linux-kernel - Re: [RFC][PATCH v3]: documentation,atomic: Add new documents

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Date:   Thu, 3 Aug 2017 22:05:16 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Will Deacon <will.deacon@....com>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [RFC][PATCH v3]: documentation,atomic: Add new documents

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();
 		r1 = READ_ONCE(*z);
 	}

	P2(int *x, int *z)
	{
		WRITE_ONCE(*z, 1);
		smp_mb();
		r2 = READ_ONCE(*x);
	}
 
 	exists
 	(0:r0=0 /\ 1:r1=0 /\ 2:r2=0 )

Should we forbid the outcome in the exists-clause? I ask because I want
to know whether we can just treat atomic_inc() as a store, because if I
replace atomic_inc() with a WRITE(*y, 6), IIUC, the current model says
this could happen.

Thoughts?

Regards,
Boqun

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