netdev - Re: Get rid of RCU callbacks in TC filters?

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Date:   Fri, 20 Oct 2017 13:52:05 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Jamal Hadi Salim <jhs@...atatu.com>,
        Chris Mi <chrism@...lanox.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>,
        Jiri Pirko <jiri@...nulli.us>
Subject: Re: Get rid of RCU callbacks in TC filters?

On Fri, Oct 20, 2017 at 1:31 PM, Cong Wang <xiyou.wangcong@...il.com> wrote:
> Actually, I just tried this approach, this way makes the core tc filter
> code harder to wait for flying callbacks, currently rcu_barrier() is
> enough, with one more schedule_work() added we probably
> need flush_workqueue()... Huh, this also means I can't use the
> global workqueue so should add a new workqueue for tc filters.
>
> Good news is I seem to make it work without adding much code.
> Stay tuned. ;)

Hmm, lockdep already complains I have a deadlock when flushing
workqueue while hodling RTNL lock, because callbacks need RTNL
too... See the rcu_barrier() in tcf_block_put().

So this approach seems to be a dead end. Is there any way out?

Probably we have to convert call_rcu() to synchronize_rcu(),
but with batching of course.