netdev - Re: [Patch net-next] vxlan: do real refcnt for vn

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Date:	Wed, 29 May 2013 10:08:53 +0800
From:	Cong Wang <amwang@...hat.com>
To:	Stephen Hemminger <stephen@...workplumber.org>
Cc:	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>
Subject: Re: [Patch net-next] vxlan: do real refcnt for vn_sock

On Tue, 2013-05-28 at 08:22 -0700, Stephen Hemminger wrote:
> On Tue, 28 May 2013 19:07:22 +0800
> Cong Wang <amwang@...hat.com> wrote:
> 
> > From: Cong Wang <amwang@...hat.com>
> > 
> > In commit 553675fb5e9ce3d71a (vxlan: listen on multiple ports),
> > we use kfree_rcu() to free ->vn_sock, but a) there is no use
> > of RCU API to access this filed, b) RCU is not enough to do refcnt
> > here, because in vxlan_leave_group() we drop RTNL lock before
> > locking the socket, it could be possible that this field is
> > freed during this period.
> > 
> > So, instead making things complex, just do basic refcnt for
> > the ->vn_sock, like we do for others.
> > 
...
> 
> Not needed all access is under RTNL

I know, this is why I had a patch (not posted) which adds the missing
rtnl_dereference(), but even if we had these, it is still not correct.

As I explained in the changelog, vxlan_leave_group() has a problem,
because it releases rtnl lock before locking the socket, _and_ it is
called after vxlan_dellink() which schedules a work to cleanup the
struct. Therefore the ->vn_sock could be freed right after rtnl lock is
released.

Am I miss anything?

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