linux-kernel - Re: [PATCH 3.16.y-ckt 009/126] sit: fix sit0 percpu double allocations

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Message-ID: <1450046747.9768.48.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Sun, 13 Dec 2015 14:45:47 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ben Hutchings <ben@...adent.org.uk>
Cc:	Luis Henriques <luis.henriques@...onical.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Steffen Klassert <steffen.klassert@...unet.com>,
	"David S. Miller" <davem@...emloft.net>,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Subject: Re: [PATCH 3.16.y-ckt 009/126] sit: fix sit0 percpu double
 allocations

On Sun, 2015-12-13 at 21:49 +0000, Ben Hutchings wrote:
> On Sun, 2015-12-13 at 13:44 -0800, Eric Dumazet wrote:
> > On Sun, 2015-12-13 at 21:22 +0000, Ben Hutchings wrote:
> > > On Sun, 2015-12-13 at 12:43 -0800, Eric Dumazet wrote:
> > 
> > > > 1) Sorry Ben, I do not understand the problem you mention.
> > > >    What is a partially initialized device exactly ?
> > > 
> > > A tunnel device which is registered but hasn't had its private
> > > structure fully initialised yet.
> > 
> > And you see this happening after my patch ? I am blind.
> > 
> > I am referring to current linux kernel, not to a backport to pre 3.18
> > kernels, that was not considered when I wrote this patch.
> > 
> > By the time ipip6_fb_tunnel_init() is called, dev->tstats had been
> > already allocated in ipip6_tunnel_init(), so what is missing ?
> 
> You moved this initialisation below the registration:
> 
> > 	ipip6_tunnel_clone_6rd(sitn->fb_tunnel_dev, sitn);
> > 	ipip6_fb_tunnel_init(sitn->fb_tunnel_dev);

Okay, so what is the exact problem you are seeing Ben ?

ipip6_tunnel_clone_6rd() looks to not contain a fatal race or mem leak.

Note that ipip6_tunnel_clone_6rd() can be called later from ioctl()
path.

ipip6_fb_tunnel_init() must be done once device is ready, as it
publishes state for packet processing.

rcu_assign_pointer(sitn->tunnels_wc[0], tunnel);

Looks like a rather correct way to register a device : init all fields,
then publish the RCU protected pointer for packets to catch it.

Really, I do not see a problem, please elaborate.



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