linux-kernel - Re: [PATCH RFC LKMM 3/7] EXP tools/memory-model: Add more LKMM limitations

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Date:   Thu, 6 Sep 2018 11:34:17 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Andrea Parri <andrea.parri@...rulasolutions.com>
Cc:     Andrea Parri <parri.andrea@...il.com>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        mingo@...nel.org, stern@...land.harvard.edu, will.deacon@....com,
        peterz@...radead.org, boqun.feng@...il.com, npiggin@...il.com,
        dhowells@...hat.com, j.alglave@....ac.uk, luc.maranget@...ia.fr,
        akiyks@...il.com
Subject: Re: [PATCH RFC LKMM 3/7] EXP tools/memory-model: Add more LKMM
 limitations

On Fri, Aug 31, 2018 at 11:43:42AM +0200, Andrea Parri wrote:
> > > > +	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
> > > > +		in litmus tests, for example, by using atomic_cmpxchg().
> > > 
> > > There is a prototype atomic_add_unless(): with current herd7,
> > > 
> > > $ cat atomic_add_unless.litmus
> > > C atomic_add_unless
> > > 
> > > {}
> > > 
> > > P0(atomic_t *u, atomic_t *v)
> > > {
> > > 	int r0;
> > > 	int r1;
> > > 
> > > 	r0 = atomic_add_unless(u, 1, 2);
> > > 	r1 = atomic_read(v);
> > > }
> > > 
> > > P1(atomic_t *u, atomic_t *v)
> > > {
> > > 	int r0;
> > > 	int r1;
> > > 
> > > 	r0 = atomic_add_unless(v, 1, 2);
> > > 	r1 = atomic_read(u);
> > > }
> > > 
> > > exists (0:r1=0 /\ 1:r1=0)
> > > 
> > > $ herd7 -conf linux-kernel.cfg atomic_add_unless.litmus
> > > Test atomic_add_unless Allowed
> > > States 3
> > > 0:r1=0; 1:r1=1;
> > > 0:r1=1; 1:r1=0;
> > > 0:r1=1; 1:r1=1;
> > > No
> > > Witnesses
> > > Positive: 0 Negative: 3
> > > Condition exists (0:r1=0 /\ 1:r1=0)
> > > Observation atomic_add_unless Never 0 3
> > > Time atomic_add_unless 0.00
> > > Hash=fa37a2359831690299e4cc394e45d966
> > > 
> > > The last commit in the herdtools7 repo. related to this implementation
> > > (AFAICT) is:
> > > 
> > >   9523c340917b6a ("herd/linux: make atomic_add_unless a primitive, so as to yield more precise dependencies for the returned boolean.")
> > > 
> > > but I can only vaguely remember those dependencies issues now :/  ...;
> > > maybe we can now solve these issues?  or should we change herd7 to re-
> > > turn a warning?  (Notice that this primitive is currently not exported
> > > to the linux-kernel.def file.)
> > 
> > Cool!  It would be good to add this to the .def file once the underlying
> > herd7 machinery is ready.  And then I would update the documentation
> > accordingly.  Or happily accept a patch updating the documentation,
> > as the case might be.  ;-)
> 
> Fair enough, ;-)  Please feel free to add:
> 
> Reviewed-by: Andrea Parri <andrea.parri@...rulasolutions.com>

Thank you!  Applied and pushed out, updating the lkmm branch in the
process.

							Thanx, Paul