linux-kernel - Re: Adding plain accesses and detecting data races in the LKMM

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Message-ID: <20190409013618.GA3824@andrea>
Date:   Tue, 9 Apr 2019 03:36:18 +0200
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     LKMM Maintainers -- Akira Yokosawa <akiyks@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Nicholas Piggin <npiggin@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>,
        Daniel Kroening <kroening@...ox.ac.uk>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Adding plain accesses and detecting data races in the LKMM

> > 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