RE: spin_unlock optimization(i386)

David Schwartz (
Tue, 30 Nov 1999 03:44:25 -0800

> Nope.
> Because when CPU0 reads 1 from valid, the value read is still
> not retired (i.e. committed) and at this point it sees that value is
> set to 45 (because it couldn't have seen valid=1 before seeing value=45
> since writes are not reordered). So it will re-exec the read of value
> (because the not-yet-committed-read is still around and its address
> is matched to the incoming writes, allowing to detect such problems).
> Now I don't know if this is part of x86 or only P6 since
> supposedly the P6 provides a slightly stronger consistency model.
> Also this solves the present case but not the one I sent earlier
> because no CPU needs to see any of the other's writes, so the
> instructions can all be retired before any communication takes place.

I couldn't find this guaranteed in the architecture manual, so I would
assume that it's specific to certain models. It's worth taking advantage of
these optimizations on chips where they're known to be safe.


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