linux-kernel - Re: [PATCH RFC tools/lkmm 10/12] tools/memory-model: Add a S lock-based external-view litmus test

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Date:   Thu, 22 Feb 2018 11:06:36 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Daniel Lustig <dlustig@...dia.com>
Cc:     Boqun Feng <boqun.feng@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        mingo@...nel.org, stern@...land.harvard.edu,
        parri.andrea@...il.com, will.deacon@....com, npiggin@...il.com,
        dhowells@...hat.com, j.alglave@....ac.uk, luc.maranget@...ia.fr,
        akiyks@...il.com, nborisov@...e.com,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH RFC tools/lkmm 10/12] tools/memory-model: Add a S
 lock-based external-view litmus test

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.