netdev - Re: [PATCH] tun: Persistent devices can get stuck in xoff state

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Date:	Wed, 09 Jul 2008 14:19:28 -0700
From:	Max Krasnyansky <maxk@...lcomm.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>
CC:	davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH] tun: Persistent devices can get stuck in xoff state

Christian Borntraeger wrote:
> Am Mittwoch, 9. Juli 2008 schrieb Max Krasnyansky:
>> The scenario goes like this. App stops reading from tun/tap.
>> TX queue gets full and driver does netif_stop_queue().
>> App closes fd and TX queue gets flushed as part of the cleanup.
>> Next time the app opens tun/tap and starts reading from it but
>> the xoff state is not cleared. We're stuck.
>> Normally xoff state is cleared when netdev is brought up. But
>> in the case of persistent devices this happens only during
>> initial setup.
> 
> I was able to reproduce the problem. The patch seems to fix it.
> 
> Tested-by: Christian Borntraeger <borntraeger@...ibm.com>

Oh it definitely fixes that problem. It's very easy to reproduce by just 
suspending (ctrl-z) an app that is ready from the tun, waiting a bit for 
the queue to fill up and then killing the app. I have a simple test app 
that simulates this. It's just that in real life that does not happen 
very often. ie Applications either constantly read from tun or they 
close it.

Thanks a lot for testing and confirming.

Max
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