netdev - Re: [Patch net-next] net_sched: avoid calling tcf_unbind_filter() in call

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Message-ID: <CAHA+R7O6cHL2csRKxsyggR+qJfLgHgf+hE4MXQakf72zLN6mOw@mail.gmail.com>
Date:	Wed, 1 Oct 2014 16:36:57 -0700
From:	Cong Wang <cwang@...pensource.com>
To:	John Fastabend <john.r.fastabend@...el.com>
Cc:	John Fastabend <john.fastabend@...il.com>,
	Cong Wang <xiyou.wangcong@...il.com>,
	netdev <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>
Subject: Re: [Patch net-next] net_sched: avoid calling tcf_unbind_filter() in
 call_rcu callback

On Wed, Oct 1, 2014 at 1:50 PM, John Fastabend
<john.r.fastabend@...el.com> wrote:
>
> Its worth noting why this is safe. Any running schedulers will either
> read the valid class field or it will be zeroed.
>
> All schedulers today when the class is 0 do a lookup using the
> same call used by the tcf_exts_bind(). So even if we have a running
> classifier hit the null class pointer it will do a lookup and get
> to the same result. This is particularly fragile at the moment because
> the only way to verify this is to audit the schedulers call sites.

Yeah, maybe David can squash this into changelog.

>
> I think we need a helper to ensure the code doesn't get broken in
> a subtle way in the future. At very least it should be documented.
> I'll try to draft a follow up patch to use a helper routine for
> this and document it.
>
> Similar patches are needed for basic, fw, route, rsvp, and tcindex.
>

Please do fix them. Meanwhile, I am trying to refactor these pieces
of ugly code.

Thanks.
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