Subject: Re: [PATCH v2] pkt_sched: sch_drr: Fix drr_dequeue() loop

From: Patrick McHardy <[email-masked]>
Date: Mon, 24 Nov 2008 13:15:07 +0100

Jarek Poplawski wrote:
> (Changelog fixed only)
> 
> pkt_sched: sch_drr: Fix drr_dequeue() loop
> 
> 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.

Thanks for the report. I don't like to overcomplicate treatment
of this broken configuration though, so this patch simply returns
NULL when the inner qdiscs is non-work-conserving.

commit f995e708c778d5c3201d769ffae636f6941654ed
Author: Patrick McHardy <[email-masked]>
Date:   Mon Nov 24 13:13:16 2008 +0100

    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 <[email-masked]>
    Signed-off-by: Patrick McHardy <[email-masked]>

diff --git a/net/sched/sch_drr.c b/net/sched/sch_drr.c
index 37e6ab9..e7a7e87 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;
 }