在Cortex-M內核中，系統(tǒng)節(jié)拍由Systick時鐘提供，當配置好系統(tǒng)滴答時鐘后，每次時鐘中斷就會觸發(fā)中斷處理數(shù)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(yōu)先級 { /* Increment the RTOS tick. */ if( xTaskIncrementTick() != pdFALSE )//時鐘計數(shù)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(tǒng)時鐘中斷時被需要。如果是，則PendSV標記置位，等待觸發(fā)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,默認為否 { /* 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(shù)使用到了pxDelayedTaskList, 這定義在本文件

PRIVILEGED_DATA static List_t * volatile pxDelayedTaskList;

該變量初始化為0，該變量正常初始化位置在創(chuàng)建 Task 對象等的函數(shù)中， 也就是說，如果在Tick中斷到來時，如果還沒有任務被創(chuàng)建，就會導致不可預期的結果，中斷服務函數(shù)會使用這個野指針，執(zhí)行任務切換。這會導致觸發(fā)棧溢出鉤子函數(shù)，或者是直接Hardfault。有些硬件初始化需要借助delay功能，不得不在初始化之前配置SysTick。而又不希望在硬件初始化階段觸發(fā)這個Bug。所以在配置SysTick之前，先創(chuàng)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，一切準備就緒后，開啟中斷。執(zhí)行vTaskStartScheduler()默認創(chuàng)建一個空閑任務。