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

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Date:   Thu, 19 Sep 2019 15:49:44 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Vlad Buslov <vladbu@...lanox.com>, netdev@...r.kernel.org
Cc:     jhs@...atatu.com, xiyou.wangcong@...il.com, jiri@...nulli.us,
        davem@...emloft.net
Subject: Re: [PATCH net v2 1/3] net: sched: sch_htb: don't call qdisc_put()
 while holding tree lock



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)