netdev - Re: [1/2] tun: Only free a netdev when all tun descriptors are closed

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Date:	Fri, 24 Apr 2009 10:55:49 +0200
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Patrick McHardy <kaber@...sh.net>,
	Matias Zabaljauregui <zabaljauregui@...il.com>, odie@...aau.dk,
	Rusty Russell <rusty@...tcorp.com.au>, lguest@...abs.org,
	virtualization@...ts.osdl.org,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	Carsten Otte <cotte@...ibm.com>
Subject: Re: [1/2] tun: Only free a netdev when all tun descriptors are closed

Am Thursday 16 April 2009 13:08:18 schrieb Herbert Xu:
> On Wed, Apr 15, 2009 at 10:38:34PM +0800, Herbert Xu wrote:
> > 
> > So how about this? We replace the dev destructor with our own that
> > doesn't immediately call free_netdev.  We only call free_netdev once
> > all tun fd's attached to the device have been closed.
> 
> Here's the patch.  I'd appreciate if everyone can review it
> and see if they can recreate the original race by
> 
> 1) Starting a process using tun and polls on it;
> 2) Doing ip tun del tunXXX while the process is polling.
> 
> tun: Only free a netdev when all tun descriptors are closed
> 
> The commit c70f182940f988448f3c12a209d18b1edc276e33 ("tun: Fix
> races between tun_net_close and free_netdev") fixed a race where
> an asynchronous deletion of a tun device can hose a poll(2) on
> a tun fd attached to that device.

Sorry for the late reply, but this patch creates another regression:
Now TUNSETIFF returns EBUSY on a persistent tap device:

open("/dev/net/tun", O_RDWR) = 11
ioctl(11, 0x400454ca, 0x3ffff8e2270) = -1 EBUSY (Device or resource busy)

Some debugging (thanks to Carsten Otte) showed that

tun_set_iff calls tun_attach (the first call inside the if(dev)). tun_attach
now checks for tun->tfile but this is already set.
Looks like we need another fix on top :-(

Christian
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