netdev - Re: [PATCH net 0/3] Fix Qdisc destroy issues caused by adding fine-grained locking to filter API

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Date:   Thu, 19 Sep 2019 12:13:46 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Vlad Buslov <vladbu@...lanox.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCH net 0/3] Fix Qdisc destroy issues caused by adding
 fine-grained locking to filter API

On Thu, Sep 19, 2019 at 1:53 AM Vlad Buslov <vladbu@...lanox.com> wrote:
>
>
> On Thu 19 Sep 2019 at 01:50, Cong Wang <xiyou.wangcong@...il.com> wrote:
> > On Wed, Sep 18, 2019 at 12:32 AM Vlad Buslov <vladbu@...lanox.com> wrote:
> >>
> >> TC filter API unlocking introduced several new fine-grained locks. The
> >> change caused sleeping-while-atomic BUGs in several Qdiscs that call cls
> >> APIs which need to obtain new mutex while holding sch tree spinlock. This
> >> series fixes affected Qdiscs by ensuring that cls API that became sleeping
> >> is only called outside of sch tree lock critical section.
> >
> > Sorry I just took a deeper look. It seems harder than just moving it
> > out of the critical section.
> >
> > qdisc_destroy() calls ops->reset() which usually purges queues,
> > I don't see how it is safe to move it out of tree spinlock without
> > respecting fast path.
> >
> > What do you think?
>
> Hmm, maybe we can split qdisc destruction in two stage process for
> affected qdiscs? Rough sketch:
>
> 1. Call qdisc_reset() (or qdisc_purge_queue()) on qdisc that are being
>    deleted under sch tree lock protection.
>
> 2. Call new qdisc_put_empty() function after releasing the lock. This
>    function would implement same functionality as a regular qdisc_put()
>    besides resetting the Qdisc and freeing skb in its queues (already
>    done by qdisc_reset())
>
> In fact, affected queues already do the same or something similar:
>
> - htb_change_class() calls qdisc_purge_queue() that calls qdisc_reset(),
>   which makes reset inside qdisc_destroy() redundant.
>
> - multiq_tune() calls qdisc_tree_flush_backlog() that has the same
>   implementation as qdisc_purge_queue() minus actually resetting the
>   Qdisc. Can we substitute first function with the second one here?
>
> - sfb_change() - same as multiq_tune().
>
> Do you think that would work?

I think they have to call qdisc_purge_queue() or whatever that calls
qdisc_reset() to reset all queues including qdisc->gso_skb and
qdisc->skb_bad_txq before releasing sch tree lock.

qdisc_tree_flush_backlog() is not sufficient.