unsigned long start_time = jiffies;
+
+	while (qdisc_restart(dev)) {
+		if (netif_queue_stopped(dev))
 			break;
-	} while (!netif_queue_stopped(dev));
+
+		/*
+		 * Postpone processing if
+		 * 1. another process needs the CPU;
+		 * 2. we've been doing it for too long.
+		 */
+		if (need_resched() || jiffies != start_time) {
+			netif_schedule(dev);
+			break;
+		}
+	}
 
 	clear_bit(__LINK_STATE_QDISC_RUNNING, &dev->state);
 }