netdev - Re: [PATCH net-next v2] net: sched: change tcf_del

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Date:   Fri, 14 Sep 2018 13:46:24 +0300
From:   Vlad Buslov <vladbu@...lanox.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCH net-next v2] net: sched: change tcf_del_walker() to take idrinfo->lock


On Thu 13 Sep 2018 at 17:13, Cong Wang <xiyou.wangcong@...il.com> wrote:
> On Wed, Sep 12, 2018 at 1:51 AM Vlad Buslov <vladbu@...lanox.com> wrote:
>>
>>
>> On Fri 07 Sep 2018 at 19:12, Cong Wang <xiyou.wangcong@...il.com> wrote:
>> > On Fri, Sep 7, 2018 at 6:52 AM Vlad Buslov <vladbu@...lanox.com> wrote:
>> >>
>> >> Action API was changed to work with actions and action_idr in concurrency
>> >> safe manner, however tcf_del_walker() still uses actions without taking a
>> >> reference or idrinfo->lock first, and deletes them directly, disregarding
>> >> possible concurrent delete.
>> >>
>> >> Add tc_action_wq workqueue to action API. Implement
>> >> tcf_idr_release_unsafe() that assumes external synchronization by caller
>> >> and delays blocking action cleanup part to tc_action_wq workqueue. Extend
>> >> tcf_action_cleanup() with 'async' argument to indicate that function should
>> >> free action asynchronously.
>> >
>> > Where exactly is blocking in tcf_action_cleanup()?
>> >
>> > From your code, it looks like free_tcf(), but from my observation,
>> > the only blocking function inside is tcf_action_goto_chain_fini()
>> > which calls __tcf_chain_put(). But, __tcf_chain_put() is blocking
>> > _ONLY_ when tc_chain_notify() is called, for tc action it is never
>> > called.
>> >
>> > So, what else is blocking?
>>
>> __tcf_chain_put() calls tc_chain_tmplt_del(), which calls
>> ops->tmplt_destroy(). This last function uses hw offload API, which is
>> blocking.
>
> Good to know.
>
> Can we just make ops->tmplt_destroy() to use workqueue?
> Making tc action to workqueue seems overkill, for me.

How about changing tcf_chain_put_by_act() to use tc_filter_wq, instead
of directly calling __tcf_chain_put()? IMO it is a better solution
because it benefits all classifiers, instead of requiring every
classifier with templates support to implement non-blocking
ops->tmplt_destroy().