netdev - Re: [Patch net 00/15] net_sched: remove RCU callbacks from TC

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Date:   Mon, 23 Oct 2017 16:23:28 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        John Fastabend <john.fastabend@...il.com>,
        Chris Mi <chrism@...lanox.com>
Subject: Re: [Patch net 00/15] net_sched: remove RCU callbacks from TC

On Mon, Oct 23, 2017 at 4:16 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Mon, 2017-10-23 at 15:02 -0700, Cong Wang wrote:
>
>> b) As suggested by Paul, we could defer the work to a workqueue and
>> gain the permission of holding RTNL again without any performance
>> impact, however, this seems impossible too, because as lockdep
>> complains we have a deadlock when flushing workqueue while hodling
>> RTNL lock, see the rcu_barrier() in tcf_block_put().
>>
>> Therefore, the simplest solution we have is probably just getting
>> rid of these RCU callbacks, because they are not necessary at all,
>> callers of these call_rcu() are all on slow paths and have RTNL
>> lock, so blocking is allowed in their contexts.
>
> I am against these pessimistic changes, sorry for not following past
> discussions last week.

So even Cc'ing you doesn't work. :-D

>
> I am asking a talk during upcoming netdev/netconf about this, if we need
> to take a decision.

I won't be able to make it.

>
> RTNL is already a big problem for many of us, adding synchronize_rcu()
> calls while holding RTNL is a no - go, unless we have clear evidence it
> can not be avoided.

You omitted too much, including the evidence I provide. In short it is very
hard to do, otherwise I should have already done it. I am very open to
any simple solution to avoid it, but so far none...

Saying no but without giving a possible solution does not help anything
here.