netdev - Re: [RACE] net: in process

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Date:	Fri, 13 Nov 2009 07:54:14 +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 12:57 AM, Stephen Hemminger
<shemminger@...tta.com> wrote:
> On Thu, 12 Nov 2009 16:50:53 +0800
> Changli Gao <xiaosuo@...il.com> wrote:
>
>
> There is are a couple of issues here, but it is not what you thought
> you saw.
>
> The receive process is always done in soft IRQ context. The backlog queue's
> are per-cpu. When a device is deleted an IPI is sent to all cpu's to
> scan there backlog queue.  What should protect the skb is the fact that
> the network device destruction process waits for an RCU grace period.
> So skb->dev points to valid data.

Yea, if the process waits for a RCU grace period, there will be no
race. But think about another case:
1. flush_backlog().
2. dev_hold(skb->dev); netif_rx(). dev_put(skb->dev);
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.

>
> BUT the flush_backlog is run too late in the device destruction process.
> It should be moved out of netdev_run_todo, to right after dev_shutdown().
> Also adding a check for skb->dev->reg_state in netif_receive_skb would
> be wise to drop packets.
>
>
> --
>



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