netdev - Re: Get rid of RCU callbacks in TC filters?

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Message-ID: <CAM_iQpW90BZaKHkFhvuYJLWDg85m6ec7secv3whc5emy7EfsMQ@mail.gmail.com>
Date:   Thu, 19 Oct 2017 20:15:41 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Chris Mi <chrism@...lanox.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>,
        Jiri Pirko <jiri@...nulli.us>
Subject: Re: Get rid of RCU callbacks in TC filters?

On Thu, Oct 19, 2017 at 8:34 AM, John Fastabend
<john.fastabend@...il.com> wrote:
>
> My take on this would be to stay with the current RCU callbacks and try
> to simplify the implementation. Falling back to sync operations seems
> like a step backwards to me. I know update/delete of filters is currently
> a pain point for some use cases so getting the RTNL out of the way may
> become a requirement to support those (alternatively maybe batching is
> good enough).

For me it looks like very hard to make tc action destroy code completely
race-free in RCU callbacks, at least looks harder than getting rid of
RCU callbacks.

>
> I guess at a high level with Cris' patches actions are now doing reference
> counting correctly. If shared filters also do reference counting similarly
> we should be OK right? (yes I know simplifying maybe too much to be
> meaningful)

I don't know what you mean by "doing reference counting correctly",
if you mean making them atomic, as I already explained to Chris, it
is not necessary at all if we remove RCU callbacks. Refcnt doesn't
have to be atomic if it is always serialized with a lock.

>
> Are we aware of any outstanding problem areas?
>

Potentially many problems, since tc action destroy code could be
called either with a RTNL lock (fine) or in a RCU callback without
RTNL lock (buggy), these two paths race with each other and RCU
callbacks race among themselves too.