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

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Date: Tue, 30 May 2023 17:29:18 -0700
From: Peilin Ye <yepeilin.cs@...il.com>
To: Vlad Buslov <vladbu@...dia.com>
Cc: Jamal Hadi Salim <jhs@...atatu.com>, Jakub Kicinski <kuba@...nel.org>,
	Pedro Tammela <pctammela@...atatu.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	Cong Wang <xiyou.wangcong@...il.com>, Jiri Pirko <jiri@...nulli.us>,
	Peilin Ye <peilin.ye@...edance.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	John Fastabend <john.fastabend@...il.com>,
	Hillf Danton <hdanton@...a.com>, netdev@...r.kernel.org,
	Cong Wang <cong.wang@...edance.com>
Subject: Re: [PATCH v5 net 6/6] net/sched: qdisc_destroy() old ingress and
 clsact Qdiscs before grafting

On Tue, May 30, 2023 at 03:18:19PM +0300, Vlad Buslov wrote:
> >> If livelock with concurrent filters insertion is an issue, then it can
> >> be remedied by setting a new Qdisc->flags bit
> >> "DELETED-REJECT-NEW-FILTERS" and checking for it together with
> >> QDISC_CLASS_OPS_DOIT_UNLOCKED in order to force any concurrent filter
> >> insertion coming after the flag is set to synchronize on rtnl lock.
> >
> > Thanks for the suggestion!  I'll try this approach.
> >
> > Currently QDISC_CLASS_OPS_DOIT_UNLOCKED is checked after taking a refcnt of
> > the "being-deleted" Qdisc.  I'll try forcing "late" requests (that arrive
> > later than Qdisc is flagged as being-deleted) sync on RTNL lock without
> > (before) taking the Qdisc refcnt (otherwise I think Task 1 will replay for
> > even longer?).
> 
> Yeah, I see what you mean. Looking at the code __tcf_qdisc_find()
> already returns -EINVAL when q->refcnt is zero, so maybe returning
> -EINVAL from that function when "DELETED-REJECT-NEW-FILTERS" flags is
> set is also fine? Would be much easier to implement as opposed to moving
> rtnl_lock there.

Ah, I see, sure.

Thanks,
Peilin Ye