NPTL implements thread priority scheduling for new threads by creating the new thread but blocking its forward progress until pthread_create in the parent can set the new thread's scheduling options. If this fails (e.g. due to EPERM), pthread_create is required to fail. To achieve this result, NPTL has pthread_create send a cancellation request to the new thread.
Unfortunately, NPTL fails to block signals during thread creation, so it's possible that a signal handler is already running (at the wrong priority) in the newly created thread by the time the failure to set the priority had been detected. The cancellation request will then cause any cancellation-point function called from the signal handler to act upon cancellation, possibly with disastrous results if the application was not written to use cancellation.
I believe the simplest solution to this issue is to block all signals before clone() is called. In the parent, signals can be unblocked immediately as soon as clone returns. In the new thread, signals cannot be unblocked until it's determined that the thread will be allowed to run.