netdev - Re: [PATCH net-next 2/4] tc/act: remove unneeded RCU lock in action callback

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Date:   Fri, 13 Jul 2018 16:08:35 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org
Cc:     Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Alexei Starovoitov <ast@...nel.org>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Subject: Re: [PATCH net-next 2/4] tc/act: remove unneeded RCU lock in action
 callback

Hi Paolo,

On 07/13/2018 11:55 AM, Paolo Abeni wrote:
> Each lockless action currently does its own RCU locking in ->act().
> This is allows using plain RCU accessor, even if the context
> is really RCU BH.
> 
> This change drops the per action RCU lock, replace the accessors
> with _bh variant, cleans up a bit the surronding code and documents
> the RCU status in the relevant header.
> No functional nor performance change is intended.
> 
> The goal of this patch is clarifying that the RCU critical section
> used by the tc actions extends up to the classifier's caller.
> 
> Signed-off-by: Paolo Abeni <pabeni@...hat.com>
[...]
> diff --git a/net/sched/act_bpf.c b/net/sched/act_bpf.c
> index 06f743d8ed41..ac20266460c0 100644
> --- a/net/sched/act_bpf.c
> +++ b/net/sched/act_bpf.c
> @@ -45,8 +45,7 @@ static int tcf_bpf(struct sk_buff *skb, const struct tc_action *act,
>  	tcf_lastuse_update(&prog->tcf_tm);
>  	bstats_cpu_update(this_cpu_ptr(prog->common.cpu_bstats), skb);
>  
> -	rcu_read_lock();
> -	filter = rcu_dereference(prog->filter);
> +	filter = rcu_dereference_bh(prog->filter);
>  	if (at_ingress) {
>  		__skb_push(skb, skb->mac_len);
>  		bpf_compute_data_pointers(skb);
> @@ -56,7 +55,6 @@ static int tcf_bpf(struct sk_buff *skb, const struct tc_action *act,
>  		bpf_compute_data_pointers(skb);
>  		filter_res = BPF_PROG_RUN(filter, skb);
>  	}
> -	rcu_read_unlock();

This conversion is not correct, BPF itself relies on RCU but not RCU-bh flavor.
You might probably see a splat if you do e.g. a map lookup with this change in
interpreter mode on tx side.

>  	/* A BPF program may overwrite the default action opcode.
>  	 * Similarly as in cls_bpf, if filter_res == -1 we use the

Thanks,
Daniel