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Date:   Tue, 16 May 2023 15:58:46 -0700
From:   Peilin Ye <yepeilin.cs@gmail.com>
To:     Jakub Kicinski <kuba@kernel.org>
Cc:     Vlad Buslov <vladbu@nvidia.com>, Jiri Pirko <jiri@resnulli.us>,
        Daniel Borkmann <daniel@iogearbox.net>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Paolo Abeni <pabeni@redhat.com>,
        Jamal Hadi Salim <jhs@mojatatu.com>,
        Cong Wang <xiyou.wangcong@gmail.com>,
        Peilin Ye <peilin.ye@bytedance.com>,
        John Fastabend <john.fastabend@gmail.com>,
        Pedro Tammela <pctammela@mojatatu.com>,
        Hillf Danton <hdanton@sina.com>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, Cong Wang <cong.wang@bytedance.com>
Subject: Re: [PATCH net 6/6] net/sched: qdisc_destroy() old ingress and
 clsact Qdiscs before grafting
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On Tue, May 16, 2023 at 02:50:10PM -0700, Jakub Kicinski wrote:
> > > Vlad, could you please clarify how you expect the unlocked filter
> > > operations to work when the qdisc has global state?  
> > 
> > Jakub, I didn't account for per-net_device pointer usage by miniqp code
> > hence this bug. I didn't comment on the fix because I was away from my
> > PC last week but Peilin's approach seems reasonable to me. When Peilin
> > brought up the issue initially I also tried to come up with some trick
> > to contain the changes to miniqp code instead of changing core but
> > couldn't think of anything workable due to the limitations already
> > discussed in this thread. I'm open to explore alternative approaches to
> > solving this issue, if that is what you suggest.
> 
> Given Peilin's investigation I think fix without changing core may
> indeed be hard. I'm not sure if returning -EBUSY when qdisc refcnt
> is elevated will be appreciated by the users, do we already have
> similar behavior in other parts of TC?

Seems like trying to delete an "in-use" cls_u32 filter returns -EBUSY:

net/sched/cls_u32.c:

	if (ht->refcnt == 1) {
		u32_destroy_hnode(tp, ht, extack);
	} else {
		NL_SET_ERR_MSG_MOD(extack, "Can not delete in-use filter");
		return -EBUSY;
	}

Thanks,
Peilin Ye