endless loop in stock interrupt handler

It seems like this problem is related to the FreeRTOS operating system I am using.

/* Standard includes. */
# include <stddef.h>
# include <stdio.h>
# include <string.h>
/* Scheduler includes. */
# include "FreeRTOS.h"
# include "task.h"
# include "queue.h"
/* NiosII includes. */
# include "system.h"
# include "altera_avalon_pio_regs.h"
int main()
{
	unsigned int i = 200;
	unsigned int j = 0;
    unsigned char led=0x01;
    portBASE_TYPE res;
    printf("hello world!\n");
    led =0xff;
   	IOWR(LED_PIO_BASE,0,led);
  	i=50000; while(i--);
    res = xTaskCreate(
                call_back1,
                ( signed char * ) "Task1",
                configMINIMAL_STACK_SIZE,
                NULL,
                1,
                NULL
    );
    if ( res != pdPASS )
         printf("erro,fail to create task:%d\n", res);
    else
          printf("success to create task\n");
    xTaskCreate(
                call_back2,
                ( signed char * ) "Task2",
                configMINIMAL_STACK_SIZE,
                NULL,
                2,
                NULL
    );
	led =0xaa;
	IOWR(LED_PIO_BASE,0,led);
    printf("start schedule\n");
    vTaskStartScheduler();
    while(1);
}
void call_back1(void* param)
{
    for(;;)
    {
    IOWR(LED_PIO_BASE,0,0x0f);
    printf("run task1\n");
    vTaskDelay(2000);
    }
}
void call_back2(void* param)
{
	for(;;)
    {
    IOWR(LED_PIO_BASE,0,0xf0);
    printf("run task2\n");
    vTaskDelay(1000);
    }
}