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From: Roland Schwarz (roland.schwarz_at_[hidden])
Date: 2006-12-01 05:33:26

After having tried to learn about atomic from reading through several
articles, processor specs and mailing lists, I am more confused than before.

I would be glad if we could (re)start a discussion about the topic.
Perhaps I am not the only one to benefit from this.

Following are some things I learned, but this might be wrong, and I
would appreciate clarification. Also some questions:

1) atomicity (in this specialized context) is about optimizing the
pattern: enter_critical_section; do_something; leave_critical_section;
by making use of processor/platform specific means. In particular in
presence of multiple processors. I.e. an atomic lib is primarily about

2) atomicity better would be addressed by the compiler, given a
suitable memory model, than as a library.

3) Despite 2) it would be possible to write a library, but it will
be hard to get processor independent semantics. E.g. there is one
concept of read/write/full memory barriers or another of acquire/release
semantics for SMP.

4) Does there exist a canonical set of atomic primitives, from which
others can be built? E.g. given atomic_test_and_set, atomic_load and
atomic_store is it possible to emulate atomic_compare_and_swap? (without
use of system mutices of course.)

5) Is it worth the effort to create a library with processor
independent semantics, at the price of not being optimal? E.g. by doing
away with the various kinds of barriers, instead simply requiring
atomicity and full memory barrier semantics for the operation? Which
operations, besides load and store would be essential?

Sorry if this is not the perfect list to discuss the topic, but I think
boost could possibly benefit from such a library, as previous
discussions let me believe.


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