netdev - Re: [Patch net-next v2 3/4] net_sched: remove tc class reference counting

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Date:   Fri, 25 Aug 2017 11:00:21 +0200
From:   Jiri Pirko <jiri@...nulli.us>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     netdev@...r.kernel.org, jhs@...atatu.com, fw@...len.de
Subject: Re: [Patch net-next v2 3/4] net_sched: remove tc class reference
 counting

Fri, Aug 25, 2017 at 01:51:29AM CEST, xiyou.wangcong@...il.com wrote:
>For TC classes, their ->get() and ->put() are always paired, and the
>reference counting is completely useless, because:
>
>1) For class modification and dumping paths, we already hold RTNL lock,
>   so all of these ->get(),->change(),->put() are atomic.

There is ongoing initiative by Florian to avoid taking RTNL for some
rtnetlink calls. I think that for dumping it could be done in tc as well.
Don't we need the refcnt then?


>
>2) For filter bindiing/unbinding, we use other reference counter than
>   this one, and they should have RTNL lock too.
>
>3) For ->qlen_notify(), it is special because it is called on ->enqueue()
>   path, but we already hold qdisc tree lock there, and we hold this
>   tree lock when graft or delete the class too, so it should not be gone
>   or changed until we release the tree lock.
>
>Therefore, this patch removes ->get() and ->put(), but:
>
>1) Adds a new ->find() to find the pointer to a class by classid, no
>   refcnt.
>
>2) Move the original class destroy upon the last refcnt into ->delete(),
>   right after releasing tree lock. This is fine because the class is
>   already removed from hash when holding the lock.
>
>For those who also use ->put() as ->unbind(), just rename them to reflect
>this change.