pkt_sched: sch_drr: fix drr_dequeue loop()

Jarek Poplawski points out:

If all child qdiscs of sch_drr are non-work-conserving (e.g. sch_tbf)
drr_dequeue() will busy-loop waiting for skbs instead of leaving the
job for a watchdog. Checking for list_empty() in each loop isn't
necessary either, because this can never be true except the first time.

Using non-work-conserving qdiscs as children of DRR makes no sense,
simply bail out in that case.

Reported-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/sched/sch_drr.c b/net/sched/sch_drr.c
index 37e6ab9..e7a7e87 100644 (file)
--- a/net/sched/sch_drr.c
+++ b/net/sched/sch_drr.c
@@ -373,11 +373,13 @@ static struct sk_buff *drr_dequeue(struct Qdisc *sch)
        struct sk_buff *skb;
        unsigned int len;
 
-       while (!list_empty(&q->active)) {
+       if (list_empty(&q->active))
+               goto out;
+       while (1) {
                cl = list_first_entry(&q->active, struct drr_class, alist);
                skb = cl->qdisc->ops->peek(cl->qdisc);
                if (skb == NULL)
-                       goto skip;
+                       goto out;
 
                len = qdisc_pkt_len(skb);
                if (len <= cl->deficit) {
@@ -390,9 +392,9 @@ static struct sk_buff *drr_dequeue(struct Qdisc *sch)
                }
 
                cl->deficit += cl->quantum;
-skip:
                list_move_tail(&cl->alist, &q->active);
        }
+out:
        return NULL;
 }