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临界区
在多用户系统上,多进程系统上,通常程序中存在部分临界代码(又称临界区、Critical Section )。我们需要确保只有一个进程或执行线程进入临界代码并拥有对资源独占式的访问权。
不论是硬件临界资源,还是软件临界资源,多个进程必须互斥地对它进行访问。每个进程中访问临界资源的那段代码称为临界区(Critical Section)。
每个进程中访问临界资源的那段程序称为临界区(Critical Section)(临界资源是一次仅允许一个进程使用的共享资源)。每次只准许一个进程进入临界区,进入后不允许其他进程进入。不论是硬件临界资源,还是软件临界资源,多个进程必须互斥地对它进行访问。
多个进程中涉及到同一个临界资源的临界区称为相关临界区。
进程进入临界区的调度原则是:
①如果有若干进程要求进入空闲的临界区,一次仅允许一个进程进入。
②任何时候,处于临界区内的进程不可多于一个。如已有进程进入自己的临界区,则其它所有试图进入临界区的进程必须等待。
③进入临界区的进程要在有限时间内退出,以便其它进程能及时进入自己的临界区。
④如果进程不能进入自己的临界区,则应让出CPU,避免进程出现“忙等”现象。
如果有多个线程试图同时访问临界区,那么在有一个线程进入后其他所有试图访问此临界区的线程将被挂起,并一直持续到进入临界区的线程离开。临界区在被释放后,其他线程可以继续抢占,并以此达到用原子方式操作共享资源的目的。
RT-Thread中的临界代码保护
在多任务实时系统中,一项工作的完成往往可以通过多个任务协调的方式共同来完成,例如一个任务从传感器中接收数据并且将数据写到共享内存中,同时另一个任务周期性的从共享内存中读取数据并发送去显示(如下图两个线程间的数据传递 )。
如果对共享内存的访问不是排他性的,那么各个线程间可能同时访问它。这将引起数据一致性的问题,例如,在显示线程试图显示数据之前,传感器线程还未完成数据的写入,那么显示将包含不同时间采样的数据,造成显示数据的迷惑。
将传感器数据写入到共享内存的代码是接收线程的关键代码段;将传感器数据从共享内存中读出的代码是显示线程的关键代码段;这两段代码都会访问共享内存。正常的操作序列应该是在一个线程对共享内存块操作完成后,才允许另一个线程去操作。对于操作/访问同一块区域,称之为临界区。任务的同步方式有很多种,其核心思想都是:在访问临界区的时候只允许一个(或一类)任务运行。
关闭中断
关闭中断也叫中断锁,是禁止多任务访问临界区最简单的一种方式,即使是在分时操作系统中也是如此。当中断关闭的时候,就意味着当前任务不会被其他事件打断(因为整个系统已经不再响应那些可以触发线程重新调度的外部事件),也就是当前线程不会被抢占,除非这个任务主动放弃了处理器控制权。关闭中断/恢复中断API接口由BSP实现,根据平台的不同其实现方式也大不相同。
关闭、打开中断接口由两个函数完成:
关闭中断
- rt_base_t rt_hw_interrupt_disable(void);
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这个函数用于关闭中断并返回关闭中断前的中断状态。
函数返回
返回调用这个函数前的中断状态。
恢复中断
- void rt_hw_interrupt_enable(rt_base_t level);
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这个函数“使能”中断,它采用恢复调用rt_hw_interrupt_disable()函数前的中断状态,进行“使能”中断状态,如果调用rt_hw_interrupt_disable() 函数前是关中断状态,那么调用此函数后依然是关中断状态。level参数是上一次调用rt_hw_interrupt_ disable()时的返回值。
函数参数
参数 描述
level 前一次rt_hw_interrupt_disable返回的中断状态。
下面在机智云gokit开发板上验证如何通过中断锁来保护临界代码,程序中创建了2个线程,线程1的优先级比线程2高,低优先级线程会执行一段耗时比较长的代码:
(1)若不启用调度锁,即注释掉rt_hw_interrupt_disable和rt_hw_interrupt_enable这2行代码,则会发现线程2会被线程1打断,字符串被修改为gokit demo而不是期望打印的Gokit Demo,说明线程2被线程1打断;
(2)若启用调度锁,即不注释掉rt_hw_interrupt_disable和rt_hw_interrupt_enable这2行代码,则会发现线程2打印的是期望的Gokit Demo,说明线程2没有被线程1打断,调度锁起到了保护临界代码的作用;
- /**
- ****************************************************
- * File Name : main.c
- * Description : Main program body
- ****************************************************
- ** This notice applies to any and all portions of this file
- * that are not between comment pairs USER CODE BEGIN and
- * USER CODE END. Other portions of this file, whether
- * inserted by the user or by software development tools
- * are owned by their respective copyright owners.
