linux-kernel - Re: [RFC][PATCH 3/3] locking,netfilter: Fix nf_conntrack

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Date:	Tue, 24 May 2016 16:42:28 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	manfred@...orfullife.com, dave@...olabs.net,
	paulmck@...ux.vnet.ibm.com, will.deacon@....com
Cc:	boqun.feng@...il.com, Waiman.Long@....com, tj@...nel.org,
	pablo@...filter.org, kaber@...sh.net, davem@...emloft.net,
	oleg@...hat.com, netfilter-devel@...r.kernel.org,
	sasha.levin@...cle.com, hofrat@...dl.org
Subject: Re: [RFC][PATCH 3/3] locking,netfilter: Fix nf_conntrack_lock()

On Tue, May 24, 2016 at 04:27:26PM +0200, Peter Zijlstra wrote:
> nf_conntrack_lock{,_all}() is borken as it misses a bunch of memory
> barriers to order the whole global vs local locks scheme.
> 
> Even x86 (and other TSO archs) are affected.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  net/netfilter/nf_conntrack_core.c |   30 +++++++++++++++++++++++++++++-
>  1 file changed, 29 insertions(+), 1 deletion(-)
> 
> --- a/net/netfilter/nf_conntrack_core.c
> +++ b/net/netfilter/nf_conntrack_core.c
> @@ -74,7 +74,18 @@ void nf_conntrack_lock(spinlock_t *lock)
>  	spin_lock(lock);
>  	while (unlikely(nf_conntrack_locks_all)) {

And note that we can replace nf_conntrack_locks_all with
spin_is_locked(nf_conntrack_locks_all_lock), since that is the exact
same state.

But I didn't want to do too much in one go.

>  		spin_unlock(lock);
> +		/*
> +		 * Order the nf_contrack_locks_all load vs the spin_unlock_wait()
> +		 * loads below, to ensure locks_all is indeed held.
> +		 */
> +		smp_rmb(); /* spin_lock(locks_all) */
>  		spin_unlock_wait(&nf_conntrack_locks_all_lock);
> +		/*
> +		 * The control dependency's LOAD->STORE order is enough to
> +		 * guarantee the spin_lock() is ordered after the above
> +		 * unlock_wait(). And the ACQUIRE of the lock ensures we are
> +		 * fully ordered against the spin_unlock() of locks_all.
> +		 */
>  		spin_lock(lock);
>  	}
>  }