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

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Date: Tue, 16 May 2023 15:58:46 -0700
From: Peilin Ye <yepeilin.cs@...il.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Vlad Buslov <vladbu@...dia.com>, Jiri Pirko <jiri@...nulli.us>,
	Daniel Borkmann <daniel@...earbox.net>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	Cong Wang <xiyou.wangcong@...il.com>,
	Peilin Ye <peilin.ye@...edance.com>,
	John Fastabend <john.fastabend@...il.com>,
	Pedro Tammela <pctammela@...atatu.com>,
	Hillf Danton <hdanton@...a.com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, Cong Wang <cong.wang@...edance.com>
Subject: Re: [PATCH net 6/6] net/sched: qdisc_destroy() old ingress and
 clsact Qdiscs before grafting

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