linux-kernel - Re: [PATCH RESEND net-next 2/5] net/sched: taprio: keep child Qdisc refcount elevated at 2 in offload mode

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Date:   Wed, 7 Jun 2023 13:16:22 +0300
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Vinicius Costa Gomes <vinicius.gomes@...el.com>,
        linux-kernel@...r.kernel.org, intel-wired-lan@...ts.osuosl.org,
        Muhammad Husaini Zulkifli <muhammad.husaini.zulkifli@...el.com>,
        Peilin Ye <yepeilin.cs@...il.com>,
        Pedro Tammela <pctammela@...atatu.com>
Subject: Re: [PATCH RESEND net-next 2/5] net/sched: taprio: keep child Qdisc
 refcount elevated at 2 in offload mode

On Wed, Jun 07, 2023 at 12:05:19PM +0200, Paolo Abeni wrote:
> Perhaps the above chunk could be:
> 
> 	*old = q->qdiscs[cl - 1];
> 	if (FULL_OFFLOAD_IS_ENABLED(q->flags)) {
> 		WARN_ON_ONCE(dev_graft_qdisc(dev_queue, new) != *old);
> 		if (new)
> 			qdisc_refcount_inc(new);
> 		if (*old)
> 			qdisc_put(*old);
> 	}
> 
> (boldly assuming I'm not completely lost, which looks a wild bet ;)

Yeah, could be like that. In full offload mode, q->qdiscs[cl - 1] is
also what's grafted to the TXQ, so the WARN_ON() would be indicative of
a serious bug if it triggered.

> > > BTW, what about including your tests from the cover letter somewhere under tc-testing?
> > 
> > I don't know about that. Does it involve adding taprio hw offload to netdevsim,
> > so that both code paths are covered?
> 
> I guess I underlooked the needed effort and we could live without new
> tests here.

Let's see how the discussion progresses.