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

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Date:   Fri, 14 Sep 2018 13:53:37 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Vlad Buslov <vladbu@...lanox.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 Fri, Sep 14, 2018 at 3:46 AM Vlad Buslov <vladbu@...lanox.com> wrote:
>
>
> 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().

My point is, there is only one filter implements ops->tmplt_destroy
so far, so there is no reason to just make all filters to adjusted
for this single one. Not to mention actions, actions are innocent
here.