Add barrier instructions to GCC CM3/4 code.

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

@@ -56,19 +56,19 @@
     ***************************************************************************
 
 
-    http://www.FreeRTOS.org - Documentation, books, training, latest versions, 
+    http://www.FreeRTOS.org - Documentation, books, training, latest versions,
     license and Real Time Engineers Ltd. contact details.
 
     http://www.FreeRTOS.org/plus - A selection of FreeRTOS ecosystem products,
     including FreeRTOS+Trace - an indispensable productivity tool, and our new
     fully thread aware and reentrant UDP/IP stack.
 
-    http://www.OpenRTOS.com - Real Time Engineers ltd license FreeRTOS to High 
-    Integrity Systems, who sell the code with commercial support, 
+    http://www.OpenRTOS.com - Real Time Engineers ltd license FreeRTOS to High
+    Integrity Systems, who sell the code with commercial support,
     indemnification and middleware, under the OpenRTOS brand.
-    
-    http://www.SafeRTOS.com - High Integrity Systems also provide a safety 
-    engineered and independently SIL3 certified version for use in safety and 
+
+    http://www.SafeRTOS.com - High Integrity Systems also provide a safety
+    engineered and independently SIL3 certified version for use in safety and
     mission critical applications that require provable dependability.
 */
 
@@ -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 );