Recursive mutex for nested locking in FreeRTOS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a recursive mutex in FreeRTOS?

A recursive mutex is a special type of mutex that allows the same task to lock it multiple times without causing a deadlock. The task must unlock it the same number of times to release the mutex.

beginner

Why do we need recursive mutexes for nested locking?

Recursive mutexes allow a task to safely enter critical sections multiple times if the code calls functions that also lock the same mutex. This prevents deadlocks caused by trying to lock a mutex already held by the same task.

beginner

How do you create a recursive mutex in FreeRTOS?

Use the function xSemaphoreCreateRecursiveMutex() to create a recursive mutex in FreeRTOS.

intermediate

How do you lock and unlock a recursive mutex in FreeRTOS?

Use xSemaphoreTakeRecursive() to lock and xSemaphoreGiveRecursive() to unlock a recursive mutex. The same task can call xSemaphoreTakeRecursive() multiple times, but must call xSemaphoreGiveRecursive() the same number of times to release it.

intermediate

What happens if a different task tries to take a recursive mutex already held by another task?

The other task will block (wait) until the mutex is fully released by the owning task. Recursive mutexes only allow the owning task to lock multiple times; other tasks must wait.

Which FreeRTOS function creates a recursive mutex?

A xSemaphoreCreateBinary()

B xSemaphoreCreateMutex()

C xSemaphoreCreateCounting()

D xSemaphoreCreateRecursiveMutex()

xSemaphoreCreateRecursiveMutex() creates a recursive mutex, allowing nested locking by the same task.

What must a task do after locking a recursive mutex multiple times?

A Unlock it once

B Unlock it the same number of times it was locked

C Do nothing, it unlocks automatically

D Call a special reset function

The task must unlock the recursive mutex the same number of times it locked it to fully release it.

Which function is used to lock a recursive mutex in FreeRTOS?

A xSemaphoreTakeRecursive()

B xSemaphoreGive()

C xSemaphoreTake()

D xSemaphoreGiveRecursive()

xSemaphoreTakeRecursive() is used to lock a recursive mutex, allowing nested locking by the same task.

What happens if a different task tries to take a recursive mutex held by another task?

A It locks immediately

B It causes a deadlock

C It blocks until the mutex is released

D It throws an error

The other task blocks and waits until the mutex is fully released by the owning task.

Why is a recursive mutex preferred over a normal mutex for nested locking?

A It prevents deadlocks when the same task locks multiple times

B It is faster

C It uses less memory

D It allows multiple tasks to lock simultaneously

Recursive mutexes prevent deadlocks caused by the same task locking the mutex multiple times.

Explain what a recursive mutex is and why it is useful in FreeRTOS.

Think about a task entering a function that locks a mutex, then calling another function that also locks the same mutex.

Describe the steps to create, lock, and unlock a recursive mutex in FreeRTOS.

Focus on the FreeRTOS API functions for recursive mutexes.

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