Add barrier instructions to GCC CM3/4 code.

FreeRTOS/Source/portable/GCC/ARM_CM3/port.c

@@ -259,6 +259,11 @@ void vPortYieldFromISR( void )
 {
 	/* Set a PendSV to request a context switch. */
 	portNVIC_INT_CTRL_REG = portNVIC_PENDSVSET_BIT;
+
+	/* Barriers are normally not required but do ensure the code is completely
+	within the specified behaviour for the architecture. */
+	__asm volatile( "dsb" );
+	__asm volatile( "isb" );
 }
 /*-----------------------------------------------------------*/
 
@@ -266,6 +271,8 @@ void vPortEnterCritical( void )
 {
 	portDISABLE_INTERRUPTS();
 	uxCriticalNesting++;
+	__asm volatile( "dsb" );
+	__asm volatile( "isb" );
 }
 /*-----------------------------------------------------------*/
 
@@ -438,6 +445,8 @@ void xPortSysTickHandler( void )
 			if( xModifiableIdleTime > 0 )
 			{
 				__asm volatile( "wfi" );
+				__asm volatile( "dsb" );
+				__asm volatile( "isb" );
 			}
 			configPOST_SLEEP_PROCESSING( xExpectedIdleTime );