Priority Inversion and Solutions in VxWorks

🚀 Introduction

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Priority inversion is a common issue in real-time systems where a low-priority task blocks a high-priority task, causing unpredictable delays.
This can severely affect system responsiveness and even lead to failure in safety-critical applications.

VxWorks addresses this problem with priority inheritance, ensuring that resource sharing doesn’t break real-time guarantees.

🧩 What is Priority Inversion?

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Imagine:

1. A low-priority task holds a shared resource (e.g., a semaphore).
2. A high-priority task needs that resource and is blocked.
3. Meanwhile, a medium-priority task (which doesn’t need the resource) keeps running.

Result:

• The high-priority task is stuck, even though the resource is only held by a low-priority task.
• This “inversion” can cause missed deadlines.

💡 Solution: Priority Inheritance in VxWorks

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VxWorks automatically elevates the priority of a low-priority task holding a resource to match that of the highest-priority task waiting.

This ensures:

• The low-priority task runs to completion.
• It releases the resource quickly.
• The high-priority task resumes execution.

💻 Example: Simulating Priority Inversion

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Code Example

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#include <vxWorks.h>
#include <semLib.h>
#include <taskLib.h>
#include <stdio.h>

SEM_ID semId;

// Low-priority task (holds the resource)
void lowPriorityTask()
{
    semTake(semId, WAIT_FOREVER);
    printf("Low priority task got the resource\n");

    // Simulate long operation
    taskDelay(200); // ~2s

    printf("Low priority task releasing the resource\n");
    semGive(semId);
}

// Medium-priority task (runs freely)
void mediumPriorityTask()
{
    while (1)
    {
        printf("Medium priority task running\n");
        taskDelay(50);
    }
}

// High-priority task (needs the resource)
void highPriorityTask()
{
    taskDelay(20); // Let low task take resource first

    printf("High priority task waiting for the resource...\n");
    semTake(semId, WAIT_FOREVER);
    printf("High priority task got the resource!\n");
    semGive(semId);
}

void usrAppInit(void)
{
    semId = semMCreate(SEM_Q_PRIORITY | SEM_INVERSION_SAFE);

    taskSpawn("tLow", 150, 0, 4000, (FUNCPTR)lowPriorityTask,
              0,0,0,0,0,0,0,0,0,0);

    taskSpawn("tMed", 100, 0, 4000, (FUNCPTR)mediumPriorityTask,
              0,0,0,0,0,0,0,0,0,0);

    taskSpawn("tHigh", 50, 0, 4000, (FUNCPTR)highPriorityTask,
              0,0,0,0,0,0,0,0,0,0);
}

📝 Explanation of the Code

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1. Semaphore Creation

   • semMCreate(SEM_Q_PRIORITY | SEM_INVERSION_SAFE) ensures priority inheritance.

2. Low-Priority Task

   • Grabs the resource and holds it for ~2s.

3. High-Priority Task

   • Tries to take the resource but gets blocked.

4. Medium-Priority Task

   • Runs continuously, but due to priority inheritance, the low-priority task is boosted and finishes quickly.

⚡ Without Priority Inheritance

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If SEM_INVERSION_SAFE is not used:

• The low-priority task stays at low priority.
• The medium-priority task runs continuously.
• The high-priority task waits much longer, suffering priority inversion.

🔍 Key Takeaways

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• Priority inversion can delay high-priority tasks unpredictably.
• VxWorks provides priority inheritance to solve the problem.
• Always use SEM_INVERSION_SAFE when creating semaphores for shared resources.

✅ Wrap-Up

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In this tutorial, you learned:

• What priority inversion is.
• How VxWorks prevents it with priority inheritance.
• Why SEM_INVERSION_SAFE is important for real-time correctness.

In the next blog, we’ll explore Task Synchronization with Events in VxWorks.