netdev - Re: [PATCH net v2 2/3] net: sched: fix action overwrite reference counting

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Date:   Thu, 8 Apr 2021 14:52:32 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Vlad Buslov <vladbu@...dia.com>
Cc:     Jamal Hadi Salim <jhs@...atatu.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        David Miller <davem@...emloft.net>,
        Jiri Pirko <jiri@...nulli.us>,
        Jakub Kicinski <kuba@...nel.org>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Davide Caratti <dcaratti@...hat.com>
Subject: Re: [PATCH net v2 2/3] net: sched: fix action overwrite reference counting

On Thu, Apr 8, 2021 at 12:50 AM Vlad Buslov <vladbu@...dia.com> wrote:
>
>
> On Thu 08 Apr 2021 at 02:50, Cong Wang <xiyou.wangcong@...il.com> wrote:
> > In my last comments, I actually meant whether we can avoid this
> > 'init_res[]' array. Since here you want to check whether an action
> > returned by tcf_action_init_1() is a new one or an existing one, how
> > about checking its refcnt? Something like:
> >
> >   act = tcf_action_init_1(...);
> >   if (IS_ERR(act)) {
> >     err = PTR_ERR(act);
> >     goto err;
> >   }
> >   if (refcount_read(&act->tcfa_refcnt) == 1) {
> >     // we know this is a newly allocated one
> >   }
> >
> > Thanks.
>
> Hmm, I don't think this would work in general case. Consider following
> cases:
>
> 1. Action existed during init as filter action(refcnt=1), init overwrote
> it setting refcnt=2, by the time we got to checking tcfa_refcnt filter
> has been deleted (refcnt=1) so code will incorrectly assume that it has
> created the action.
>
> 2. We need this check in tcf_action_add() to release the refcnt in case
> of overwriting existing actions, but by that time actions are already
> accessible though idr, so even in case when new action has been created
> (refcnt=1) it could already been referenced by concurrently created
> filter (refcnt=2).

Hmm, I nearly forgot RTNL is lifted for some cases along TC filter
and action control paths... It seems we have no better way to work
around this.

Thanks.