Attached are two source files (one .c, one .hs), which are compiled (using mk.sh) into DsoHsDemo.dll. When this library is loaded, using the following C snippet:

int main(int argc, char* argv[])
{
    printf("Started LoadLibrary\n");
    LoadLibrary("DsoHsDemo.dll");
    printf("Finished LoadLibrary\n");
}

The program freezes:

$ ./mk.sh && ./CSnippet
The Glorious Glasgow Haskell Compilation System, version 6.12.0.20091010
Creating library file: DsoHsDemo.dll.a
Started LoadLibrary
pre hs_init
<program freezes>

The freeze only occurs when the dll is linked with -threaded, and happens within either startupHaskell, or (if you call hs_init/hs_add_root instead) within hs_add_root. This happens with GHC 6.10.4 and 6.12rc1 on Windows XP.

This bug appears to make it impossible to successfully build a DLL for multithreaded use.