netdev - Re: [patch net-next v4 03/12] net: sched: introduce chain object to uapi

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Date:   Wed, 25 Jul 2018 09:40:44 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Jiri Pirko <jiri@...nulli.us>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Simon Horman <simon.horman@...ronome.com>,
        john.hurley@...ronome.com, David Ahern <dsahern@...il.com>,
        mlxsw@...lanox.com, sridhar.samudrala@...el.com
Subject: Re: [patch net-next v4 03/12] net: sched: introduce chain object to uapi

On Tue, Jul 24, 2018 at 11:49 PM Jiri Pirko <jiri@...nulli.us> wrote:
>
> Wed, Jul 25, 2018 at 01:20:08AM CEST, xiyou.wangcong@...il.com wrote:
> >So, you only send out notification when the last refcnt is gone.
> >
> >If the chain that is being deleted by a user is still used by an action,
> >you return 0 or -EPERM?
>
> 0 and the chain stays there until the action is removed. Hmm, do you thing
> that -EPERM should be returned in that case? The thing is, we have to
> flush the chain in order to see the action references are there. We would
> have to have 2 ref counters, one for filter, one for actions.
> What do you think?

_If_ RTM_DELCHAIN does decrease the chain refcnt, then it is
broken:

# tc chain add X... (refcnt == 1)
# tc action add ... goto chain X (refcnt==2)
# tc chain del X ... (refcnt== 1)
# tc chain del X ... (refcnt==0)

RTM_DELCHAIN should just test if refcnt is 1, if it is, delete it,
otherwise return -EPERM. This is how we handle tc standalone
actions, see tcf_idr_delete_index().

Yes, you might need two refcnt's here.