Re: Uses for memory barriers

[Alan Stern <stern@rowland.harvard.edu>]

On Mon, 11 Sep 2006, Paul E. McKenney wrote:

> This is a summary of the Linux memory-barrier semantics as I understand
> them:
> 
> 1.	A given CPU will always perceive its own memory operations
> 	as occuring in program order.
> 
> 2.	All stores to a given single memory location will be perceived
> 	as having occurred in the same order by all CPUs.  This is
> 	"coherence".  (And this is the property that I was forgetting
> 	about when I first looked at your second example.)
...

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.

Alan Stern

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