为什么我只写了循环20次，但是线程一直执行？

th1 线程为什么会永远执行下去不停呢？
第35行我只之让它循环了20次
补上46-48行才让它运行一段时间后停下来

/*
 * Copyright (c) 2006-2021, RT-Thread Development Team
 *
 * SPDX-License-Identifier: Apache-2.0
 *
 * Change Logs:
 * Date           Author       Notes
 * 2021-08-24     RT-Thread    first version
 */
#include <rtthread.h>

#define DBG_TAG "main"
#define DBG_LVL DBG_LOG
#define MB1_SIZE_MAX 10
#include <rtdbg.h>

rt_thread_t th1 , th2;
rt_mq_t mymsq = NULL;
rt_mailbox_t mb1 = NULL;

typedef struct
{
    uint16_t buf;
    rt_mailbox_t mb_t;
}MYMSG;

void th1_entry(void *parameter)
{
    uint16_t mbrx = 0;
    MYMSG myMsg = {10, mb1};
    rt_thread_mdelay(5000);
    rt_kprintf("\nTx: %d\n", myMsg.buf);
    rt_mq_send(mymsq, (void*)&myMsg, sizeof(MYMSG));
    for (int i=0;i<20;i++){
        if (rt_mb_recv(mb1, (rt_ubase_t *)&mbrx, 1500) != RT_EOK){
            mbrx = 0;
            rt_kprintf("Tx: %d      ack: %d     resending...\n", myMsg.buf, mbrx);
        }
        if (mbrx > 0) {
            if (mbrx == 1) {
                myMsg.buf++;
            } else if (mbrx == 2) {
                myMsg.buf += 2;
            }
//            if (myMsg.buf > 35){
//                break;
//            }
            rt_kprintf("Tx: %d      ack: %d     sending next msg...\n", myMsg.buf, mbrx);
            rt_mq_send(mymsq, (void*)&myMsg, sizeof(MYMSG));
        }
    }
}

void th2_entry(void *parameter)
{
    MYMSG myMsg;
    for(;;){
        rt_thread_mdelay(50);
        rt_mq_recv(mymsq, (void*)&myMsg, sizeof(MYMSG), RT_WAITING_FOREVER);
        if ((myMsg.buf % 3) == 0){
            rt_mb_send(myMsg.mb_t, 1);
            rt_kprintf("\nRx: %d      ack: 1\n",myMsg.buf);
        } else {
            rt_mb_send(myMsg.mb_t, 2);
            rt_kprintf("\nRx: %d      ack: 2\n",myMsg.buf);
        }
    }
}

int main(void)
{
    mymsq = rt_mq_create("mq1", 10, sizeof(MYMSG), RT_IPC_FLAG_FIFO);
    mb1 = rt_mb_create("mb1", MB1_SIZE_MAX, RT_IPC_FLAG_FIFO);

    th1 = rt_thread_create("th1", th1_entry,RT_NULL,512, 21, 0);
    if(th1 == RT_NULL){
        LOG_E("th1 rt_thread_create failed...\n");
        return -ENOMEM;
    }

    th2 = rt_thread_create("th2", th2_entry,RT_NULL,512, 20, 81);
    if(th1 == RT_NULL){
       LOG_E("th2 rt_thread_create failed...\n");
       return -ENOMEM;
    }

    rt_thread_startup(th1);
    rt_thread_startup(th2);

    return 0;
}

问题补充。

1. th1线程去掉rt_mb_recv()就好了
2. rtt studio新建的rtt项目会出现这种情况，而使用正点潘多拉提供的demo模板就不会

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