linux-kernel - Re: Uses for memory barriers

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Message-ID: <Pine.LNX.4.44L0.0609081723350.7953-100000@iolanthe.rowland.org>
Date:	Fri, 8 Sep 2006 17:26:01 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Oliver Neukum <oliver@...kum.org>
cc:	paulmck@...ibm.com, David Howells <dhowells@...hat.com>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Uses for memory barriers

On Fri, 8 Sep 2006, Oliver Neukum wrote:

> It seems you are correct.
> Therefore the correct code on CPU 1 would be:
> 
> y = -1;
> b = 1;
> //mb();
> //x = a;
> while (y < 0) relax();
> 
> mb();
> x = a;
> 
> assert(x==1 || y==1);   //???
> 
> And yes, it is confusing. I've been forced to change my mind twice.

Again you have misunderstood.  The original code was _not_ incorrect.  I 
was asking: Given the code as stated, would the assertion ever fail?

The code _was_ correct for my purposes, namely, to illustrate a technical 
point about the behavior of memory barriers.

Alan

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