netdev - Re: [PATCH] [RFC] netfilter: nf_conntrack: don't relase a conntrack with non-zero refcnt

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Date:	Mon, 03 Feb 2014 08:22:04 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Pablo Neira Ayuso <pablo@...filter.org>
Cc:	Florian Westphal <fw@...len.de>,
	Andrew Vagin <avagin@...allels.com>,
	Andrey Vagin <avagin@...nvz.org>,
	netfilter-devel@...r.kernel.org, netfilter@...r.kernel.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	vvs@...nvz.org, Cyrill Gorcunov <gorcunov@...nvz.org>,
	Vasiliy Averin <vvs@...allels.com>
Subject: Re: [PATCH] [RFC] netfilter: nf_conntrack: don't relase a conntrack
 with non-zero refcnt

On Mon, 2014-02-03 at 00:30 +0100, Pablo Neira Ayuso wrote:
>          */
>         smp_wmb();
> -       atomic_set(&ct->ct_general.use, 1);
> +       atomic_set(&ct->ct_general.use, 0);
>         return ct; 

Hi Pablo !

I think your patch is the way to go, but might need some extra care
with memory barriers.

I believe the smp_wmb() here is no longer needed.

If its a newly allocated memory, no other users can access to ct,
if its a recycled ct, content is already 0 anyway.

After your patch, nf_conntrack_get(&tmpl->ct_general) should increment 
an already non zero refcnt, so no memory barrier is needed.

But one smp_wmb() is needed right before this point :

	/* The caller holds a reference to this object */
	atomic_set(&ct->ct_general.use, 2);

Thanks !


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