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>

1 files changed, 5 insertions, 3 deletions

diff --git a/net/sched/sch_drr.c b/net/sched/sch_drr.c
index 37e6ab99bbea..e7a7e87b141a 100644
--- 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;
 }