Recursive mutex for nested locking in FreeRTOS - Mini Project: Build & Apply

Recursive mutex for nested locking in FreeRTOS

📖 Scenario: You are developing a FreeRTOS application where a task needs to safely access a shared resource multiple times in a nested manner without causing a deadlock.Using a recursive mutex allows the same task to lock the mutex multiple times before unlocking it.

🎯 Goal: Build a FreeRTOS program snippet that creates a recursive mutex, locks it twice in nested calls, and then unlocks it twice, demonstrating safe nested locking.

📋 What You'll Learn

Create a recursive mutex using FreeRTOS API

Lock the recursive mutex twice in nested calls

Unlock the recursive mutex twice

Print messages showing the locking and unlocking sequence

💡 Why This Matters

🌍 Real World

Recursive mutexes are useful in embedded systems where a task may need to enter multiple critical sections that require the same mutex without causing deadlocks.

💼 Career

Understanding recursive mutexes is important for embedded software engineers working with FreeRTOS or similar real-time operating systems to write safe multitasking code.

1

Create a recursive mutex

Create a recursive mutex called xRecursiveMutex using xSemaphoreCreateRecursiveMutex().

FreeRTOS

# Create a recursive mutex called xRecursiveMutex
// Your code here

Hint

Use xSemaphoreCreateRecursiveMutex() to create the recursive mutex.

2

Lock the recursive mutex twice

Inside the setup() function, lock the recursive mutex xRecursiveMutex twice using xSemaphoreTakeRecursive() with portMAX_DELAY as the block time.

FreeRTOS

#include "FreeRTOS.h"
#include "semphr.h"

SemaphoreHandle_t xRecursiveMutex;

void setup() {
    xRecursiveMutex = xSemaphoreCreateRecursiveMutex();
    // Lock the recursive mutex twice
    // Your code here
}

Hint

Call xSemaphoreTakeRecursive(xRecursiveMutex, portMAX_DELAY); twice to lock it twice.

3

Unlock the recursive mutex twice

Inside the setup() function, after locking, unlock the recursive mutex xRecursiveMutex twice using xSemaphoreGiveRecursive().

FreeRTOS

#include "FreeRTOS.h"
#include "semphr.h"
#include "stdio.h"

SemaphoreHandle_t xRecursiveMutex;

void setup() {
    xRecursiveMutex = xSemaphoreCreateRecursiveMutex();
    xSemaphoreTakeRecursive(xRecursiveMutex, portMAX_DELAY);
    xSemaphoreTakeRecursive(xRecursiveMutex, portMAX_DELAY);
    // Unlock the recursive mutex twice
    // Your code here
}

Hint

Call xSemaphoreGiveRecursive(xRecursiveMutex); twice to unlock it twice.

4

Print locking and unlocking messages

Add printf statements before and after each lock and unlock call inside setup() to show the sequence:
- Before first lock: "Locking first time"
- Before second lock: "Locking second time"
- Before first unlock: "Unlocking first time"
- Before second unlock: "Unlocking second time"

FreeRTOS

#include "FreeRTOS.h"
#include "semphr.h"
#include "stdio.h"

SemaphoreHandle_t xRecursiveMutex;

void setup() {
    xRecursiveMutex = xSemaphoreCreateRecursiveMutex();
    // Print and lock twice
    // Your code here
    // Print and unlock twice
    // Your code here
}

Hint

Use printf() before each lock and unlock call to show the sequence.

Practice

1.

What is the main advantage of using a recursive mutex in FreeRTOS?

easy

A. It increases the priority of the task holding the mutex.

B. It automatically unlocks after a fixed time.

C. It allows the same task to lock the mutex multiple times without deadlock.

D. It can be shared between tasks without any locking.

Solution

1. Step 1: Understand mutex behavior

   Normal mutexes block if the same task tries to lock again, causing deadlock.

2. Step 2: Role of recursive mutex

   Recursive mutexes allow the same task to lock multiple times safely, avoiding deadlock.

3. Final Answer:

   It allows the same task to lock the mutex multiple times without deadlock. -> Option C

4. Quick Check:

   Recursive mutex = multiple locks by same task [OK]

Hint: Recursive mutex lets one task lock repeatedly safely [OK]

Common Mistakes:

• Thinking mutex unlocks automatically
• Confusing recursive mutex with priority inheritance
• Assuming mutex can be shared without locking

2.

Which FreeRTOS function is used to lock a recursive mutex?

SemaphoreHandle_t xMutex = xSemaphoreCreateRecursiveMutex();
// Lock the mutex here

easy

A. xSemaphoreTakeRecursive(xMutex, portMAX_DELAY);

B. xSemaphoreGiveRecursive(xMutex);

C. xSemaphoreTake(xMutex, portMAX_DELAY);

D. xSemaphoreCreateMutex();

Solution

1. Step 1: Identify locking function for recursive mutex

   Recursive mutexes require xSemaphoreTakeRecursive() to lock safely multiple times.

