can't release mutex

[Wakan]

Hi,
I'm using commoncpp library in a project that includes a multithread 
server that
can accept many tcp connection and each connection generates a thread.
In this server I'm using a thirdy party proprietary library (compiled), 
that (as the producer says)
can't work in multithread environment.
As each connection thread needs to call functions of this thirdy party 
library, I've used a mutex to protect the piece of
code where this external library function is called.
In this way I can use this library fine, because all calls are 
serialized and the protected function is called by one thread each time.
This is an example:
...
   while(1) {
       if(isPending(pendingInput)) {
               mutex.enterMutex();
               ---> protected function call (a function that can't work 
in multithread env) <---       
               mutex.leaveMutex();
      }
   }
...
in 99% of cases it works fine. But it accidentally happens that a thread 
can't exit from the mutex
maybe because the thirdy party function freezes itself.
When this happens, the server can accept other connections, soit still 
works, but each thread can't enter in the mutex proteced piece of code.
I'm thinking about something, like a timeout, that exits the mutex after 
a time, if the thread is not exiting...
Can someone help me to resolve this problem? Are there other solutions?
Thanks in advance,
Regards
Carlo