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From: Peter Dimov (pdimov_at_[hidden])
Date: 2007-10-19 06:52:58

Markus Schöpflin:
> Ion Gaztañaga wrote:
> > Markus Schöpflin escribió:

> >> 1) Currently it is not specified whether an atomic operation implies a
> >> memory barrier or not. This should be explicitly stated.
> >
> > They should imply a barrier. Since this was an internal header, I
> > haven't documented anything. Now that people is contributing, at least,
> > I should add a comment to the header.
> OK, I need to add memory barriers to the code then.

You need to know what kind of memory synchronization is implied. Acquire for
atomic load, release for atomic store and acquire+release is a reasonably
safe bet in situations such as this one where the author isn't quite sure.

Last I looked at the various implementations, most of the routines did not
provide these guarantees, though.

Your use of volatile is also a bit suspect. Volatile operations may be
atomic without implying an acquire (release) constraint, both for the
hardware and for the compiler.

On Alpha, you'll probably need to add a memory barrier after the loads, a
memory barrier before the stores, and one before and one after the
read/modify/write operations. This would require that the compiler is smart
enough to recognize the barrier and not move code across.

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