The signal handler runs at interrupt priority level 0, which is the highest priority. Therefore, it cannot be interrupted by any other activity.
Since the signal handler runs at a higher interrupt priority (lower numerical value for the priority level) than the SVC calls (see Interrupt priority levels), SVC calls are not available in the signal handler.
The signal handler may be called several times during a timeslot. It is recommended to use the signal handler only for real time signal handling. When the application has handled the signal, it can exit the signal handler and wait for the next signal, if it wants to do other (less time critical) processing at lower interrupt priority (higher numerical value for the priority level) while waiting.