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

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Message-Id: <200807091715.25464.borntraeger@de.ibm.com>
Date:	Wed, 9 Jul 2008 17:15:25 +0200
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Max Krasnyansky <maxk@...lcomm.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH] tun: Persistent devices can get stuck in xoff state

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.

Thats interesting. I believe we have seen exactly this behaviour
with KVM and lots of preallocated tap devices. 
[...]
> +++ b/drivers/net/tun.c
> @@ -576,6 +576,11 @@ static int tun_set_iff(struct file *file, struct ifreq 
*ifr)
>  	file->private_data = tun;
>  	tun->attached = 1;
> 
> +	/* Make sure persistent devices do not get stuck in
> +	 * xoff state */
> +	if (netif_running(tun->dev))
> +		netif_wake_queue(tun->dev);
> +
>  	strcpy(ifr->ifr_name, tun->dev->name);
>  	return 0;

I think that patch looks ok, but I am curious why you dont clear the xoff 
state on application close at the same time when the TX queue gets flushed?

Christian
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