linux-kernel - Re: [PATCH 8/7] net/netfilter/nf_conntrack

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Date:   Fri, 2 Sep 2016 14:17:50 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Will Deacon <will.deacon@....com>,
        Manfred Spraul <manfred@...orfullife.com>,
        benh@...nel.crashing.org, paulmck@...ux.vnet.ibm.com,
        Ingo Molnar <mingo@...e.hu>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, 1vier1@....de,
        Davidlohr Bueso <dave@...olabs.net>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        netfilter-devel@...r.kernel.org
Subject: Re: [PATCH 8/7] net/netfilter/nf_conntrack_core: Remove another
 memory barrier

Hi Manfred,

On Thu, Sep 01, 2016 at 06:41:26PM +0200, Peter Zijlstra wrote:
> On Thu, Sep 01, 2016 at 04:30:39PM +0100, Will Deacon wrote:
> > On Thu, Sep 01, 2016 at 05:27:52PM +0200, Manfred Spraul wrote:
> > > Since spin_unlock_wait() is defined as equivalent to spin_lock();
> > > spin_unlock(), the memory barrier before spin_unlock_wait() is
> > > also not required.
> 

As Peter said below, ACQUIRE+RELEASE is not a barrier.

What we rely here is that spin_unlock_wait() could pair with another
LOCK or UNLOCK(as spin_unlock_wait() acts as spin_lock();
spin_unlock()). And once paired, we could have the necessary order
guarantee between the code preceding or following unlock_wait() and the
code in the lock critical sections.

Regards,
Boqun

> Note that ACQUIRE+RELEASE isn't a barrier.
> 
> Both are semi-permeable and things can cross in the middle, like:
> 
> 
> 	x = 1;
> 	LOCK
> 	UNLOCK
> 	r = y;
> 
> can (validly) get re-ordered like:
> 
> 	LOCK
> 	r = y;
> 	x = 1;
> 	UNLOCK
> 
> So if you want things ordered, as I think you do, I think the smp_mb()
> is still needed.
> 
> RELEASE + ACQUIRE otoh, that is a load-store barrier (but not
> transitive).

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