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

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

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);

>
>> +	rcu_read_unlock();
>> +
>> +	return f;
>> +}
>> +
>>  static bool __fl_delete(struct tcf_proto *tp, struct cls_fl_filter *f,
>>  			struct netlink_ext_ack *extack)
>>  {
>> @@ -456,10 +503,7 @@ static bool __fl_delete(struct tcf_proto *tp, struct cls_fl_filter *f,
>>  	if (!tc_skip_hw(f->flags))
>>  		fl_hw_destroy_filter(tp, f, extack);
>>  	tcf_unbind_filter(tp, &f->res);
>> -	if (async)
>> -		tcf_queue_work(&f->rwork, fl_destroy_filter_work);
>> -	else
>> -		__fl_destroy_filter(f);
>> +	__fl_put(f);
>>
>>  	return last;
>>  }
>> @@ -494,11 +538,18 @@ static void fl_destroy(struct tcf_proto *tp, bool rtnl_held,
>>  	tcf_queue_work(&head->rwork, fl_destroy_sleepable);
>>  }
>>
>> +static void fl_put(struct tcf_proto *tp, void *arg)
>> +{
>> +	struct cls_fl_filter *f = arg;
>> +
>> +	__fl_put(f);
>> +}
>> +
>>  static void *fl_get(struct tcf_proto *tp, u32 handle)
>>  {
>>  	struct cls_fl_head *head = fl_head_dereference(tp);
>>
>> -	return idr_find(&head->handle_idr, handle);
>> +	return __fl_get(head, handle);
>>  }
>>
>>  static const struct nla_policy fl_policy[TCA_FLOWER_MAX + 1] = {
>> @@ -1321,12 +1372,16 @@ static int fl_change(struct net *net, struct sk_buff *in_skb,
>>  	struct nlattr **tb;
>>  	int err;
>>
>> -	if (!tca[TCA_OPTIONS])
>> -		return -EINVAL;
>> +	if (!tca[TCA_OPTIONS]) {
>> +		err = -EINVAL;
>> +		goto errout_fold;
>> +	}
>>
>>  	mask = kzalloc(sizeof(struct fl_flow_mask), GFP_KERNEL);
>> -	if (!mask)
>> -		return -ENOBUFS;
>> +	if (!mask) {
>> +		err = -ENOBUFS;
>> +		goto errout_fold;
>> +	}
>>
>>  	tb = kcalloc(TCA_FLOWER_MAX + 1, sizeof(struct nlattr *), GFP_KERNEL);
>>  	if (!tb) {
>> @@ -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.