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

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Date:   Mon, 23 Oct 2017 16:16:39 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     netdev@...r.kernel.org, paulmck@...ux.vnet.ibm.com,
        jhs@...atatu.com, 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 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.

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

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.

Thanks !