netdev - Re: [PATCH net-next] net: sched: set dedicated tcf

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Message-ID: <CAM_iQpWyUDtnyhqj=qrUVcqFqY3hz9z+XH7F2EVaxR8gMvn=rg@mail.gmail.com>
Date:   Wed, 27 Feb 2019 15:09:50 -0800
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] net: sched: set dedicated tcf_walker flag when
 tp is empty

On Wed, Feb 27, 2019 at 6:28 AM Vlad Buslov <vladbu@...lanox.com> wrote:
>
>
> On Tue 26 Feb 2019 at 22:38, Cong Wang <xiyou.wangcong@...il.com> wrote:
> > On Tue, Feb 26, 2019 at 7:08 AM Vlad Buslov <vladbu@...lanox.com> wrote:
> >>
> >>
> >> On Mon 25 Feb 2019 at 22:52, Cong Wang <xiyou.wangcong@...il.com> wrote:
> >> > On Mon, Feb 25, 2019 at 7:38 AM Vlad Buslov <vladbu@...lanox.com> wrote:
> >> >>
> >> >> Using tcf_walker->stop flag to determine when tcf_walker->fn() was called
> >> >> at least once is unreliable. Some classifiers set 'stop' flag on error
> >> >> before calling walker callback, other classifiers used to call it with NULL
> >> >> filter pointer when empty. In order to prevent further regressions, extend
> >> >> tcf_walker structure with dedicated 'nonempty' flag. Set this flag in
> >> >> tcf_walker->fn() implementation that is used to check if classifier has
> >> >> filters configured.
> >> >
> >> >
> >> > So, after this patch commits like 31a998487641 ("net: sched: fw: don't
> >> > set arg->stop in fw_walk() when empty") can be reverted??
> >>
> >> Yes, it is safe now to revert following commits:
> >>
> >> 3027ff41f67c ("net: sched: route: don't set arg->stop in route4_walk() when empty")
> >> 31a998487641 ("net: sched: fw: don't set arg->stop in fw_walk() when empty")
> >
> > Yeah, and probably commit d66022cd1623
> > ("net: sched: matchall: verify that filter is not NULL in mall_walk()").
> >
> > Please send a patch to revert them all.
> >
> > Thanks.
>
> I think commit d66022cd1623 ("net: sched: matchall: verify that filter
> is not NULL in mall_walk()") and commit 8b58d12f4ae1 ("net: sched:
> cgroup: verify that filter is not NULL during walk") shouldn't be
> reverted. They are still necessary to prevent tcf_chain_dump() from
> dumping NULL filter pointer. It can happen when dump is initiated in
> parallel with inserting first filter to unlocked classifier.
> tcf_fill_node() verifies that filter pointer is not NULL, so it will not
> crash, but will output tcf_proto info for second time. This might
> "confuse" user-space.

I don't get this.

First of all, what's confused here?

Secondly, if there is something confusing, isn't it all because of
your parallel algorithm? That is, the retry logic. I don't see how
commit d66022cd1623 could be useful in this context, it helps
to prevent a NULL crash which isn't a concern as long as it is
checked in tcf_fill_node() as you described.


Thanks.