netdev - Re: [Lguest] [PATCH 4/5] lguest: use KVM hypercalls

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Message-ID: <20090415143834.GA12384@gondor.apana.org.au>
Date:	Wed, 15 Apr 2009 22:38:34 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	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,
	Christian Borntraeger <borntraeger@...ibm.com>
Subject: Re: [Lguest] [PATCH 4/5] lguest: use KVM hypercalls

On Wed, Apr 15, 2009 at 07:18:44AM -0700, Eric W. Biederman wrote:
>
> So holding the reference only blocks us indefinitely in
> netdev_wait_allrefs, blocking the network namespace exit, and holding
> net_mutex indefinitely.

OK that's a killer because process A in my previous scenario can
kill the device itself and cause a dead-lock.

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.

This can be achieved by simply adding a counter to tun_struct.
We'd also change the async detach path to set a marker instead
of detaching.  That marker can then be checked in places like
tun_get.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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