Difference between revisions of "Binary Semaphore 04: Non-Recursive Locks"

(Created page with "category: Free RTOS with Arduino In the earlier example, we created 2 high priority task and an Idle task.<br> In this tutorial, we will see how to use the scheduler Idle...")
 
 
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[[category: Free RTOS with Arduino]]
 
[[category: Free RTOS with Arduino]]
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In the earlier example, we created 2 high priority task and an Idle task.<br>
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In this tutorial, we will see one of the demerits of binary semaphore ie. Non-Recursive Locks.<br>
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In this tutorial, we will see how to use the scheduler Idle task to run the user function.<br><br>
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Here we will see what happens if a task tries to acquire same binary semaphore more than ones.<br><br>
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==Teminologies==
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#<b>LPT:</b> Low Priority Task
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#<b>MPT:</b> Medium Priority Task
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#<b>HPT:</b> High Priority Task
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=Prerequisites=
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Please check [[Semaphore|this tutorial]] for detailed explanation on Semaphores.<br> <br>
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=API Details=
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Below is the list of the API's used in this tutorial.
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<b>1.xTaskCreate()</b>: 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 [http://www.freertos.org/a00125.html this link] for more details.<br>
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<b>2.vTaskDelay()</b>: 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 [http://www.freertos.org/a00127.html this link] for more details.<br>
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<b>3.vTaskDelete():</b>This function is used to delete as task. We need to pass the taskHandle of the task to be deleted.<br> To delete the own task we should pass NULL as parameter.<br>
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Please check [http://www.freertos.org/a00126.html this link] for details. <br>
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<b>4.vSemaphoreCreateBinary():</b> This interface is used to create a binary semaphore. We need to pass a handle of type SemaphoreHandle_t to this function.<br>  
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Please check [http://www.freertos.org/a00121.html this link] for details. <br>
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<b>5.xSemaphoreTake():</b> 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.<br>
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Please check [http://www.freertos.org/a00122.html this link] for details. <br>
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<b>6.xSemaphoreGive():</b> This interface is used to release a binary semaphore. We need to pass a semaphore handle to this function.<br>
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Please check [http://www.freertos.org/a00123.html this link] for details. <br>
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<b>7.vSemaphoreDelete ():</b> This interface is used to delete a binary semaphore. We need to pass a semaphore handle to this function.<br>
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Please check [http://www.freertos.org/a00113.html#vSemaphoreDelete this link] for details. <br>
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=Example=
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In this example, we will be creating an LPT. LPT will acquire a semaphore and creates an HPT.
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HPT will preempt LPT as starts running. HPT tries to acquire a semaphore used by LPT and goes to blocked state resulting in priority inversion. Now LPT starts running and requests for the same semaphore.
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<html><script src="https://gist.github.com/SaheblalBagwan/dbcd698228010f3a95c4927f18c365e8.js"></script></html>
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[[File:BinarySemaphoreNonRecursiveLock.png]]
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#LPT starts running and acquires the semaphore.
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#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.
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#LPT starts executing again and tries to take the same semaphore. Since the semaphore is not available, it gets blocked.
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#Finally the scheduler is left out with the idle task and it keeps running. The LPT and HPT will in blocked state until someone releases the semaphore.
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Have an opinion, suggestion , question or feedback about the article let it out here!
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{{DISQUS}}

Latest revision as of 11:52, 15 July 2016

In this tutorial, we will see one of the demerits of binary semaphore ie. Non-Recursive Locks.
Here we will see what happens if a task tries to acquire same binary semaphore more than ones.

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, we will be creating an LPT. LPT will acquire a semaphore and creates an HPT. HPT will preempt LPT as starts running. HPT tries to acquire a semaphore used by LPT and goes to blocked state resulting in priority inversion. Now LPT starts running and requests for the same semaphore.

BinarySemaphoreNonRecursiveLock.png

  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 tries to take the same semaphore. Since the semaphore is not available, it gets blocked.
  4. Finally the scheduler is left out with the idle task and it keeps running. The LPT and HPT will in blocked state until someone releases the semaphore.


Have an opinion, suggestion , question or feedback about the article let it out here!

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