linux-kernel - Re: Uses for memory barriers

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Message-ID: <20060919181919.GG1310@us.ibm.com>
Date:	Tue, 19 Sep 2006 11:19:20 -0700
From:	"Paul E. McKenney" <paulmck@...ibm.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	David Howells <dhowells@...hat.com>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Uses for memory barriers

On Wed, Sep 20, 2006 at 03:51:29AM +1000, Nick Piggin wrote:
> Alan Stern wrote:
> >On Wed, 20 Sep 2006, Nick Piggin wrote:
> 
> >>I don't think that need be the case if one of the CPUs that has written
> >>the variable forwards the store to a subsequent load before it reaches
> >>the cache coherency (I could be wrong here). So if that is the case, then
> >>your above example would be correct.
> >
> >I don't understand your comment.  Are you saying it's possible for two 
> >CPUs to observe the same two writes and see them occurring in opposite 
> >orders?
> 
> If store forwarding is able to occur outside cache coherency protocol,
> then I don't see why not. I would also be interested to know if this
> is the case on real systems.

We are discussing multiple writes to the same variable, correct?

Just checking...

							Thanx, Paul
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