- *
- * COPYRIGHT(c) 2017 STMicroelectronics
- *
- * Redistribution and use in source and binary forms, with or without modification,
- * are permitted provided that the following conditions are met:
- * 1. Redistributions of source code must retain the above copyright notice,
- * this list of conditions and the following disclaimer.
- * 2. Redistributions in binary form must reproduce the above copyright notice,
- * this list of conditions and the following disclaimer in the documentation
- * and/or other materials provided with the distribution.
- * 3. Neither the name of STMicroelectronics nor the names of its contributors
- * may be used to endorse or promote products derived from this software
- * without specific prior written permission.
- *
- * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
- * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
- * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
- * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
- * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
- * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
- * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
- * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
- * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
- * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
- *
- ****************************************************
- */
- /* Includes ------------------------------------------------------------------*/
- #include "main.h"
- #include "STM32f1xx_hal.h"
- #include "usart.h"
- #include "gpio.h"
- /* USER CODE BEGIN Includes */
- #include "rtthread.h"
- #include "string.h"
- /* USER CODE END Includes */
- /* Private variables ---------------------------------------------------------*/
- /* USER CODE BEGIN PV */
- /* Private variables ---------------------------------------------------------*/
- /* USER CODE END PV */
- /* Private function prototypes -----------------------------------------------*/
- void SystemClock_Config(void);
- /* USER CODE BEGIN PFP */
- /* Private function prototypes -----------------------------------------------*/
- /* USER CODE END PFP */
- /* USER CODE BEGIN 0 */
- //重映射串口1到rt_kprintf
- void rt_hw_console_output(const char *str)
- {
- /* empty console output */
- char aa='\r';
- rt_enter_critical();
- while(*str!='\0')
- {
- if(*str=='\n')
- {
- HAL_UART_Transmit(&huart1, (uint8_t *)&aa, 1, 10);
- }
- HAL_UART_Transmit(&huart1, (uint8_t *)(str++), 1, 10);
- }
-
- rt_exit_critical();
- }
- void rt_hw_us_delay(int us)
- {
- rt_uint32_t delta;
- /* 获得延时经过的tick数 */
- us = us * (SysTick->LOAD/(1000000/RT_TICK_PER_SECOND));
- /* 获得当前时间 */
- delta = SysTick->VAL;
- /* 循环获得当前时间,直到达到指定的时间后退出循环 */
- while (delta - SysTick->VAL< us);
- }
- void rt_hw_ms_delay(int ms)
- {
- int i=0,j=0;
- for(j=0;j<ms;j++)
- {
- for (i=0;i<2;i++)
- rt_hw_us_delay(500);
- }
- }
- /*
- * 程序清单:通过关闭中断保护临界区代码
- *线程1的优先级比线程2高,低优先级线程会执行一段耗时比较长的代码;
- *(1)若不启用调度锁,即注释掉rt_hw_interrupt_disable和rt_hw_interrupt_enable这2行代码,
- *则会发现线程2会被线程1打断,字符串被修改为gokit demo而不是期望打印的Gokit Demo,说明线程2被线程1打断;
- *(2)若启用调度锁,即不注释掉rt_hw_interrupt_disable和rt_hw_interrupt_enable这2行代码,
- *则会发现线程2打印的是期望的Gokit Demo,说明线程2没有被线程1打断,调度锁起到了保护临界代码的作用;
- */
- /* 指向线程控制块的指针 */
- static rt_thread_t tid1 = RT_NULL;
- static rt_thread_t tid2 = RT_NULL;
- char str[]="GOKIT DEMO";
- /* 线程1入口 */
- static void thread1_entry(void* parameter)
- {
- /* 高优先级线程1开始运行,会修改str内容 */
- while(1)
- {
- strcpy(str,"gokit demo");
- rt_kprintf("thread1: %s\n",str);
- rt_thread_delay(1000);
- }
- }
- /* 线程2入口 */
- static void thread2_entry(void* parameter)
- {
- rt_base_t level;
- /* 关闭中断*/
- level = rt_hw_interrupt_disable();
- strcpy(str,"Gokit Demo");
- /*低优先级线程2执行耗时有点长的代码*/
- rt_hw_ms_delay(4000);
- rt_kprintf("thread2: %s\n",str);
- /* 打开中断*/
- rt_hw_interrupt_enable(level);
- }
- /* USER CODE END 0 */
- int main(void)
- {
- /* USER CODE BEGIN 1 */
- /* USER CODE END 1 */
