linux-kernel - [RFC][PATCH 0/3] locking/qspinlock: Improve determinism for x86

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Date:   Wed, 26 Sep 2018 13:01:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     will.deacon@....com, mingo@...nel.org
Cc:     linux-kernel@...r.kernel.org, longman@...hat.com,
        andrea.parri@...rulasolutions.com, tglx@...utronix.de,
        Peter Zijlstra <peterz@...radead.org>
Subject: [RFC][PATCH 0/3] locking/qspinlock: Improve determinism for x86

Back when Will did his qspinlock determinism patches, we were left with one
cmpxchg loop on x86 due to the use of atomic_fetch_or(). Will proposed a nifty
trick:

  http://lkml.kernel.org/r/20180409145409.GA9661@arm.com

But at the time we didn't pursue it. This series implements that and argues for
its correctness. In particular it places an smp_mb__after_atomic() in
between the two operations, which forces the load to come after the
store (which is free on x86 anyway).

In particular this ordering ensures a concurrent unlock cannot trigger
the uncontended handoff. Also it ensures that if the xchg() happens
after a (successful) trylock, we must observe that LOCKED bit.