1. 临界段代码

//任务级的临界段代码保护

taskENTER_CRITICAL()
taskEXIT_CRITICAL()

//中断级的临界段代码保护

taskENTER_CRITICAL_FROM_ISR()
taskEXIT_CRITICAL_FROM_ISR()

2. 以STM32为例

（1）STM32有0~15，共16级中断，可嵌套。数字小，优先级高。
（2) FreeRTOS 系统可管理的最大优先级为5，表示可以开关5-15优先级的中断，即5-15优先级的中断可以调用 FreeRTOS 的 API 函数（带FromISR）。
（3) 优先级(0~4)的中断 FreeRTOS 是不能禁止的，中断服务函数也不能调用 FreeRTOS 的 API 函数！
（4) 中断级临界段代码保护
taskENTER_CRITICAL_FROM_ISR()和 taskEXIT_CRITICAL_FROM_ISR()中断级别临 界 段 代 码 保 护 ， 是 用 在 中 断 服 务 程 序 中 的 ， 而 且 这 个 中 断 的 优 先 级 一 定 要 低 于configMAX_SYSCALL_INTERRUPT_PRIORITY！举例，中断优先级为7的uart接收中断，可以使用taskENTER_CRITICAL_FROM_ISR()和 taskEXIT_CRITICAL_FROM_ISR()。

3. 相关代码

//FreeRTOSConfig.h

/* Cortex-M specific definitions. */
#ifdef __NVIC_PRIO_BITS
	/* __BVIC_PRIO_BITS will be specified when CMSIS is being used. */
	#define configPRIO_BITS       		__NVIC_PRIO_BITS
#else
	#define configPRIO_BITS       		4        /* 15 priority levels */
#endif

/* The lowest interrupt priority that can be used in a call to a "set priority"
function. */
#define configLIBRARY_LOWEST_INTERRUPT_PRIORITY			0xf

/* The highest interrupt priority that can be used by any interrupt service
routine that makes calls to interrupt safe FreeRTOS API functions.  DO NOT CALL
INTERRUPT SAFE FREERTOS API FUNCTIONS FROM ANY INTERRUPT THAT HAS A HIGHER
PRIORITY THAN THIS! (higher priorities are lower numeric values. */
#define configLIBRARY_MAX_SYSCALL_INTERRUPT_PRIORITY	5

/* Interrupt priorities used by the kernel port layer itself.  These are generic
to all Cortex-M ports, and do not rely on any particular library functions. */
#define configKERNEL_INTERRUPT_PRIORITY 		( configLIBRARY_LOWEST_INTERRUPT_PRIORITY << (8 - configPRIO_BITS) )
/* !!!! configMAX_SYSCALL_INTERRUPT_PRIORITY must not be set to zero !!!!
See http://www.FreeRTOS.org/RTOS-Cortex-M3-M4.html. */
#define configMAX_SYSCALL_INTERRUPT_PRIORITY 	( configLIBRARY_MAX_SYSCALL_INTERRUPT_PRIORITY << (8 - configPRIO_BITS) )

//port.c

#define portNVIC_PENDSV_PRI					( ( ( uint32_t ) configKERNEL_INTERRUPT_PRIORITY ) << 16UL )
#define portNVIC_SYSTICK_PRI				( ( ( uint32_t ) configKERNEL_INTERRUPT_PRIORITY ) << 24UL )

void vPortEnterCritical( void )
{
	portDISABLE_INTERRUPTS();
	uxCriticalNesting++;
	if( uxCriticalNesting == 1 )
	{
		configASSERT( ( portNVIC_INT_CTRL_REG & portVECTACTIVE_MASK ) == 0 );
	}
}
void vPortExitCritical( void )
{
	configASSERT( uxCriticalNesting );
	uxCriticalNesting--;
	if( uxCriticalNesting == 0 )
	{
		portENABLE_INTERRUPTS();
	}
}

//portmacro.h

static portFORCE_INLINE void vPortSetBASEPRI( uint32_t ulBASEPRI )
{
	__asm
	{
		/* Barrier instructions are not used as this function is only used to
		lower the BASEPRI value. */
		msr basepri, ulBASEPRI
	}
}
static portFORCE_INLINE void vPortRaiseBASEPRI( void )
{
uint32_t ulNewBASEPRI = configMAX_SYSCALL_INTERRUPT_PRIORITY;

	__asm
	{
		/* Set BASEPRI to the max syscall priority to effect a critical
		section. */
		msr basepri, ulNewBASEPRI
		dsb
		isb
	}
}

configMAX_SYSCALL_INTERRUPT_PRIORITY = 0x50

执行到HAL_Init()前面

程序运行后，systick优先级，240/16=15