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

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Message-Id: <20170803145514.GS3730@linux.vnet.ibm.com>
Date:   Thu, 3 Aug 2017 07:55:14 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Will Deacon <will.deacon@....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

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.  ;-)

							Thanx, Paul

>  		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