Improvements to the Cortex-M ports:

- Clear the SysTick current value register before starting the SysTick (only required if something uses SysTick before starting the scheduler).
- Ensure atomic operations are thread safe by executing clrex in the context switch.

FreeRTOS/Source/portable/RVDS/ARM_CM4F/port.c

@@ -479,6 +479,9 @@ __asm void xPortPendSVHandler( void )
 	/* Save the new top of stack into the first member of the TCB. */
 	str r0, [r2]
 
+	/* Ensure thread safety of atomic operations. */
+	clrex
+
 	stmdb sp!, {r3}
 	mov r0, #configMAX_SYSCALL_INTERRUPT_PRIORITY
 	msr basepri, r0
@@ -696,7 +699,7 @@ void xPortSysTickHandler( void )
  * Setup the SysTick timer to generate the tick interrupts at the required
  * frequency.
  */
-#if configOVERRIDE_DEFAULT_TICK_CONFIGURATION == 0
+#if( configOVERRIDE_DEFAULT_TICK_CONFIGURATION == 0 )
 
 	void vPortSetupTimerInterrupt( void )
 	{
@@ -709,6 +712,10 @@ void xPortSysTickHandler( void )
 		}
 		#endif /* configUSE_TICKLESS_IDLE */
 
+		/* Stop and clear the SysTick. */
+		portNVIC_SYSTICK_CTRL_REG = 0UL;
+		portNVIC_SYSTICK_CURRENT_VALUE_REG = 0UL;
+
 		/* Configure SysTick to interrupt at the requested rate. */
 		portNVIC_SYSTICK_LOAD_REG = ( configSYSTICK_CLOCK_HZ / configTICK_RATE_HZ ) - 1UL;
 		portNVIC_SYSTICK_CTRL_REG = ( portNVIC_SYSTICK_CLK_BIT | portNVIC_SYSTICK_INT_BIT | portNVIC_SYSTICK_ENABLE_BIT );