linux-kernel - [Question] atomic_fetch_andnot() in nohz_idle

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Message-ID: <20181121223453.GA4016@andrea>
Date:   Wed, 21 Nov 2018 23:34:53 +0100
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     linux-kernel@...r.kernel.org
Subject: [Question] atomic_fetch_andnot() in nohz_idle_balance()

Hi,

The comment for the atomic_fetch_andnot() in nohz_idle_balance() says:

  "barrier, pairs with nohz_balance_enter_idle(), ensures ..."

which, well, does sound a note of warning... ;-)

I see that nohz_balance_enter_idle() has an smp_mb__after_atomic() but
the comment for the latter suggests that this barrier is pairing with
the smp_mb() in _nohz_idle_balance().

So, what is the intended pairing barrier for the atomic_fetch_andnot()?
what (which memory accesses) do you want "to order" here?

Thanks,
  Andrea