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

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Message-ID: <20190413213938.GA4371@andrea>
Date:   Sat, 13 Apr 2019 23:39:38 +0200
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        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>,
        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

On Tue, Apr 09, 2019 at 08:01:32AM -0700, Paul E. McKenney wrote:
> On Tue, Apr 09, 2019 at 03:36:18AM +0200, Andrea Parri wrote:
> > > > 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.
> 
> But don't all implementations of smp_mb__before_atomic() and
> smp_mb__after_atomic() currently supply a compiler barrier?

Yes, AFAICS, all implementations of smp_mb__{before,after}_atomic() currently
supply a compiler barrier.

Nevertheless, there's a difference between:  (1) Specify that these barriers
supply a compiler barrier,  (2) Specify that (certain) combinations of these
barriers and RMWs supply a compiler barrier, and  (3) This patch...  ;-)

FWIW, I'm not aware of current/informal documentation following (the arguably
simpler but slightly stronger) (1).  But again (amending my last remark): (1)
and (2) both make sense to me.

Thanx,
  Andrea


> 
> 							Thanx, Paul
>