linux-kernel - Re: Uses for memory barriers

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Message-ID: <20060911190431.GC1295@us.ibm.com>
Date:	Mon, 11 Sep 2006 12:04:31 -0700
From:	"Paul E. McKenney" <paulmck@...ibm.com>
To:	Segher Boessenkool <segher@...nel.crashing.org>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Oliver Neukum <oliver@...kum.org>,
	David Howells <dhowells@...hat.com>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Uses for memory barriers

On Mon, Sep 11, 2006 at 07:23:49PM +0200, Segher Boessenkool wrote:
> >This can't be right.  Together 1 and 2 would obviate the need for  
> >wmb().
> >The CPU doing "STORE A; STORE B" will always see the operations  
> >occuring
> >in program order by 1, and hence every other CPU would always see them
> >occurring in the same order by 2 -- even without wmb().
> >
> >Either 2 is too strong, or else what you mean by "perceived" isn't
> >sufficiently clear.
> 
> 2. is only for multiple stores to a _single_ memory location -- you
> use wmb() to order stores to _separate_ memory locations.

Precisely!!!

						Thanx, Paul
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