netdev - Re: [RACE] net: in process

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Message-ID: <412e6f7f0911121717kfc892a5mbe16279d7d41776d@mail.gmail.com>
Date:	Fri, 13 Nov 2009 09:17:19 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Patrick McHardy <kaber@...sh.net>, netdev@...r.kernel.org
Subject: Re: [RACE] net: in process_backlog

On Fri, Nov 13, 2009 at 8:11 AM, Stephen Hemminger
<shemminger@...tta.com> wrote:
> On Fri, 13 Nov 2009 07:54:14 +0800
> Changli Gao <xiaosuo@...il.com> wrote:
>
>>
>> Yea, if the process waits for a RCU grace period, there will be no
>> race. But think about another case:
>> 1. flush_backlog().
> After flush backlog there should be no more skb's with that device
> in the queue, and if more are added, the device is buggy.
>
>> 2. dev_hold(skb->dev); netif_rx(). dev_put(skb->dev);
> There is no dev_hold in netif_rx path.
>

If the caller of netif_rx or netif_rx_ni isn't the device driver self,
it must find and hold the dev, then call netif_rx and put it.

>> 3. wait_for_refs();
>> 4. free(dev);
>> 5. netif_receive_skb(); //skb->dev doesn't present.
>> flush_backlog() can't prevent new skbs are added to backlog. If we
>> swap the flush_backlog() and wait_for_refs(), this case will be OK
>> too.
>
> It is still up to device driver not to add skb's to queue when stopped.
>

do you mean before calling netif_rx, the caller should check the dev
is alive or not? If so, why not add the check into netif_rx or
netif_rx_ni()? Could you explains why swapping the flush_backlog() and
wait_for_refs() can't work?


-- 
Regards，
Changli Gao(xiaosuo@...il.com)
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