Binary Semaphore 03: Worst Case Priority Inversion

In this tutorial, we will see how to create and use binary semaphore.
Later we will be looking into priority inversion using the LPT,MPT and HPT.

Teminologies

1. LPT: Low Priority Task
2. MPT: Medium Priority Task
3. HPT: High Priority Task

Prerequisites

Please check this tutorial for detailed explanation on Semaphores.

API Details

Below is the list of the API's used in this tutorial.

1.xTaskCreate(): This interface is used to create a new Task, if the task is successfully created then it returns pdPass(1) or else errCOULD_NOT_ALLOCATE_REQUIRED_MEMORY(-1). Check this link for more details.

2.vTaskDelay(): This function is used to delay/block the task for specified delay time(ticks). INCLUDE_vTaskDelay needs to be set to 1 in FreeRtosConfig.h file for using this function. Check this link for more details.

3.vTaskDelete():This function is used to delete as task. We need to pass the taskHandle of the task to be deleted.
To delete the own task we should pass NULL as parameter.
Please check this link for details.

4.vSemaphoreCreateBinary(): This interface is used to create a binary semaphore. We need to pass a handle of type SemaphoreHandle_t to this function.
Please check this link for details.

5.xSemaphoreTake(): This interface is used to acquire a binary semaphore. We need to pass a semaphore handle and the amount of time to wait for semaphore to become available.
Please check this link for details.

6.xSemaphoreGive(): This interface is used to release a binary semaphore. We need to pass a semaphore handle to this function.
Please check this link for details.

7.vSemaphoreDelete (): This interface is used to delete a binary semaphore. We need to pass a semaphore handle to this function.
Please check this link for details.

Example

In this example, HPT waits for the LPT as it is holding the resource(semaphore). In between an MPT starts executing thereby blocking the LPT and HPT. This makes the HPT wait for MPT and LPT.

	/***************************************************************************************************
	ExploreEmbedded Copyright Notice
	****************************************************************************************************
	* File: BinarySemaphoreWorstCasePriorityInversion
	* Version: 16.0
	* Author: ExploreEmbedded
	* Website: http://www.exploreembedded.com/wiki
	* Description: Program to demonstrate the Binary semaphore usage and worst case priority inversion
	This code has been developed and tested on ExploreEmbedded boards.
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	**************************************************************************************************/
	#include <Arduino_FreeRTOS.h>
	TaskHandle_t LPT_TaskHandle;
	TaskHandle_t HPT_TaskHandle;
	TaskHandle_t MPT_TaskHandle;
	SemaphoreHandle_t binSemaphore_A = NULL;
	#define printMsg(taskhandle,str) {\
	Serial.print(F("Priority "));\ // Print task priority
	Serial.print(uxTaskPriorityGet(taskhandle));\
	Serial.print(F(" : "));\
	Serial.println(F(str));\ // Print user string
	}
	void setup()
	{
	Serial.begin(9600);
	Serial.println(F("In Setup function, Creating Binary Semaphore"));
	vSemaphoreCreateBinary(binSemaphore_A); /* Create binary semaphore */
	if(binSemaphore_A != NULL)
	{
	Serial.println(F("Creating low priority task"));
	xTaskCreate(LPT_Task, "LPT_Task", 100, NULL, 1, &LPT_TaskHandle);
	}
	else
	{
	Serial.println(F("Failed to create Semaphore"));
	}
	}
	void loop()
	{ // Hooked to Idle Task, will run when CPU is Idle
	Serial.println(F("Loop function"));
	delay(50);
	}
	/*LPT: Low priority task*/
	void LPT_Task(void* pvParameters)
	{
	printMsg(LPT_TaskHandle,"LPT_Task Acquiring semaphore");
	xSemaphoreTake(binSemaphore_A,portMAX_DELAY);
	printMsg(LPT_TaskHandle,"LPT_Task Creating HPT");
	xTaskCreate(HPT_Task, "HPT_Task", 100, NULL, 3, &HPT_TaskHandle);
	printMsg(LPT_TaskHandle,"LPT_Task Creating MPT");
	xTaskCreate(MPT_Task, "MPT_Task", 100, NULL, 2, &MPT_TaskHandle);
	printMsg(LPT_TaskHandle,"LPT_Task Releasing the semaphore");
	xSemaphoreGive(binSemaphore_A);
	printMsg(LPT_TaskHandle,"LPT_Task Finally Exiting");
	vTaskDelete(LPT_TaskHandle);
	}
	/*MPT: Medium priority task*/
	void MPT_Task(void* pvParameters)
	{
	printMsg(MPT_TaskHandle,"MPT_Task Done and About to Exit");
	vTaskDelete(MPT_TaskHandle);
	}
	/*HPT: High priority task*/
	void HPT_Task(void* pvParameters)
	{
	printMsg(HPT_TaskHandle,"HPT_Task Trying to Acquire the semaphore");
	xSemaphoreTake(binSemaphore_A,portMAX_DELAY);
	printMsg(HPT_TaskHandle,"HPT_Task Acquired the semaphore");
	printMsg(HPT_TaskHandle,"HPT_Task Releasing the semaphore");
	xSemaphoreGive(binSemaphore_A);
	printMsg(HPT_TaskHandle,"HPT_Task About to Exit");
	vTaskDelete(HPT_TaskHandle);
	}

view raw 13-BinarySemaphoreWorstCasePriorityInversion.c hosted with ❤ by GitHub

1. LPT starts running and acquires the semaphore.
2. Now HPT is created and it preempts LPT and starts running. It makes the request to acquire the semaphore. Since the semaphore is already with LPT, HPT goes to blocked state.
3. LPT starts executing again and creates an MPT tasks.
4. MPT preempts the LPT and starts running. At this point, the HPT is starved because of LPT and MPT. MPT completes its job and deletes itself.
5. LPT comes out of blocked state and releases the semaphore.
6. Immediately the HPT comes out of the blocked state and starts executing. It releases the semaphore and runs for some time and deletes itself.
7. Now control goes back to LPT which completes its job and deletes itself.
8. Finally the scheduler is left out with the idle task and it keeps running.

In this scenario, the HPT task waits for LPT and MPT from 2-6 which is the extended priority inversion period.

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