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

[Paolo Abeni <pabeni@redhat.com>]

On Fri, 2018-07-13 at 16:08 +0200, Daniel Borkmann wrote:
> 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@redhat.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.

Thank you for your review.

I actually tested with lockdep, and lockdep is happy about it.

The not so nice fact is that many TC modules already use plain RCU
primitives in the control path (call_rcu, kfree_rcu, etc.) and
rcu_derefence_bh() in the datapath (e.g. all the classifiers). AFACS,
despite the mix, this use is safe.

Cheers,

Paolo