On Tue, 2015-12-01 at 11:17 -0800, Cong Wang wrote:
> On Tue, Dec 1, 2015 at 11:09 AM, Eric Dumazet <eric.duma...@gmail.com> wrote:
> > On Tue, 2015-12-01 at 10:43 -0800, Cong Wang wrote:
> >
> >> This smells hacky... Another way to fix this is to hold the qdisc tree
> >> lock in mq_dump(), since it is not a hot path (comparing with
> >> enqueue/dequeue)?
> >
> > Really ? Which qdisc tree lock will protect you exactly ???
> >
> > Whole point of MQ is that each TX queue has its own lock.
> >
> > So multiple cpus can call qdisc_tree_decrease_qlen() at the same time,
> > holding their own lock.
> >
> > Clearly modifying mq 'data' is wrong.
>
> Ah, yeah, but mq _seems_ also the only one who modifies sch->q.qlen
> in ->dump(), which is the root cause of this bug. I am wondering if it should
> just compute the qlen and return it without modifying sch->q.qlen.
Sure, but then we still would get PAX underflows warnings ...
Also need to take care of sch->qstats.drops += count;
Also that would require a change of ->dump() api, since tc_fill_qdisc()
does :
if (q->ops->dump && q->ops->dump(q, skb) < 0)
goto nla_put_failure;
qlen = q->q.qlen;
Not sure it is worth the pain, changing signature of all ->dump()
handlers...
What about adding TCQ_F_NOPARENT and then :
Note : Seems to be more invasive patch for net tree (need to properly
set TCQ_F_NOPARENT)
diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c
index f43c8f33f09e..20c462b48404 100644
--- a/net/sched/sch_api.c
+++ b/net/sched/sch_api.c
@@ -753,6 +753,9 @@ void qdisc_tree_decrease_qlen(struct Qdisc *sch, unsigned
int n)
while ((parentid = sch->parent)) {
if (TC_H_MAJ(parentid) == TC_H_MAJ(TC_H_INGRESS))
return;
+ /* This qdisc has no 'parent', we are at the top of the tree */
+ if (sch->flags & TCQ_F_NOPARENT)
+ return;
sch = qdisc_lookup(qdisc_dev(sch), TC_H_MAJ(parentid));
if (sch == NULL) {
--
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