- /* MCU Configuration----------------------------------------------------------*/
- // /* Reset of all peripherals, Initializes the Flash interface and the Systick. */
- // HAL_Init();
- // /* USER CODE BEGIN Init */
- // /* USER CODE END Init */
- // /* Configure the system clock */
- // SystemClock_Config();
- // /* USER CODE BEGIN SysInit */
- // /* USER CODE END SysInit */
- // /* Initialize all configured peripherals */
- // MX_GPIO_Init();
- // MX_USART1_UART_Init();
- /* USER CODE BEGIN 2 */
-
- /* 创建线程1 */
- tid1 = rt_thread_create("t1",
- thread1_entry,
- RT_NULL,
- 512,
- 2,
- 20);
- if (tid1 != RT_NULL)
- rt_thread_startup(tid1);
-
- /* 创建线程2 */
- tid2 = rt_thread_create("t2",
- thread2_entry,
- RT_NULL,
- 512,
- 3,
- 20);
- if (tid2 != RT_NULL)
- rt_thread_startup(tid2);
-
- printf("\r\n机智云 只为智能硬件而生\r\n");
- printf("Gizwits Smart Cloud for Smart Products\r\n");
- printf("链接|增值|开放|中立|安全|自有|自由|生态\r\n");
- printf("www.gizwits.com\r\n");
- printf("\r\nGokit RT-Thread Demo\r\n\r\n");
-
- return 0;
-
-
- /* USER CODE END 2 */
- /* Infinite loop */
- /* USER CODE BEGIN WHILE */
- // while (1)
- // {
- /* USER CODE END WHILE */
- /* USER CODE BEGIN 3 */
- //
- // }
- /* USER CODE END 3 */
- }
- /** System Clock Configuration
- */
- void SystemClock_Config(void)
- {
- RCC_OscInitTypeDef RCC_OscInitStruct;
- RCC_ClkInitTypeDef RCC_ClkInitStruct;
- /**Initializes the CPU, AHB and APB busses clocks
- */
- RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSE;
- RCC_OscInitStruct.HSEState = RCC_HSE_ON;
- RCC_OscInitStruct.HSEPredivValue = RCC_HSE_PREDIV_DIV1;
- RCC_OscInitStruct.HSIState = RCC_HSI_ON;
- RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
- RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSE;
- RCC_OscInitStruct.PLL.PLLMUL = RCC_PLL_MUL9;
- if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK)
- {
- _Error_Handler(__FILE__, __LINE__);
- }
- /**Initializes the CPU, AHB and APB busses clocks
- */
- RCC_ClkInitStruct.ClockType = RCC_CLOCKTYPE_HCLK|RCC_CLOCKTYPE_SYSCLK
- |RCC_CLOCKTYPE_PCLK1|RCC_CLOCKTYPE_PCLK2;
- RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_PLLCLK;
- RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV1;
- RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV2;
- RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV1;
- if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_2) != HAL_OK)
- {
- _Error_Handler(__FILE__, __LINE__);
- }
- /**Configure the Systick interrupt time
- */
- HAL_SYSTICK_Config(HAL_RCC_GetHCLKFreq()/1000);
- /**Configure the Systick
- */
- HAL_SYSTICK_CLKSourceConfig(SYSTICK_CLKSOURCE_HCLK);
- /* SysTick_IRQn interrupt configuration */
- HAL_NVIC_SetPriority(SysTick_IRQn, 0, 0);
- }
- /* USER CODE BEGIN 4 */
- /* USER CODE END 4 */
- /**
- * @brief This function is executed in case of error occurrence.
- * @param None
- * @retval None
- */
- void _Error_Handler(char * file, int line)
- {
- /* USER CODE BEGIN Error_Handler_Debug */
- /* User can add his own implementation to report the HAL error return state */
- while(1)
- {
- }
- /* USER CODE END Error_Handler_Debug */
- }
- #ifdef USE_FULL_ASSERT
- /**
- * @brief Reports the name of the source file and the source line number
- * where the assert_param error has occurred.
- * @param file: pointer to the source file name
- * @param line: assert_param error line source number
- * @retval None
- */
- void assert_failed(uint8_t* file, uint32_t line)
- {
- /* USER CODE BEGIN 6 */
- /* User can add his own implementation to report the file name and line number,
- ex: printf("Wrong parameters value: file %s on line %d\r\n", file, line) */
- /* USER CODE END 6 */
- }
- #endif
- /**
- * @}
- */
- /**
- * @}
- */
- /**************** (C) COPYRIGHT STMicroelectronics ****END OF FILE***/
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通过串口信息可以看到实验结果与预期一致:
源码下载:
Gokit_RT-Thread.zip
(8.7 MB, 下载次数: 2, 售价: 1 金钱)
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