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Date:   Tue, 9 Apr 2019 03:36:18 +0200
From:   Andrea Parri <andrea.parri@amarulasolutions.com>
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     LKMM Maintainers -- Akira Yokosawa <akiyks@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Daniel Lustig <dlustig@nvidia.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        Nicholas Piggin <npiggin@gmail.com>,
        "Paul E. McKenney" <paulmck@linux.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will.deacon@arm.com>,
        Daniel Kroening <kroening@cs.ox.ac.uk>,
        Kernel development list <linux-kernel@vger.kernel.org>
Subject: Re: Adding plain accesses and detecting data races in the LKMM
Message-ID: <20190409013618.GA3824@andrea>
References: <20190408055117.GA25135@andrea>
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> > The formula was more along the line of "do not assume either of these
> > cases to hold; use barrier() is you need an unconditional barrier..."
> > AFAICT, all current implementations of smp_mb__{before,after}_atomic()
> > provides a compiler barrier with either barrier() or "memory" clobber.
> 
> Well, we have two reasonable choices: Say that 
> smp_mb__{before,after}_atomic will always provide a compiler barrier, 
> or don't say this.  I see no point in saying that the combination of 
> Before-atomic followed by RMW provides a barrier.

;-/ I'm fine with the first choice. I don't see how the second choice
(this proposal/patch) would be consistent with some documentation and
with the current implementations; for example,

1) Documentation/atomic_t.txt says:

Thus:

  atomic_fetch_add();

is equivalent to:

  smp_mb__before_atomic();
  atomic_fetch_add_relaxed();
  smp_mb__after_atomic();

[...]

2) Some implementations of the _relaxed() variants do not provide any
compiler barrier currently.

  Andrea