linux-kernel - Re: Uses for memory barriers

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Message-Id: <DA530D09-FD88-4BAF-996B-00E900F6CA51@kernel.crashing.org>
Date:	Mon, 11 Sep 2006 19:23:49 +0200
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	"Paul E. McKenney" <paulmck@...ibm.com>,
	Oliver Neukum <oliver@...kum.org>,
	David Howells <dhowells@...hat.com>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Uses for memory barriers

> 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.


Segher

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