FreeRTOS在Cortex-M系列內核中遇到的關於系統滴答中斷的問題

眾所周知，在Cortex-M內核中，系統節拍由Systick時鐘提供，當配置好系統滴答時鐘後，每次時鐘中斷就會觸發中斷處理函數 xPortSysTickHandler(),

void xPortSysTickHandler( void )
{
    /* The SysTick runs at the lowest interrupt priority, so when this interrupt
     * executes all interrupts must be unmasked.  There is therefore no need to
     * save and then restore the interrupt mask value as its value is already
     * known - therefore the slightly faster vPortRaiseBASEPRI() function is used
     * in place of portSET_INTERRUPT_MASK_FROM_ISR(). */
    vPortRaiseBASEPRI();//屏蔽歸屬FreeRTOS的中斷優先順序
    {
        /* Increment the RTOS tick. */
        if( xTaskIncrementTick() != pdFALSE )//時鐘計數處理
        {
            /* A context switch is required.  Context switching is performed in
             * the PendSV interrupt.  Pend the PendSV interrupt. */
            portNVIC_INT_CTRL_REG = portNVIC_PENDSVSET_BIT;//如果需要切換上下文操作，PendSV標記置位
        }
    }

    vPortClearBASEPRIFromISR();
}

　　這部分主要是依靠 xTaskIncrementTick()，來判斷任務切換是否在此次系統時鐘中斷時被需要。如果是，則PendSV標記置位，等待觸發PendSV中斷。

　　來看看 xTaskIncrementTick()。

BaseType_t xTaskIncrementTick( void )
{
    TCB_t * pxTCB;
    TickType_t xItemValue;
    BaseType_t xSwitchRequired = pdFALSE;

    /* Called by the portable layer each time a tick interrupt occurs.
     * Increments the tick then checks to see if the new tick value will cause any
     * tasks to be unblocked. */
    traceTASK_INCREMENT_TICK( xTickCount );

    if( uxSchedulerSuspended == ( UBaseType_t ) pdFALSE ) //調度是否被掛起,預設為否
    {
        /* Minor optimisation.  The tick count cannot change in this
         * block. */
        const TickType_t xConstTickCount = xTickCount + ( TickType_t ) 1;

        /* Increment the RTOS tick, switching the delayed and overflowed
         * delayed lists if it wraps to 0. */
        xTickCount = xConstTickCount;

        if( xConstTickCount == ( TickType_t ) 0U ) /*lint !e774 'if' does not always evaluate to false as it is looking for an overflow. 如果xConstTickCount為0，說明溢出了*/
        {
            taskSWITCH_DELAYED_LISTS();/*切換延遲列表*/
        }
        else
        {
            mtCOVERAGE_TEST_MARKER();
        }

        /* See if this tick has made a timeout expire.  Tasks are stored in
         * the  queue in the order of their wake time - meaning once one task
         * has been found whose block time has not expired there is no need to
         * look any further down the list. */
        if( xConstTickCount >= xNextTaskUnblockTime )
        {
            for( ; ; )
            {
                if( listLIST_IS_EMPTY( pxDelayedTaskList ) != pdFALSE )
                {
                    /* The delayed list is empty.  Set xNextTaskUnblockTime
                     * to the maximum possible value so it is extremely
                     * unlikely that the
                     * if( xTickCount >= xNextTaskUnblockTime ) test will pass
                     * next time through. */
                    xNextTaskUnblockTime = portMAX_DELAY; /*lint !e961 MISRA exception as the casts are only redundant for some ports. */
                    break;
                }
                else
                {
                    /* The delayed list is not empty, get the value of the
                     * item at the head of the delayed list.  This is the time
                     * at which the task at the head of the delayed list must
                     * be removed from the Blocked state. */
                    pxTCB = listGET_OWNER_OF_HEAD_ENTRY( pxDelayedTaskList ); /*lint !e9079 void * is used as this macro is used with timers and co-routines too.  Alignment is known to be fine as the type of the pointer stored and retrieved is the same. */
                    xItemValue = listGET_LIST_ITEM_VALUE( &( pxTCB->xStateListItem ) );
......
   

　　關鍵問題是，這個函數使用到了 pxDelayedTaskList, 這定義在本文件

PRIVILEGED_DATA static List_t * volatile pxDelayedTaskList; 

　　該變數初始化為0，該變數正常初始化位置在創建 Task 對象等的函數中， 也就是說，如果在Tick中斷到來時，如果還沒有任務被創建，就會導致不可預期的結果，中斷服務函數會使用這個野指針，執行任務切換。

　　這會導致觸發棧溢出鉤子函數，或者是直接 Hardfault。

　　有些硬體初始化需要藉助delay功能，不得不在初始化之前配置SysTick。而又不希望在硬體初始化階段觸發這個Bug。

　　所以在配置SysTick之前，先創建一個初始化任務，初始化 pxDelayedTaskList 這個指針，在初始化任務里配置SysTick，和其他初始化，這樣能夠避免此類問題。

　　或者是在配置SysTick的時候屏蔽中斷，一切準備就緒後，開啟中斷。

　　執行 vTaskStartScheduler()，預設創建一個空閑任務。