netdev - Re: [Xen-devel] [PATCH 3/8] netback: get/put module along with vif connect/disconnect

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Date:	Tue, 5 Mar 2013 14:07:42 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	Wei Liu <wei.liu2@...rix.com>
CC:	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
	"annie.li@...cle.com" <annie.li@...cle.com>
Subject: Re: [Xen-devel] [PATCH 3/8] netback: get/put module along with vif
 connect/disconnect

On 05/03/13 13:30, Wei Liu wrote:
> On Tue, 2013-03-05 at 10:02 +0000, David Vrabel wrote:
>> On 15/02/13 16:00, Wei Liu wrote:
>>> If there is vif running and user unloads netback, guest's network interface
>>> just mysteriously stops working. So we need to prevent unloading netback
>>> module if there is vif running.
>>
>> It's not mysterious -- it is cleanly disconnected, and will reconnect
>> when the module is reinserted.
>>
> 
> From a guest's POV, it just stops without any sign. This should be
> prevented IMHO.

This is a bug in the frontend or a bug in the backend failing to
disconnect correctly.

I posted a series of "xen-foofront: handle backend CLOSED without
CLOSING" patches that may help here. (I didn't get applied to netfront
for some reason.)

Disabling module unload doesn't prevent this from happening away.  You
can always manually unbind the backend device from the xen-netback
driver which has the same effect as unloading the module.

> Netback / netfront lose all states when netback is unloaded. And
> netfront doesn't support reconfiguration at the moment. My guess is that
> this is the reason why netback doesn't even have unload function at
> first.

If netfront cannot handle reconnect then that's a bug in the frontend or
a bug in the backend xenbus code not setting up the reconnect correctly.

>> Being able to unload modules while they are in use is standard so I
>> don't think this should be applied.
> 
> I don't think this is true from a module dependency point of view - just
> try to unload any in use module, rmmod / modprobe will give you a fatal
> error.

Try it with any other network interface driver and it will unload just fine.

David
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