netdev - Re: [Patch net 2/2] net_sched: always call ->destroy when ->init() fails

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Date:	Fri, 24 Oct 2014 17:14:13 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Cong Wang <xiyou.wangcong@...il.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net,
	Wang Bo <wang.bo116@....com.cn>,
	John Fastabend <john.r.fastabend@...el.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Patrick McHardy <kaber@...sh.net>, Terry Lam <vtlam@...gle.com>
Subject: Re: [Patch net 2/2] net_sched: always call ->destroy when ->init()
 fails

On Fri, 2014-10-24 at 16:55 -0700, Cong Wang wrote:
> In qdisc_create(), when ->init() exists and it fails, we should
> call ->destroy() to clean up the potentially partially initialized
> qdisc's. This will also make the ->init() implementation easier.
> 

Why is this patch needed ?

You are adding bugs, its not clear what bug you are fixing.

I really do not like the idea of ->init() relying on a ->destroy() to
cleanup a failed ->init().

This is not what most management functions do in our stack.

I very much prefer that a function returning an error has no side
effect, like if it hadnt be called at all.

> qdisc_create_dflt() already does that. Most callers of qdisc_create_dflt()
> simply use noop_qdisc when it fails.
> 
> And, most of the ->destroy() implementations are already able to clean up
> partially initialization, we don't need to worry.
> 
> The following ->init()'s need to catch up:
> fq_codel_init(), hhf_init(), multiq_init(),  sfq_init(), mq_init(),
> mqprio_init().
> 

...

>  
>  static int multiq_dump(struct Qdisc *sch, struct sk_buff *skb)
> diff --git a/net/sched/sch_qfq.c b/net/sched/sch_qfq.c
> index 3ec7e88..55ac6a4 100644
> --- a/net/sched/sch_qfq.c
> +++ b/net/sched/sch_qfq.c
> @@ -522,7 +522,6 @@ static int qfq_change_class(struct Qdisc *sch, u32 classid, u32 parentid,
>  	return 0;
>  
>  destroy_class:
> -	qdisc_destroy(cl->qdisc);
>  	kfree(cl);
>  	return err;
>  }


Really ? I am calling this a leak of cl->qdisc.


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