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

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Date:   Mon, 23 Oct 2017 16:31:17 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Cong Wang <xiyou.wangcong@...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, 2017-10-23 at 16:23 -0700, Cong Wang wrote:
> 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

Nope. At the end of the day, there are only 24 hours per day.

> 
> >
> > 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.

I did not pretend to give a bug fix, I simply said your patch series was
probably not the right way.

Sure, we could add back BKL and solve a lot of lockdep issues.