2. Step 2: Match function usage

   xSemaphoreTakeRecursive(xMutex, portMAX_DELAY) locks the recursive mutex properly.

3. Final Answer:

   xSemaphoreTakeRecursive(xMutex, portMAX_DELAY); -> Option A

4. Quick Check:

   Lock recursive mutex = xSemaphoreTakeRecursive() [OK]

Hint: Use xSemaphoreTakeRecursive() to lock recursive mutex [OK]

Common Mistakes:

• Using xSemaphoreTake() instead of recursive version
• Confusing give and take functions
• Trying to create mutex again instead of locking

3.

What will be the output of the following FreeRTOS code snippet?

SemaphoreHandle_t xMutex = xSemaphoreCreateRecursiveMutex();

void Task(void *params) {
    xSemaphoreTakeRecursive(xMutex, portMAX_DELAY);
    xSemaphoreTakeRecursive(xMutex, portMAX_DELAY);
    xSemaphoreGiveRecursive(xMutex);
    xSemaphoreGiveRecursive(xMutex);
    if (xSemaphoreGiveRecursive(xMutex) == pdFALSE) {
        printf("Mutex fully released\n");
    } else {
        printf("Mutex still locked\n");
    }
}

medium

A. Mutex fully released

B. Mutex still locked

C. Compilation error

D. Deadlock occurs

Solution

1. Step 1: Count lock and unlock calls

   The mutex is locked twice and unlocked twice, so it is fully released after two gives.

2. Step 2: Analyze extra unlock call

   The third give returns pdFALSE because mutex is already fully released, triggering "Mutex fully released" print.

3. Final Answer:

   Mutex fully released -> Option A

4. Quick Check:

   Extra unlock returns false = fully released [OK]

Hint: Unlock same times as lock to fully release mutex [OK]

Common Mistakes:

• Assuming extra unlock succeeds
• Ignoring return value of xSemaphoreGiveRecursive()
• Thinking deadlock occurs on unlock

4.

Identify the error in this FreeRTOS recursive mutex usage:

SemaphoreHandle_t xMutex = xSemaphoreCreateRecursiveMutex();

void Task(void *params) {
    xSemaphoreTakeRecursive(xMutex, portMAX_DELAY);
    // Some code
    xSemaphoreGive(xMutex);
}

medium

A. Not locking the mutex before unlocking.

B. Using xSemaphoreGive() instead of xSemaphoreGiveRecursive() to unlock.

C. Creating mutex inside the task instead of globally.

D. Using portMAX_DELAY as timeout.

Solution

1. Step 1: Check unlock function for recursive mutex

   Recursive mutexes must be unlocked with xSemaphoreGiveRecursive(), not xSemaphoreGive().

2. Step 2: Identify mismatch in lock/unlock

   Using xSemaphoreGive() causes undefined behavior or errors because it is for normal mutexes.

3. Final Answer:

   Using xSemaphoreGive() instead of xSemaphoreGiveRecursive() to unlock. -> Option B

4. Quick Check:

   Unlock recursive mutex = xSemaphoreGiveRecursive() [OK]

Hint: Unlock recursive mutex with xSemaphoreGiveRecursive() only [OK]

Common Mistakes:

• Mixing normal and recursive mutex functions
• Ignoring correct unlock function
• Assuming portMAX_DELAY causes error here

5.

You have a recursive mutex locked three times by the same task. What must you do to avoid deadlock when unlocking?

// Pseudocode
xSemaphoreTakeRecursive(xMutex, portMAX_DELAY);
xSemaphoreTakeRecursive(xMutex, portMAX_DELAY);
xSemaphoreTakeRecursive(xMutex, portMAX_DELAY);
// ... critical section ...
// Unlock here

hard

A. Call xSemaphoreTakeRecursive() again to balance locks.

B. Call xSemaphoreGiveRecursive() once to release the mutex fully.

C. Call xSemaphoreGive() three times to release the mutex.

D. Call xSemaphoreGiveRecursive() exactly three times before releasing the mutex.

Solution

1. Step 1: Understand recursive mutex lock count

   Each lock increments a count; unlocking must decrement the same number of times.

2. Step 2: Proper unlock calls

   To fully release, call xSemaphoreGiveRecursive() three times matching the three locks.

3. Final Answer:

   Call xSemaphoreGiveRecursive() exactly three times before releasing the mutex. -> Option D

4. Quick Check:

   Unlock count = lock count to avoid deadlock [OK]

Hint: Unlock recursive mutex as many times as locked [OK]

Common Mistakes:

• Unlocking fewer times causing deadlock
• Using xSemaphoreGive() instead of recursive version
• Trying to unlock more times than locked