question about concurrency implementation

[Dean Herington]

I'm curious about the implementation of Concurrent Haskell in GHC and
Hugs.  Does access to values possibly shared among threads cost the same
in Concurrent Haskell as in regular Haskell?  I'm guessing the answer is
"yes", because Concurrent Haskell is provided by default in GHC.  If the
costs are the same, does that rely on there being no true concurrency in
the current implementations?  How would the cost change if true
concurrency were provided?  Wouldn't thunk evaluation involve mutual
exclusion?

Dean Herington