COM calls reentering in STA

I wrote an exe COM server and its client, which is an Office COM addin. All
unmanaged using ATL. Client and server use STA. The client initially passes
in a callback pointer the server will use to call its methods. The server is
multithreaded so it uses the GIT table to marshall calls to the client. The
problem I have it that calls from multiple server threads, made to the same
client object and method, reenter in the client instead of waiting until the
first call has returned. For example, if during the first call the client
does some trivial COM call like QueryInterface, a second call from server
comes in, reentering the method which is stilll processing the first call.
(All works well if I slow things down so the server does only one operation
at a time.)

Trace statements in the server confirm that it is making the second call
(from a second thread) before the first has returned, and that the two calls
are using different marshalled pointers, indicating that each thread has a
different proxy pointer from the GIT.

So I wonder if this is expected behavior. The idea that using the
marshalled pointer switches threads so the calls all originate from one
thread may be true behind the scenes, but does not seem to be enough to
provide serialization of the calls. Why does the main thread not wait for
the COM call to return before resuming execution? If COM does not serialize
the calls, what do I have to do to serialize them?

--
Scott McPhillips [VC++ MVP]