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

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Message-ID: <20160901164126.GZ10153@twins.programming.kicks-ass.net>
Date:   Thu, 1 Sep 2016 18:41:26 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will.deacon@....com>
Cc:     Manfred Spraul <manfred@...orfullife.com>,
        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 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.

RELEASE + ACQUIRE otoh, that is a load-store barrier (but not
transitive).