Comment 1 for bug 451111

"A process shall be created with a single thread. If a multi-threaded process calls fork(), the new process shall contain a replica of the calling thread and its entire address space, possibly including the states of mutexes and other resources. Consequently, to avoid errors, the child process may only execute async-signal-safe operations until such time as one of the exec functions is called."

The "consequently" part basically means that you are extremely limited in what you can do after forking a threaded program. I'm tempted to say that having the wrong value for *all-threads* is the least of your problems.