netdev - Re: [PATCH net-next] xen-netback: Rework rx_work

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Date:	Wed, 15 Jan 2014 11:47:02 +0000
From:	Zoltan Kiss <zoltan.kiss@...rix.com>
To:	Wei Liu <wei.liu2@...rix.com>
CC:	<ian.campbell@...rix.com>, <xen-devel@...ts.xenproject.org>,
	<netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<jonathan.davies@...rix.com>
Subject: Re: [PATCH net-next] xen-netback: Rework rx_work_todo

On 15/01/14 10:37, Wei Liu wrote:
> On Tue, Jan 14, 2014 at 07:28:39PM +0000, Zoltan Kiss wrote:
>> The recent patch to fix receive side flow control (11b57f) solved the spinning
>> thread problem, however caused an another one. The receive side can stall, if:
>> - xenvif_rx_action sets rx_queue_stopped to false
>> - interrupt happens, and sets rx_event to true
>> - then xenvif_kthread sets rx_event to false
>>
>
> If you mean "rx_work_todo" returns false.
>
> In this case
>
> (!skb_queue_empty(&vif->rx_queue) && !vif->rx_queue_stopped) || vif->rx_event;
>
> can still be true, can't it?
Sorry, I should wrote rx_queue_stopped to true

>
>> Also, through rx_event a malicious guest can force the RX thread to spin. This
>> patch ditch that two variable, and rework rx_work_todo. If the thread finds it
>
> This seems to be a bigger problem. Can you elaborate?
My mistake too. I forgot that rx_action set it to false, so it's not 
really a spinning. However the thread should still run xenvif_rx_action 
to figure out there is no space in the ring before it sets rx_event to 
false. In my patch we can quit earlier.

Zoli
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