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Date:   Thu, 22 Feb 2018 11:06:36 +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>,
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        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 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.