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

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Date:   Thu, 26 Jul 2018 12:06:14 +0200
From:   Jiri Pirko <jiri@...nulli.us>
To:     Cong Wang <xiyou.wangcong@...il.com>
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

Thu, Jul 26, 2018 at 09:38:39AM CEST, jiri@...nulli.us wrote:
>Wed, Jul 25, 2018 at 06:40:44PM CEST, xiyou.wangcong@...il.com wrote:
>>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.
>
>Okay. Sounds good. I'm on it. 

Actually, I found an issue. The action to "goto chain" might be attached
to a filter in the same chain. That is completely legitimate usage.
When I do:
# tc chain del X
I expect the chain to be flushed and removed. If there is an action
there with "goto" to the same chain, the command should be successful.
However, I don't see any easy way to find out if the chain is referenced
only by actions used by filters in the same chain :/

Thoughts?