linux-kernel - Re: BUG: sleeping function called from invalid context in tcf_chain0_head_change_cb

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Date:   Tue, 17 Sep 2019 19:57:19 +0000
From:   Vlad Buslov <vladbu@...lanox.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
CC:     Vlad Buslov <vladbu@...lanox.com>,
        syzbot <syzbot+ac54455281db908c581e@...kaller.appspotmail.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>,
        David Ahern <dsahern@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "hawk@...nel.org" <hawk@...nel.org>,
        Ido Schimmel <idosch@...lanox.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...lanox.com>, Jiri Pirko <jiri@...nulli.us>,
        John Fastabend <john.fastabend@...il.com>,
        Martin KaFai Lau <kafai@...com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Nikolay Aleksandrov <nikolay@...ulusnetworks.com>,
        Petr Machata <petrm@...lanox.com>,
        Roopa Prabhu <roopa@...ulusnetworks.com>,
        Song Liu <songliubraving@...com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        "xdp-newbies@...r.kernel.org" <xdp-newbies@...r.kernel.org>,
        "yhs@...com" <yhs@...com>
Subject: Re: BUG: sleeping function called from invalid context in
 tcf_chain0_head_change_cb_del


On Tue 17 Sep 2019 at 20:03, Cong Wang <xiyou.wangcong@...il.com> wrote:
> On Tue, Sep 17, 2019 at 1:27 AM Vlad Buslov <vladbu@...lanox.com> wrote:
>> Hi Cong,
>>
>> Don't see why we would need qdisc tree lock while releasing the
>> reference to (or destroying) previous Qdisc. I've skimmed through other
>> scheds and it looks like sch_multiq, sch_htb and sch_tbf are also
>> affected. Do you want me to send patches?
>
> Yes, please do.

It looks like tbf is not affected by the bug after all. Relevant part of
code from tbf_change():

	if (q->qdisc != &noop_qdisc) {
		err = fifo_set_limit(q->qdisc, qopt->limit);
		if (err)
			goto done;
	} else if (qopt->limit > 0) {
		child = fifo_create_dflt(sch, &bfifo_qdisc_ops, qopt->limit,
					 extack);
		if (IS_ERR(child)) {
			err = PTR_ERR(child);
			goto done;
		}

		/* child is fifo, no need to check for noop_qdisc */
		qdisc_hash_add(child, true);
	}

	sch_tree_lock(sch);
	if (child) {
		qdisc_tree_flush_backlog(q->qdisc);
		qdisc_put(q->qdisc);
		q->qdisc = child;
	}

It seems that qdisc_put() is redundant here because it is only called
q->qdisc == &noop_qdisc, which is a noop.