linux-kernel - Re: [PATCH 3/4] net/netfilter/nf_conntrack

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Date:   Mon, 29 Aug 2016 12:51:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Manfred Spraul <manfred@...orfullife.com>
Cc:     benh@...nel.crashing.org, paulmck@...ux.vnet.ibm.com,
        Ingo Molnar <mingo@...e.hu>, Boqun Feng <boqun.feng@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, 1vier1@....de,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH 3/4] net/netfilter/nf_conntrack_core: update memory
 barriers.

On Sun, Aug 28, 2016 at 01:56:15PM +0200, Manfred Spraul wrote:
> +++ b/net/netfilter/nf_conntrack_core.c
> @@ -80,20 +80,29 @@ static __read_mostly bool nf_conntrack_locks_all;
>  
>  void nf_conntrack_lock(spinlock_t *lock) __acquires(lock)
>  {
> +	/* Step 1: Acquire the lock */
>  	spin_lock(lock);
> -	while (unlikely(nf_conntrack_locks_all)) {
> -		spin_unlock(lock);
>  
> -		/*
> -		 * Order the 'nf_conntrack_locks_all' load vs. the
> -		 * spin_unlock_wait() loads below, to ensure
> -		 * that 'nf_conntrack_locks_all_lock' is indeed held:
> -		 */
> -		smp_rmb(); /* spin_lock(&nf_conntrack_locks_all_lock) */
> -		spin_unlock_wait(&nf_conntrack_locks_all_lock);
> -		spin_lock(lock);
> -	}
> +	/* Step 2: make it visible to all CPUs that we hold the lock */
> +	smp_mb__after_spin_lock();

I hate this comment. A barrier does _not_ make visible anything.

A barrier _orders_ things.