netdev - Re: [PATCH net-next 1/5] net/sched: wrap open coded Qdics class filter counter

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Date: Mon, 24 Jul 2023 14:05:55 -0300
From: Pedro Tammela <pctammela@...atatu.com>
To: Cong Wang <xiyou.wangcong@...il.com>
Cc: netdev@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
 kuba@...nel.org, pabeni@...hat.com, jhs@...atatu.com, jiri@...nulli.us
Subject: Re: [PATCH net-next 1/5] net/sched: wrap open coded Qdics class
 filter counter

On 23/07/2023 15:10, Cong Wang wrote:
> On Fri, Jul 21, 2023 at 04:13:28PM -0300, Pedro Tammela wrote:
>> The 'filter_cnt' counter is used to control a Qdisc class lifetime.
>> Each filter referecing this class by its id will eventually
>> increment/decrement this counter in their respective
>> 'add/update/delete' routines.
>> As these operations are always serialized under rtnl lock, we don't
>> need an atomic type like 'refcount_t'.
>>
>> It also means that we lose the overflow/underflow checks already
>> present in refcount_t, which are valuable to hunt down bugs
>> where the unsigned counter wraps around as it aids automated tools
>> like syzkaller to scream in such situations.
>>
>> Wrap the open coded increment/decrement into helper functions and
>> add overflow checks to the operations.
> 
> So what's the concern of using refcount_t here? Since we have RTNL lock,
> I don't think performance is a concern.
> 
> I'd prefer to reuse the overflow/underflow with refcount_t than
> open-coding new ones.

I see.
There's a need for another minor adaption as well.
As the 'filter_cnt' starts from 0, it can do 0->1 transitions, which the 
refcount API warns.
So in a refcount_t 'filter_cnt' we would need to initialize it to 1.

I think it's confusing for a variable that represents a count of filters 
have 1 as a nil value.

> 
> 
>> diff --git a/include/net/tc_class.h b/include/net/tc_class.h
>> new file mode 100644
>> index 000000000000..2ab4aa2dba30
>> --- /dev/null
>> +++ b/include/net/tc_class.h
> 
> Why not put these helpers togethre with other qdisc class helpers in
> include/net/sch_generic.h?

OK

> 
> Thanks.