Signalling all threads in a process

Without keeping a list of current threads, I'm trying to see that a realtime signal gets delivered to all threads in my process. My idea is to go about it like this:

• Initially the signal handler is installed and the signal is unblocked in all threads.
• When one thread wants to send the 'broadcast' signal, it acquires a mutex and sets a global flag that the broadcast is taking place.
• The sender blocks the signal (using pthread_sigmask) for itself, and enters a loop repeatedly calling raise(sig) until sigpending indicates that the signal is pending (there were no threads remaining with the signal blocked).
• As threads receive the signal, they act on it but wait in the signal handler for the broadcast flag to be cleared, so that the signal will remain masked.
• The sender finishes the loop by unblocking the signal (in order to get its own delivery).
• When the sender handles its own signal, it clears the global flag so that all the other threads can continue with their business.

The problem I'm running into is that pthread_sigmask is not being respected. Everything works right if I run the test program under strace (presumably due to different scheduling timing), but as soon as I run it alone, the sender receives its own signal (despite having blocked it..?) and none of the other threads ever get scheduled.

Any ideas what might be wrong? I've tried using sigqueue instead of raise, probing the signal mask, adding sleep all over the place to make sure the threads are patiently waiting for their signals, etc. and now I'm at a loss.

Edit: Thanks to psmears' answer, I think I understand the problem. Here's a potential solution. Feedback would be great:

• At any given time, I can know the number of threads running, and I can prevent all thread creation and ex