Date: Tue, 19 Dec 2017 11:05:44 -0500 (EST)
From: Alan Stern <stern@rowland.harvard.edu>
To: afzal mohammed <afzal.mohd.ma@gmail.com>
cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>, <parri.andrea@gmail.com>,
        <will.deacon@arm.com>, <boqun.feng@gmail.com>, <npiggin@gmail.com>,
        <dhowells@redhat.com>, <j.alglave@ucl.ac.uk>, <luc.maranget@inria.fr>,
        <linux-kernel@vger.kernel.org>, <elena.reshetova@intel.com>
Subject: Re: Prototype patch for Linux-kernel memory model
In-Reply-To: <20171219083650.GA10215@afzalpc>
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On Tue, 19 Dec 2017, afzal mohammed wrote:

> Hi,
> 
> A trivial & late (sorry) comment,
> 
> On Wed, Nov 15, 2017 at 08:37:49AM -0800, Paul E. McKenney wrote:
> 
> > +THE HAPPENS-BEFORE RELATION: hb
> > +-------------------------------
> 
> > +Less trivial examples of prop all involve fences.  Unlike the simple
> > +examples above, they can require that some instructions are executed
> > +out of program order.  This next one should look familiar:
> > +
> > +	int buf = 0, flag = 0;
> > +
> > +	P0()
> > +	{
> > +		WRITE_ONCE(buf, 1);
> > +		smp_wmb();
> > +		WRITE_ONCE(flag, 1);
> > +	}
> > +
> > +	P1()
> > +	{
> > +		int r1;
> > +		int r2;
> > +
> > +		r1 = READ_ONCE(flag);
> > +		r2 = READ_ONCE(buf);
> > +	}
> > +
> > +This is the MP pattern again, with an smp_wmb() fence between the two
> > +stores.  If r1 = 1 and r2 = 0 at the end then there is a prop link
> > +from P1's second load to its first (backwards!).  The reason is
> > +similar to the previous examples: The value P1 loads from buf gets
> > +overwritten by P1's store to buf,
> 
>                   P0's store to buf
> 
> afzal

Thank you; I have applied the change.

Alan Stern