netdev - Re: [patch net] net: sched: fix memleak for chain zero

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Date:   Wed, 6 Sep 2017 16:37:59 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Jiri Pirko <jiri@...nulli.us>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jakub Kicinski <kubakici@...pl>, mlxsw@...lanox.com
Subject: Re: [patch net] net: sched: fix memleak for chain zero

On Wed, Sep 6, 2017 at 1:33 PM, Jiri Pirko <jiri@...nulli.us> wrote:
> Wed, Sep 06, 2017 at 07:40:02PM CEST, xiyou.wangcong@...il.com wrote:
>>On Wed, Sep 6, 2017 at 4:14 AM, Jiri Pirko <jiri@...nulli.us> wrote:
>>> From: Jiri Pirko <jiri@...lanox.com>
>>>
>>> There's a memleak happening for chain 0. The thing is, chain 0 needs to
>>> be always present, not created on demand. Therefore tcf_block_get upon
>>> creation of block calls the tcf_chain_create function directly. The
>>> chain is created with refcnt == 1, which is not correct in this case and
>>> causes the memleak. So move the refcnt increment into tcf_chain_get
>>> function even for the case when chain needs to be created.
>>>
>>
>>Your approach could work but you just make the code even
>>uglier than it is now:
>>
>>1. The current code is already ugly for special-casing chain 0:
>>
>>        if (--chain->refcnt == 0 && !chain->filter_chain && chain->index != 0)
>>                tcf_chain_destroy(chain);
>>
>>2. With your patch, chain 0 has a different _initial_ refcnt with others.
>
> No. Initial refcnt is the same. ! for every action that holds the chain.
> So actually, it returns it back where it should be.

Not all all.

tcf_block_get() calls tcf_chain_create(, 0), after your patch
chain 0 has refcnt==0 initially.

Non-0 chain? They are created via tcf_chain_get(), aka,
refcnt==0 initially.

>
>
>>
>>3. Allowing an object (chain 0) exists with refcnt==0
>
> So? That is for every chain that does not have goto_chain action
> pointing at. Please read the code.

So you are pretending to be GC but you are apparently not.

You create all the troubles by setting yourself to believe chain 0
is special and refcnt==0 is okay. Both are wrong.

Actually the !list_empty() check is totally unnecessary too,
it is yet another place you get it wrong, you hide the race
condition in commit 744a4cf63e52 which makes it harder
to expose.

I understand you don't trust me. Look at DaveM's reaction
to your refcnt==0 madness.

Remember, refcnt can be very simple, you just want to
make it harder by abusing it or attempting to invent a GC.

I am going to update my patch (to remove all your madness)
since this is horribly wrong to me. Sorry.