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

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Date: Sun, 23 Jul 2023 11:10:31 -0700
From: Cong Wang <xiyou.wangcong@...il.com>
To: Pedro Tammela <pctammela@...atatu.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 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.


> 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?

Thanks.