C语言Freertos的递归锁详解

假设有 2 个互斥量 M1、 M2， 2 个任务 A、 B：
A 获得了互斥量 M1
B 获得了互斥量 M2
A 还要获得互斥量 M2 才能运行，结果 A 阻塞
B 还要获得互斥量 M1 才能运行，结果 B 阻塞
A、 B 都阻塞，再无法释放它们持有的互斥量
死锁发生！

任务 A 获得了互斥锁 M
它调用一个函数
函数要去获取同一个互斥锁 M，于是它阻塞：任务 A 休眠，等待任务 A
来释放互斥锁！
死锁发生！

SemaphoreHandle_t xMutex;
int main( void )
{
	prvSetupHardware();
    
    xMutex = xSemaphoreCreateRecursiveMutex( );
	if( xMutex != NULL )
	{
		
		vTaskStartScheduler();
	}
	else
	{
		
	}
	return 0;
}

static void vTakeTask( void *pvParameters )
{
	const TickType_t xTicksToWait = pdMS_TO_TICKS( 100UL );	
	BaseType_t xStatus;
	int i;
	
	
	for( ;; )
	{	
		
		xStatus = xSemaphoreTakeRecursive(xMutex, portMAX_DELAY);
		printf("Task1 take the Mutex in main loop %s\r\n", \
			(xStatus == pdTRUE)? "Success" : "Failed");
		
		vTaskDelay(xTicksToWait);
		for (i = 0; i < 10; i++)
		{
			
			xStatus = xSemaphoreTakeRecursive(xMutex, portMAX_DELAY);
			printf("Task1 take the Mutex in sub loop %s, for time %d\r\n", \
				(xStatus == pdTRUE)? "Success" : "Failed", i);
			
			xSemaphoreGiveRecursive(xMutex);
		}
		
		xSemaphoreGiveRecursive(xMutex);
	}
}

static void vGiveAndTakeTask( void *pvParameters )
{
	const TickType_t xTicksToWait = pdMS_TO_TICKS( 10UL );	
	BaseType_t xStatus;
	
	xStatus = xSemaphoreTakeRecursive(xMutex, 0);
	printf("Task2: at first, take the Mutex %s\r\n", \
		(xStatus == pdTRUE)? "Success" : "Failed");
	
	if (xStatus != pdTRUE)
	{
		
		xStatus = xSemaphoreGiveRecursive(xMutex);
		printf("Task2: give Mutex %s\r\n", \
			(xStatus == pdTRUE)? "Success" : "Failed");
	}
	
	xStatus = xSemaphoreTakeRecursive(xMutex, portMAX_DELAY);
	printf("Task2: and then, take the Mutex %s\r\n", \
		(xStatus == pdTRUE)? "Success" : "Failed");
	
	for( ;; )
	{	
		
		vTaskDelay(xTicksToWait);
	}
}