linux-kernel - Re: Does Itanium permit speculative stores?

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Message-ID: <20131127171909.GO4137@linux.vnet.ibm.com>
Date:	Wed, 27 Nov 2013 09:19:10 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jon Masters <jonathan@...masters.org>
Cc:	tony.luck@...el.com, peterz@...radead.org,
	linux-kernel@...r.kernel.org
Subject: Re: Does Itanium permit speculative stores?

On Tue, Nov 26, 2013 at 11:55:58PM -0500, Jon Masters wrote:
> On 11/11/2013 12:13 PM, Paul E. McKenney wrote:
> > Hello, Tony,
> > 
> > Does Itanium permit speculative stores?  For example, on Itanium what are
> > the permitted outcomes of the following litmus test, where both x and y
> > are initially zero?
> > 
> > 	CPU 0				CPU 1
> > 
> > 	r1 = ACCESS_ONCE(x);		r2 = ACCESS_ONCE(y);
> > 	if (r1)				if (r2)
> > 		ACCESS_ONCE(y) = 1;		ACCESS_ONCE(x) = 1;
> > 
> > In particular, is the outcome (r1 == 1 && r2 == 1) possible on Itanium
> > given this litmus test?
> > 
> > 							Thanx, Paul
> > 
> > --
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> 
> Btw, I was reading through some UEFI docs and noticed a reference to "A
> Formal Specification of Intel Itanium Processor Family Memory Ordering",
> then remembered this thread. In case it's of use:
> 
> http://www.intel.com/design/itanium/downloads/251429.htm

I have seen this, but there have been too many times when I have fooled
myself about what the words mean (with DEC Alpha back in the late 90s
being the most impressive example).  So while I do learn what I can from
them, they are unfortunately not a substitute for asking.  ;-)

Besides, some of the Itanium locking code uses instructions that the
above manual is silent about.

							Thanx, Paul

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