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Date: Tue, 5 Jul 2022 16:29:51 +0100
From: Kajetan Puchalski <kajetan.puchalski@arm.com>
To: Will Deacon <will@kernel.org>
Cc: Florian Westphal <fw@strlen.de>,
	Pablo Neira Ayuso <pablo@netfilter.org>,
	Jozsef Kadlecsik <kadlec@netfilter.org>,
	"David S. Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	Mel Gorman <mgorman@suse.de>, lukasz.luba@arm.com,
	dietmar.eggemann@arm.com, mark.rutland@arm.com, broonie@kernel.org,
	netfilter-devel@vger.kernel.org, coreteam@netfilter.org,
	netdev@vger.kernel.org, stable@vger.kernel.org,
	regressions@lists.linux.dev, linux-kernel@vger.kernel.org,
	peterz@infradead.org
Subject: Re: [Regression] stress-ng udp-flood causes kernel panic on Ampere
 Altra
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In-Reply-To: <20220705112449.GA931@willie-the-truck>

> > > Sorry, but I have absolutely no context here. We have a handy document
> > > describing the differences between atomic_t and refcount_t:
> > > 
> > > 	Documentation/core-api/refcount-vs-atomic.rst
> > > 
> > > What else do you need to know?
> > 
> > Hmm, and I see a tonne of *_inc_not_zero() conversions in 719774377622
> > ("netfilter: conntrack: convert to refcount_t api") which mean that you
> > no longer have ordering to subsequent reads in the absence of an address
> > dependency.
> 
> I think the patch above needs auditing with the relaxed behaviour in mind,
> but for the specific crash reported here possibly something like the diff
> below?
> 
> Will
> 
> --->8
> 
> diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
> index 082a2fd8d85b..5ad9fcc84269 100644
> --- a/net/netfilter/nf_conntrack_core.c
> +++ b/net/netfilter/nf_conntrack_core.c
> @@ -1394,6 +1394,7 @@ static unsigned int early_drop_list(struct net *net,
>                  * already fired or someone else deleted it. Just drop ref
>                  * and move to next entry.
>                  */
> +               smp_rmb();      /* XXX: Why? */
>                 if (net_eq(nf_ct_net(tmp), net) &&
>                     nf_ct_is_confirmed(tmp) &&
>                     nf_ct_delete(tmp, 0, 0))
> 

With this patch applied the issue goes away as well. The test runs fine
well beyond where it would crash previously so looks good, thanks!