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Date:   Thu, 22 Feb 2018 11:20:26 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Daniel Lustig <dlustig@nvidia.com>
Cc:     Boqun Feng <boqun.feng@gmail.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        mingo@kernel.org, stern@rowland.harvard.edu,
        parri.andrea@gmail.com, will.deacon@arm.com, npiggin@gmail.com,
        dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr,
        akiyks@gmail.com, nborisov@suse.com,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>
Subject: Re: [PATCH RFC tools/lkmm 10/12] tools/memory-model: Add a S
 lock-based external-view litmus test
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On Thu, Feb 22, 2018 at 11:06:36AM +0100, Peter Zijlstra wrote:
> On Wed, Feb 21, 2018 at 09:42:08PM -0800, Daniel Lustig wrote:
> > And yes, if we go with a purely RCpc interpretation of acquire and
> > release, then I don't believe the writes in the previous critical
> > section would be ordered with the writes in the subsequent critical
> > section.
> 
> Excuse my ignorance (also jumping in the middle of things), but how can
> this be?
> 
> spin_unlock() is a store-release, this means the write to the lock word
> must happen after any stores inside the critical section.
> 
> spin_lock() is a load-acquire + test-and-set-ctrl-dep, we'll only
> proceed with the critical section if we observe the lock 'unlocked',
> which also means we must observe the stores prior to the unlock.
> 
> And both the ctrl-dep and the ACQUIRE ensure future stores cannot happen
> before.
> 
> So while the lock store and subsequent critical section stores are
> unordered, I don't see how it would be possible to not be ordered
> against stores from a previous critical section.
> 

Or are we talking about a third party observing while not partaking in
the lock-chain? Then I agree, the stores can be observed out of order by
this 3rd actor.