linux-kernel - Re: [RFC 2/3] tools/memory-model: Add a litmus test for atomic

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Message-ID: <20200225130102.wsz3bpyhjmcru7os@yquem.inria.fr>
Date:   Tue, 25 Feb 2020 14:01:02 +0100
From:   Luc Maranget <luc.maranget@...ia.fr>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Andrea Parri <parri.andrea@...il.com>,
        Luc Maranget <luc.maranget@...ia.fr>,
        linux-kernel@...r.kernel.org,
        Alan Stern <stern@...land.harvard.edu>,
        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>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Jonathan Corbet <corbet@....net>, linux-arch@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [RFC 2/3] tools/memory-model: Add a litmus test for atomic_set()

Hi,

As far as I can remember I have implemented atomic_add_unless in herd7.

As to your test, I have first run a slightly modified version of your test
as a kernel module (using klitmus7).

C atomic_add_unless-dependency
{
        atomic_t y = ATOMIC_INIT(1);
}
  P0(int *x, atomic_t *y, int *z)
{
        int r0;
        r0 = READ_ONCE(*x);
        if (atomic_add_unless((atomic_t *)y, 2, r0))
                WRITE_ONCE(*z, 42);
        else
                WRITE_ONCE(*z, 1);
}
  P1(int *x, int *z)
{
        int r0;
        r0 = smp_load_acquire(z);
        WRITE_ONCE(*x, 1);
}
locations [y]
exists
(1:r0 = 1 /\ 0:r0 = 1)


The test is also accepted by herd7, here producing teh same final values
as actual run on a raspberry PI4B.

--Luc

> Luc,
> 
> Could you have a look at the problem Andrea and I discuss here? It seems
> that you have done a few things in herd for atomic_add_unless() in
> particular, and based on the experiments of Andrea and me, seems
> atomic_add_unless() works correctly. So can you confirm that herd now
> can handle atomic_add_unless() or there is still something missing?
> 
> Thanks!
> 
> Regards,
> Boqun
> 
> On Fri, Feb 14, 2020 at 06:40:03PM +0800, Boqun Feng wrote:
> > On Fri, Feb 14, 2020 at 09:12:13AM +0100, Andrea Parri wrote:
> > > > @@ -0,0 +1,24 @@
> > > > +C Atomic-set-observable-to-RMW
> > > > +
> > > > +(*
> > > > + * Result: Never
> > > > + *
> > > > + * Test of the result of atomic_set() must be observable to atomic RMWs.
> > > > + *)
> > > > +
> > > > +{
> > > > +	atomic_t v = ATOMIC_INIT(1);
> > > > +}
> > > > +
> > > > +P0(atomic_t *v)
> > > > +{
> > > > +	(void)atomic_add_unless(v,1,0);
> > > 
> > > We blacklisted this primitive some time ago, cf. section "LIMITATIONS",
> > > entry (6b) in tools/memory-model/README; the discussion was here:
> > > 
> > >   https://lkml.kernel.org/r/20180829211053.20531-3-paulmck@linux.vnet.ibm.com
> > > 
> > 
> > And in an email replying to that email, you just tried and seemed
> > atomic_add_unless() works ;-)
> > 
> > > but unfortunately I can't remember other details at the moment: maybe
> > > it is just a matter of or the proper time to update that section.
> > > 
> > 
> > I spend a few time looking into the changes in herd, the dependency
> > problem seems to be as follow:
> > 
> > For atomic_add_unless(ptr, a, u), the return value (true or false)
> > depends on both *ptr and u, this is different than other atomic RMW,
> > whose return value only depends on *ptr. Considering the following
> > litmus test:
> > 
> > 	C atomic_add_unless-dependency
> > 
> > 	{
> > 		int y = 1;
> > 	}
> > 
> > 	P0(int *x, int *y, int *z)
> > 	{
> > 		int r0;
> > 		int r1;
> > 		int r2;
> > 
> > 		r0 = READ_ONCE(*x);
> > 		if (atomic_add_unless(y, 2, r0))
> > 			WRITE_ONCE(*z, 42);
> > 		else
> > 			WRITE_ONCE(*z, 1);
> > 	}
> > 
> > 	P1(int *x, int *y, int *z)
> > 	{
> > 		int r0;
> > 
> > 		r0 = smp_load_acquire(z);
> > 
> > 		WRITE_ONCE(*x, 1);
> > 	}
> > 
> > 	exists
> > 	(1:r0 = 1 /\ 0:r0 = 1)
> > 
> > , the exist-clause will never trigger, however if we replace
> > "atomic_add_unless(y, 2, r0)" with "atomic_add_unless(y, 2, 1)", the
> > write on *z and the read from *x on CPU 0 are not ordered, so we could
> > observe the exist-clause triggered.
> > 
> > I just tried with the latest herd, and herd can work out this
> > dependency. So I think we are good now and can change the limitation
> > section in the document. But I will wait for Luc's input for this. Luc,
> > did I get this correct? Is there any other limitation on
> > atomic_add_unless() now?
> > 
> > Regards,
> > Boqun
> > 
> > > Thanks,
> > >   Andrea