Deadlock in ntdll.dll

Hello,

I want to know what ntdll.dll does in Windows 2000?? Because my application
which contains many threads and needs to use modems frequently, I find it
sometimes will deadlock in the ntdll.dll as shown in the debugger. So I want
to know what parts in my application cause such problem so that I can
specifically look at the parts, I suspect it is caused by threads...

Or is there any tools to trace such problem?

Thanks.

Danny