Use __get_PRIMASK to determine is interrupts should be enabled in Serial_::readBreak

Author: sandeepmistry

cores/arduino/USB/CDC.cpp

@@ -298,6 +298,8 @@ Serial_::operator bool()
 }
 
 int32_t Serial_::readBreak() {
+	uint8_t enableInterrupts = (__get_PRIMASK() == 0);
+
 	// disable interrupts,
 	// to avoid clearing a breakValue that might occur 
 	// while processing the current break value
@@ -307,8 +309,10 @@ int32_t Serial_::readBreak() {
 
 	breakValue = -1;
 
-	// re-enable the interrupts
-	interrupts();
+	if (enableInterrupts) {
+		// re-enable the interrupts
+		interrupts();
+	}
 
 	return ret;
 }