Protect processing break value by disabling and re-enabling interrupts

Author: sandeepmistry

hardware/arduino/sam/cores/arduino/USB/CDC.cpp

@@ -309,18 +309,23 @@ Serial_::operator bool()
 }
 
 int32_t Serial_::readBreak() {
-	// read the breakValue, using exclusive read
-	int32_t ret = __LDREXW((volatile uint32_t*)&breakValue);
-
-	if (ret == -1) {
-		// no new break value, clear exclusive access record
-		__CLREX();
-	} else {
-		// only set it to -1 (no break),
-		// if a new value has not been written in the ISR,
-		// using exclusive write
-		__STREXW(-1, (volatile uint32_t*)&breakValue);
-	}
+	uint8_t enableInterrupts = ((__get_PRIMASK() & 0x1) == 0 && 
+								(__get_FAULTMASK() & 0x1) == 0);
+
+	// disable interrupts,
+	// to avoid clearing a breakValue that might occur 
+	// while processing the current break value
+	__disable_irq();
+
+	int ret = breakValue;
+
+	breakValue = -1;
+
+	if (enableInterrupts)
+	{
+		// re-enable the interrupts
+		__enable_irq();
+    }
 
 	return ret;
 }