linux-kernel - Re: [PATCH v3 1/5] tools/memory-model: Add an exception for limitations on

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Date:   Thu, 27 Feb 2020 17:49:02 +0100
From:   Luc Maranget <luc.maranget@...ia.fr>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org,
        Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Jonathan Corbet <corbet@....net>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Rob Herring <robh@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        linux-arch@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v3 1/5] tools/memory-model: Add an exception for
 limitations on _unless() family

> On Thu, 27 Feb 2020, Boqun Feng wrote:
> 
> > According to Luc, atomic_add_unless() is directly provided by herd7,
> > therefore it can be used in litmus tests. So change the limitation
> > section in README to unlimit the use of atomic_add_unless().
> 
> Is this really true?  Why does herd treat atomic_add_unless() different
> from all the other atomic RMS ops?  All the other ones we support do
> have entries in linux-kernel.def.

I think this to be true :)

As far as I remember atomic_add_unless is quite different fron other atomic
RMW ops and called for a specific all-OCaml implementation, without an
entry in linux-kernel.def. As to  atomic_long_add_unless, I was not aware
of its existence.

--Luc

> 
> Alan
> 
> PS: It seems strange to support atomic_add_unless but not 
> atomic_long_add_unless.  The difference between the two is trivial.
> 
> > 
> > Cc: Luc Maranget <luc.maranget@...ia.fr>
> > Signed-off-by: Boqun Feng <boqun.feng@...il.com>
> > ---
> >  tools/memory-model/README | 10 +++++++---
> >  1 file changed, 7 insertions(+), 3 deletions(-)
> > 
> > diff --git a/tools/memory-model/README b/tools/memory-model/README
> > index fc07b52f2028..409211b1c544 100644
> > --- a/tools/memory-model/README
> > +++ b/tools/memory-model/README
> > @@ -207,11 +207,15 @@ The Linux-kernel memory model (LKMM) has the following limitations:
> >  		case as a store release.
> >  
> >  	b.	The "unless" RMW operations are not currently modeled:
> > -		atomic_long_add_unless(), atomic_add_unless(),
> > -		atomic_inc_unless_negative(), and
> > -		atomic_dec_unless_positive().  These can be emulated
> > +		atomic_long_add_unless(), atomic_inc_unless_negative(),
> > +		and atomic_dec_unless_positive().  These can be emulated
> >  		in litmus tests, for example, by using atomic_cmpxchg().
> >  
> > +		One exception of this limitation is atomic_add_unless(),
> > +		which is provided directly by herd7 (so no corresponding
> > +		definition in linux-kernel.def). atomic_add_unless() is
> > +		modeled by herd7 therefore it can be used in litmus tests.
> > +
> >  	c.	The call_rcu() function is not modeled.  It can be
> >  		emulated in litmus tests by adding another process that
> >  		invokes synchronize_rcu() and the body of the callback
> >