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Date: Thu, 3 Aug 2017 17:12:24 +0100
From: Will Deacon <will.deacon@arm.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Boqun Feng <boqun.feng@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>, linux-kernel@vger.kernel.org,
        Ingo Molnar <mingo@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
        Randy Dunlap <rdunlap@infradead.org>
Subject: Re: [RFC][PATCH v3]: documentation,atomic: Add new documents
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On Thu, Aug 03, 2017 at 07:55:14AM -0700, Paul E. McKenney wrote:
> On Thu, Aug 03, 2017 at 10:05:16PM +0800, Boqun Feng wrote:
> > Hi Will,
> > 
> > On Wed, Aug 02, 2017 at 10:45:32AM +0100, Will Deacon wrote:
> > [...]
> > > 
> > > It's worth noting that we don't have the problem with any value-returning
> > > atomics, so all flavours of xchg in this test would be forbidden on arm64
> > > too.
> > > 
> > > > 	C C-WillDeacon-MP+o-r+ai-rmb-o.litmus
> > > > 
> > > > 	(*
> > > > 	 * Expected result: Never.
> > > > 	 *
> > > > 	 * Desired litmus test, with atomic_inc() emulated by xchg_relaxed():
> > > > 	 *
> > > > 	 *     WRITE_ONCE(x, 1);               atomic_inc(&y);
> > > > 	 *     r0 = xchg_release(&y, 5);       smp_rmb();
> > > > 	 *                                     r1 = READ_ONCE(x);
> > > > 	 *
> > > > 	 *
> > > > 	 *     WARN_ON(r0 == 0 && r1 == 0);
> > > > 	 *)
> > > > 
> > > > 	{
> > > > 	}
> > > > 
> > > > 	P0(int *x, int *y)
> > > > 	{
> > > > 		WRITE_ONCE(*x, 1);
> > > > 		r0 = xchg_release(y, 5);
> > > > 	}
> > > > 
> > > > 	P1(int *x, int *y)
> > > > 	{
> > > > 		r2 = xchg_relaxed(y, 1);
> > > > 		smp_rmb();
> > > > 		r1 = READ_ONCE(*x);
> > > > 	}
> > > > 
> > > > 	exists
> > > > 	(0:r0=0 /\ 1:r1=0)
> > > > 
> > 
> > How about a litmus test like this?
> > 
> >  	C C-AMO-global-transitivity.litmus
> >  
> >  	{
> >  	}
> >  
> >  	P0(int *x, int *y)
> >  	{
> >  		WRITE_ONCE(*x, 1);
> >  		r0 = xchg_release(y, 5);
> >  	}
> >  
> >  	P1(int *y, int *z)
> >  	{
> >  		atomic_inc(y);
> >  		smp_mb();
> 
> I am going to guess that the smp_mb() enforces the needed ordering,
> but Will will let me know.  ;-)

Yup, that would be forbidden on arm64, and would also be forbidden if
you used WRITE_ONCE instead of atomic_inc (remember that we recently
became multi-copy atomic).

Will