netdev - Re: [PATCH net v2 1/3] net: sched: sch_htb: don't call qdisc

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Date:   Fri, 20 Sep 2019 06:34:28 +0000
From:   Vlad Buslov <vladbu@...lanox.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
CC:     Vlad Buslov <vladbu@...lanox.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "jhs@...atatu.com" <jhs@...atatu.com>,
        "xiyou.wangcong@...il.com" <xiyou.wangcong@...il.com>,
        "jiri@...nulli.us" <jiri@...nulli.us>,
        "davem@...emloft.net" <davem@...emloft.net>
Subject: Re: [PATCH net v2 1/3] net: sched: sch_htb: don't call qdisc_put()
 while holding tree lock


On Fri 20 Sep 2019 at 01:49, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On 9/19/19 1:14 PM, Vlad Buslov wrote:
>> Recent changes that removed rtnl dependency from rules update path of tc
>> also made tcf_block_put() function sleeping. This function is called from
>> ops->destroy() of several Qdisc implementations, which in turn is called by
>> qdisc_put(). Some Qdiscs call qdisc_put() while holding sch tree spinlock,
>> which results sleeping-while-atomic BUG.
>>
>
>
> Note that calling qdisc_put() while holding sch tree lock can also
> trigger deadlocks.
>
> For example sch_pie.c has a del_timer_sync() in pie_destroy(),
> while the pie_timer() timer handler acquires the root_lock.
>
> (there are other cases like that, SFQ for example)

These and other examples of sleeping calls in cls APIs used by
tcf_block_put() that I described in one of my previous emails make me
think that putting might_sleep() at the beginning of qdisc_put() would
be a good idea, instead of waiting for syzbot to find correct
combination to trigger a crash.