Messages in this thread | | | Date | Mon, 11 Sep 2006 12:03:35 -0700 | From | "Paul E. McKenney" <> | Subject | Re: Uses for memory barriers |
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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.
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