netdev - RE: [patch net v2 1/4] net/sched: Change tc_action refcnt and bindcnt to atomic

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Date:   Mon, 23 Oct 2017 02:47:22 +0000
From:   Chris Mi <chrism@...lanox.com>
To:     Cong Wang <xiyou.wangcong@...il.com>,
        Jamal Hadi Salim <jhs@...atatu.com>
CC:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Lucas Bates <lucasb@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        David Miller <davem@...emloft.net>
Subject: RE: [patch net v2 1/4] net/sched: Change tc_action refcnt and bindcnt
 to atomic

> -----Original Message-----
> From: Cong Wang [mailto:xiyou.wangcong@...il.com]
> Sent: Friday, October 20, 2017 11:00 AM
> To: Jamal Hadi Salim <jhs@...atatu.com>
> Cc: Chris Mi <chrism@...lanox.com>; Linux Kernel Network Developers
> <netdev@...r.kernel.org>; Lucas Bates <lucasb@...atatu.com>; Jiri Pirko
> <jiri@...nulli.us>; David Miller <davem@...emloft.net>
> Subject: Re: [patch net v2 1/4] net/sched: Change tc_action refcnt and
> bindcnt to atomic
> 
> On Thu, Oct 19, 2017 at 7:21 AM, Jamal Hadi Salim <jhs@...atatu.com>
> wrote:
> > On 17-10-18 12:43 PM, Cong Wang wrote:
> >>
> >> On Tue, Oct 17, 2017 at 6:03 PM, Chris Mi <chrism@...lanox.com> wrote:
> >>>>
> >>>> -----Original Message-----
> >
> >
> >>
> >> You listed 3 problems, and you think they are 3 different ones, here
> >> I argue problem 3 (using RCU callbacks) is the cause of problem 1
> >> (refcnt not atomic). This is why I mentioned I have been thinking
> >> about removing RCU callbacks, because it probably could fix all of them.
> >>
> >
> > Cong,
> > Given this is a known bug (the test case Chris presented crashes the
> > kernel) - would it make sense to have a patch that goes to -net to fix
> > this while your approach and discussion outcome goes into net-next?
> 
> I am not sure. Because Chris' patchset is large too and I don't think it could fix
> all crashes, so it has little value to just apply them for -net.

It seems it is not easy to discard call_rcu().  I'm afraid even if we have a final solution
without call_rcu(), it is not mature at the beginning as well. I mean we also need time
to fix the possible bugs of the new design. And maybe to destroy the filters in parallel
is the right direction. If this bug is the last bug brought by call_rcu(), then changing it
may not be a good idea.
 
Patch 1 is straightforward to use atomic. Patch 2 is to convert the list to array.
I think there is no harm to the new design.  Patch 3 and 4 are useful test case.
We also need it with new design to make sure there is no regression.

So I think my patch set should not be held so long time.

My two cents.