FreeRTOS中Systick的问题

在Cortex-M内核中，系统节拍由Systick时钟提供，当配置好系统滴答时钟后，每次时钟中断就会触发中断处理数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();//屏蔽归属FreeRTOS的中断优先级 { /* Increment the RTOS tick. */ if( xTaskIncrementTick() != pdFALSE )//时钟计数处理 { /* A context switch is required. Context switching is performed in * the PendSV interrupt. Pend the PendSV interrupt. */ portNVIC_INT_CTRL_REG = portNVIC_PENDSVSET_BIT;//如果需要切换上下文操作，PendSV标记置位 } }

vPortClearBASEPRIFromISR();}

这部分主要是依靠 xTaskIncrementTick()，来判断任务切换是否在此次系统时钟中断时被需要。如果是，则PendSV标记置位，等待触发PendSV中断。来看看 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 ) //调度是否被挂起,默认为否 { /* 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为0，说明溢出了*/ { taskSWITCH_DELAYED_LISTS();/*切换延迟列表*/ } 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 ) );

关键问题是，这个函数使用到了pxDelayedTaskList, 这定义在本文件

PRIVILEGED_DATA static List_t * volatile pxDelayedTaskList;

该变量初始化为0，该变量正常初始化位置在创建 Task 对象等的函数中， 也就是说，如果在Tick中断到来时，如果还没有任务被创建，就会导致不可预期的结果，中断服务函数会使用这个野指针，执行任务切换。这会导致触发栈溢出钩子函数，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阶段触发这个Bug。所以在配置SysTick之前，先创建一个初始化任务，初始化 pxDelayedTaskList 这个指针，在初始化任务里配置SysTick，和其他初始化，这样能够避免此类问题。或者是在配置SysTick的时候屏蔽中断，一切准备就绪后，开启中断。执行vTaskStartScheduler()默认创建一个空闲任务。