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

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Date:   Fri, 2 Sep 2016 08:35:55 +0200
From:   Manfred Spraul <manfred@...orfullife.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....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>,
        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

On 09/01/2016 06:41 PM, 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.
> 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.
CPU1:
x=1; /* without WRITE_ONCE */
LOCK(l);
UNLOCK(l);
<do_semop>
smp_store_release(x,0)


CPU2;
LOCK(l)
if (smp_load_acquire(x)==1) goto slow_path
<do_semop>
UNLOCK(l)

Ordering is enforced because both CPUs access the same lock.

x=1 can't be reordered past the UNLOCK(l), I don't see that further 
guarantees are necessary.

Correct?

--
     Manfred