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

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Message-ID: <20131112212904.GR4138@linux.vnet.ibm.com>
Date:	Tue, 12 Nov 2013 13:29:04 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Does Itanium permit speculative stores?

On Tue, Nov 12, 2013 at 06:46:20PM +0000, Luck, Tony wrote:
> > So the point we're having a discussion on is if any architecture has
> > visible speculative STORES and if there's an architecture that doesn't
> > have control dependencies.
> >
> > On the visible speculative STORES; can, if in the above example we have
> > regular loads/stores:
> >
> >  LOAD r1, x			LOAD r2, y
> >  IF (r1)			IF (r2)
> >	STORE y, 1			STORE x, 1
> >
> > we observe: r1==1 && r2==1
> >
> > In order for that to be true; we must be able to observe the stores
> > before the loads are complete -- and therefore before the branches are a
> > certainty.
> 
> Even without the ".acq" and ".rel" this code is still safe.
> 
> Quoting ia64 SDM vol 1, Section 4.4.7 Memory Access Ordering"
> "In addition, memory writes and flushes must observe control dependencies"
> 
> which I take to mean that the STORE can't be visible until we are certain of
> the outcome of the conditional.

Even better!

							Thanx, Paul

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