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

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Message-ID: <ZH/V5gf+YjKuC0bn@C02FL77VMD6R.googleapis.com>
Date: Tue, 6 Jun 2023 17:57:10 -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 Thu, Jun 01, 2023 at 09:20:39AM +0300, Vlad Buslov wrote:
> On Wed 31 May 2023 at 20:57, Peilin Ye <yepeilin.cs@...il.com> wrote:
> > +static inline bool qdisc_is_destroying(const struct Qdisc *qdisc)
> > +{
> > +       return qdisc->flags & TCQ_F_DESTROYING;
> 
> Hmm, do we need at least some kind of {READ|WRITE}_ONCE() for accessing
> flags since they are now used in unlocked filter code path?

Thanks, after taking another look at cls_api.c, I noticed this code in
tc_new_tfilter():

	err = tp->ops->change(net, skb, tp, cl, t->tcm_handle, tca, &fh,
			      flags, extack);
	if (err == 0) {
		tfilter_notify(net, skb, n, tp, block, q, parent, fh,
			       RTM_NEWTFILTER, false, rtnl_held, extack);
		tfilter_put(tp, fh);
		/* q pointer is NULL for shared blocks */
		if (q)
			q->flags &= ~TCQ_F_CAN_BYPASS;
	}               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

TCQ_F_CAN_BYPASS is cleared after e.g. adding a filter to the Qdisc, and it
isn't atomic [1].

We also have this:

  ->dequeue()
    htb_dequeue()
      htb_dequeue_tree()
        qdisc_warn_nonwc():

  void qdisc_warn_nonwc(const char *txt, struct Qdisc *qdisc)
  {
          if (!(qdisc->flags & TCQ_F_WARN_NONWC)) {
                  pr_warn("%s: %s qdisc %X: is non-work-conserving?\n",
                          txt, qdisc->ops->id, qdisc->handle >> 16);
                  qdisc->flags |= TCQ_F_WARN_NONWC;
          }       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  }
  EXPORT_SYMBOL(qdisc_warn_nonwc);

Also non-atomic; isn't it possible for the above 2 underlined statements to
race with each other?  If true, I think we need to change Qdisc::flags to
use atomic bitops, just like what we're doing for Qdisc::state and
::state2.  It feels like a separate TODO, however.

I also thought about adding the new DELETED-REJECT-NEW-FILTERS flag to
::state2, but not sure if it's okay to extend it for our purpose.

Thanks,
Peilin Ye

[1] Compiled to this on my Intel 64:

   0x0000000000017788 <+6472>:	83 62 10 fb        	andl   $0xfffffffb,0x10(%rdx)