Re: [PATCH v5 net 6/6] net/sched: qdisc_destroy() old ingress and clsact Qdiscs before grafting

Hi Jakub and all,

On Fri, May 26, 2023 at 07:33:24PM -0700, Jakub Kicinski wrote:

On Fri, 26 May 2023 16:09:51 -0700 Peilin Ye wrote:

quoted

Thanks a lot, I'll get right on it.

Any insights? Is it just a live-lock inherent to the retry scheme
or we actually forget to release the lock/refcnt?

I think it's just a thread holding the RTNL mutex for too long (replaying
too many times).  We could replay for arbitrary times in
tc_{modify,get}_qdisc() if the user keeps sending RTNL-unlocked filter
requests for the old Qdisc.

I tested the new reproducer Pedro posted, on:

1. All 6 v5 patches, FWIW, which caused a similar hang as Pedro reported

2. First 5 v5 patches, plus patch 6 in v1 (no replaying), did not trigger
   any issues (in about 30 minutes).

3. All 6 v5 patches, plus this diff:

diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c
index 286b7c58f5b9..988718ba5abe 100644
--- a/net/sched/sch_api.c
+++ b/net/sched/sch_api.c

@@ -1090,8 +1090,11 @@ static int qdisc_graft(struct net_device *dev, struct Qdisc *parent,
                         * RTNL-unlocked filter request(s).  This is the counterpart of that
                         * qdisc_refcount_inc_nz() call in __tcf_qdisc_find().
                         */
-                       if (!qdisc_refcount_dec_if_one(dev_queue->qdisc_sleeping))
+                       if (!qdisc_refcount_dec_if_one(dev_queue->qdisc_sleeping)) {
+                               rtnl_unlock();
+                               rtnl_lock();
                                return -EAGAIN;
+                       }
                }

                if (dev->flags & IFF_UP)

   Did not trigger any issues (in about 30 mintues) either.

What would you suggest?

Thanks,
Peilin Ye