Subject: Re: [Patch net-next] net_sched: move the empty tp check from ->destroy() to ->delete()

From: Daniel Borkmann <[email-masked]>
Date: Sun, 27 Nov 2016 01:33:39 +0100

On 11/26/2016 12:09 PM, Daniel Borkmann wrote:
> On 11/26/2016 07:46 AM, Cong Wang wrote:
>> On Thu, Nov 24, 2016 at 7:20 AM, Daniel Borkmann <[email-masked]> wrote:
[...]
>>> Ok, strange, qdisc_destroy() calls into ops->destroy(), where ingress
>>> drops its entire chain via tcf_destroy_chain(), so that will be NULL
>>> eventually. The tps are freed by call_rcu() as well as qdisc itself
>>> later on via qdisc_rcu_free(), where it frees per-cpu bstats as well.
>>> Outstanding readers should either bail out due to if (!cl) or can still
>>> process the chain until read section ends, but during that time, cl->q
>>> resp. bstats should be good. Do you happen to know what's at address
>>> ffff880a68b04028? I was wondering wrt call_rcu() vs call_rcu_bh(), but
>>> at least on ingress (netif_receive_skb_internal()) we hold rcu_read_lock()
>>> here. The KASAN report is reliably happening at this location, right?
>>
>> I am confused as well, I don't see how it could be related to my patch yet.
>> I will take a deep look in the weekend.
>
> Ok, I'm currently on the run. Got too late yesterday night, but I'll
> write what I found in the evening today, not related to ingress though.

Just pushed out my analysis to netdev under "[PATCH net] net, sched: respect
rcu grace period on cls destruction". My conclusion is that both issues are
actually separate, and that one is small enough where we could route it via
net actually. Perhaps this at the same time shrinks your "[PATCH net-next]
net_sched: move the empty tp check from ->destroy() to ->delete()" to a
reasonable size that it's suitable to net as well. Your ->delete()/->destroy()
one is definitely needed, too. The tp->root one is independant of ->delete()/
->destroy() as they are different races and tp->root could also happen when
you just destroy the whole tp directly. I think that seems like a good path
forward to me.

Thanks,
Daniel