Multiple C threads -> one unthreaded GHC RTS

[Albert Y. C. Lai]

Suppose I have a C program with multiple C threads calling Haskell 
functions, maybe the same Haskell function too, and the GHC RTS used is 
the unthreaded one. What will go wrong?

I ask because Cabal/cabal-install has recently added building of 
export-to-C shared libraries, i.e., the "foreign-library" section. It 
sees to all the technical details, very nice, I'm very happy about this 
addition, but it chooses the unthreaded GHC RTS.

Thanks.