netdev - Re: [PATCH net-next] hyperv: Move state setting for link query

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Date:	Wed, 05 Mar 2014 13:57:23 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	Haiyang Zhang <haiyangz@...rosoft.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:	KY Srinivasan <kys@...rosoft.com>,
	"olaf@...fle.de" <olaf@...fle.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"driverdev-devel@...uxdriverproject.org" 
	<driverdev-devel@...uxdriverproject.org>
Subject: Re: [PATCH net-next] hyperv: Move state setting for link query

On 03/05/2014 12:57 AM, Haiyang Zhang wrote:
>
>> -----Original Message-----
>> From: Jason Wang [mailto:jasowang@...hat.com]
>> Sent: Monday, March 3, 2014 10:10 PM
>> To: Haiyang Zhang; davem@...emloft.net; netdev@...r.kernel.org
>> Cc: KY Srinivasan; olaf@...fle.de; linux-kernel@...r.kernel.org; driverdev-
>> devel@...uxdriverproject.org
>> Subject: Re: [PATCH net-next] hyperv: Move state setting for link query
>>
>> On 03/04/2014 07:54 AM, Haiyang Zhang wrote:
>>> It moves the state setting for query into rndis_filter_receive_response().
>>> All callbacks including query-complete and status-callback are
>>> synchronized by channel->inbound_lock. This prevents pentential race
>> between them.
>>
>> This still looks racy to me. The problem is workqueue is not synchronized with
>> those here.
>>
>> Consider the following case in netvsc_link_change():
>>
>>     if (rdev->link_state) {
>>     ... receive interrupt ...
>>     rndis_filter_receice_response() which changes rdev->link_state
>>     ...
>>         netif_carrier_off()
>>     }
>>
>> And also it need to schedule a work otherwise the link status is out of sync.
> The rndis_filter_query_device_link_status() makes the query and wait for the
> complete message, including set state, before returning.
>
> The rndis_filter_query_device_link_status() is called from rndis_filter_device_add(),
> which is called from either netvsc_change_mtu() or netvsc_probe().
>
> The change_mtu() and netvsc_link_change() are synchronized by rtnl_lock().
> In netvsc_probe(), the status query & complete happens before register_netdev(), and
> the netvsc_linkstatus_callback() schedules the work only after netdevice is registered.
> So, there are no race in either case.
>
> The carrier_on/off work will be scheduled when netvsc_open() is called. Then,
> the status will be updated based on the latest link_state.
>
> Thanks,
> - Haiyang
>

I see. Then if the link status is changing during mtu changing in guest.
Looks like we may miss a link status updating?
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