On Thu, Jul 30, 2026 at 1:24 PM Eric Dumazet <[email protected]> wrote: > I dislike this patch; it will force dump operations to restart on busy netns. > > Most devices are created with a new ifindex (dev_index_reserve() is > called with ifindex == 0) > > Forcing a given ifindex is rather unusual.
Agreed. For context on why I went looking: we hit this on production hosts running kernels predating 748bbef5fc6a. An agent that reconciles tc configuration against a qdisc dump saw qdiscs that did exist as absent, tried to create them again (-EEXIST), and tore down configuration it believed had gone stale. Because nothing sets cb->seq there, a truncated reply is indistinguishable from a complete one, so there was no way to detect it and retry. 748bbef5fc6a makes that far less likely, and I have not been able to reproduce the missing-qdisc case on 7.2-rc5. One case does remain, which this patch would not have fixed either: ctx->q_idx is a position within the per-device qdisc_hash walk, so a qdisc added or removed on the device at a batch boundary misaligns the skip count. Is that worth reporting to userspace, and how would you want it detected? On Thu, Jul 30, 2026 at 12:25 PM Eric Dumazet <[email protected]> wrote: > > On Thu, Jul 30, 2026 at 12:00 PM Reshma Sreekumar <[email protected]> > wrote: > > > > tc_dump_qdisc() walks every netdev in the netns and can span many netlink > > batches. RTNL is only held while a single batch is filled, so devices can > > be registered or unregistered in between. Since 748bbef5fc6a ("net/sched: > > switch tc_dump_qdisc() to for_each_netdev_dump()") the walk resumes on an > > ifindex rather than a list position, but ifindexes are reused: a device > > created between two batches can be given an ifindex the dump has already > > walked past, and is then missed entirely. Before that commit the resume > > used a position in the device list, so any unregister shifted every later > > device and silently dropped a run of qdiscs from the reply. > > I dislike this patch; it will force dump operations to restart on busy netns. > > Most devices are created with a new ifindex (dev_index_reserve() is > called with ifindex == 0) > > Forcing a given ifindex is rather unusual. > > I suggest detecting this specific use case instead of slowing down everything.
