Subject: Re: [PATCH net-next 03/12] net: sched: flower: introduce reference counting for filters

From: Stefano Brivio <[email-masked]>
Date: Fri, 15 Feb 2019 13:32:10 +0100

On Fri, 15 Feb 2019 11:22:45 +0000
Vlad Buslov <[email-masked]> wrote:

> On Thu 14 Feb 2019 at 20:34, Stefano Brivio <[email-masked]> wrote:
> > On Thu, 14 Feb 2019 09:47:03 +0200
> > Vlad Buslov <[email-masked]> wrote:
> >  
> >> +static struct cls_fl_filter *fl_get_next_filter(struct tcf_proto *tp,
> >> +						unsigned long *handle)
> >> +{
> >> +	struct cls_fl_head *head = fl_head_dereference(tp);
> >> +	struct cls_fl_filter *f;
> >> +
> >> +	rcu_read_lock();
> >> +	/* don't return filters that are being deleted */
> >> +	while ((f = idr_get_next_ul(&head->handle_idr,
> >> +				    handle)) != NULL &&
> >> +	       !refcount_inc_not_zero(&f->refcnt))
> >> +		++(*handle);  
> >
> > This... hurts :) What about:
> >
> > 	while ((f = idr_get_next_ul(&head->handle_idr, &handle))) {
> > 		if (refcount_inc_not_zero(&f->refcnt))
> > 			break;
> > 		++(*handle);
> > 	}
> >
> > ?  
> 
> I prefer to avoid using value of assignment as boolean and
> non-structured jumps, when possible. In this case it seems OK either
> way, but how about:
> 
> 	for (f = idr_get_next_ul(&head->handle_idr, handle);
> 	     f && !refcount_inc_not_zero(&f->refcnt);
> 	     f = idr_get_next_ul(&head->handle_idr, handle))
> 		++(*handle);

Honestly, I preferred the original, this is repeating idr_get_next_ul()
twice.

Maybe, just:

	[...]
	struct idr *idr;

	[...]
	idr = &head->handle_idr;
	while ((f = idr_get_next_ul(idr, handle)) != NULL &&
	       !refcount_inc_not_zero(&f->refcnt))
		++(*handle);

also rather ugly, but not entirely unreadable. I tried drafting a
helper for this, but it just ends up hiding what this does.

> >> @@ -1349,6 +1404,7 @@ static int fl_change(struct net *net, struct sk_buff *in_skb,
> >>  		err = -ENOBUFS;
> >>  		goto errout_tb;
> >>  	}
> >> +	refcount_set(&fnew->refcnt, 1);
> >>
> >>  	err = tcf_exts_init(&fnew->exts, TCA_FLOWER_ACT, 0);
> >>  	if (err < 0)
> >> @@ -1381,6 +1437,7 @@ static int fl_change(struct net *net, struct sk_buff *in_skb,
> >>  	if (!tc_in_hw(fnew->flags))
> >>  		fnew->flags |= TCA_CLS_FLAGS_NOT_IN_HW;
> >>
> >> +	refcount_inc(&fnew->refcnt);  
> >
> > I guess I'm not getting the semantics but... why is it 2 now?  
> 
> As soon as fnew is inserted into head->handle_idr (one reference), it
> becomes accessible to concurrent users, which means that it can be
> deleted at any time. However, tp->change() returns a reference to newly
> created filter to cls_api by assigning "arg" parameter to it (second
> reference). After tp->change() returns, cls API continues to use fnew
> and releases it with tfilter_put() when finished.

I see, thanks for the explanation!

-- 
Stefano