On Mon, Sep 11, 2006 at 12:50:07PM -0400, Alan Stern wrote:
> 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().
Not so. A and B are different memory locations, hence #2 does not
apply to the "STORE A; STORE B" sequence.
> Either 2 is too strong, or else what you mean by "perceived" isn't
> sufficiently clear.
The key phrase is "to a given -single- memory location". ;-)
A and B are presumably -different- memory locations. However, if A and
B are aliases for the same memory location, then the wmb() would in fact
be unnecessary. But, again, I am assuming that they are different, so
that #2 does not apply.
Thanx, Paul
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