linux-kernel - Re: debug: nt_conntrack and KVM crash

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Date:	Mon, 1 Feb 2010 16:48:16 +0200
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Jon Masters <jonathan@...masters.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	netdev <netdev@...r.kernel.org>,
	netfilter-devel <netfilter-devel@...r.kernel.org>,
	Patrick McHardy <kaber@...sh.net>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: debug: nt_conntrack and KVM crash

On Mon, Feb 1, 2010 at 1:23 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> I wrote the algos, I know that we need different slab caches, for sure,
> this is not something I can _measure_, but theory can predict.
>
> SLAB_DESTROY_BY_RCU has very special semantics, you can ask Paul E.
> McKenny for details if you dont trust me.
>
> If you use a shared slab cache, one object can instantly flight between
> one hash table (netns ONE) to another one (netns TWO), and concurrent
> reader (doing a lookup in netns ONE, 'finding' an object of netns TWO)
> can be fooled without notice, because no RCU grace period has to be
> observed between object freeing and its reuse.
>
> We dont have this problem with UDP/TCP slab caches because TCP/UDP
> hashtables are global to the machine (and each object has a pointer to
> its netns).

conntracks also have netns pointer (->ct_net). This should be enough, yes?

> If we use per netns conntrack hash tables, we also *must* use per netns
> conntrack slab caches, to guarantee an object can not escape from one
> namespace to another one.
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