Recursive mutex for nested locking in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a recursive mutex in FreeRTOS.

FreeRTOS

SemaphoreHandle_t xMutex = xSemaphoreCreate[1]Mutex();

Drag options to blanks, or click blank then click option'

ASemaphoreCreate

BCounting

CBinary

DRecursive

Attempts:

3 left

💡 Hint

Common Mistakes

Using xSemaphoreCreateBinary() instead of xSemaphoreCreateRecursiveMutex()

Using xSemaphoreCreateCounting() which is not recursive

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

2fill in blank

medium

Complete the code to take the recursive mutex safely.

FreeRTOS

if(xSemaphoreTakeRecursive(xMutex, [1]) == pdTRUE) {
    // Critical section
}

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A100

B0

CportMAX_DELAY

DpdFALSE

Attempts:

3 left

💡 Hint

Common Mistakes

Passing 0 which means no wait

Passing pdFALSE which is not a valid block time

Use portMAX_DELAY to wait indefinitely for the mutex.

3fill in blank

hard

Fix the error in releasing the recursive mutex.

FreeRTOS

xSemaphore[1]Recursive(xMutex);

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AGive

BTake

CRelease

DUnlock

Attempts:

3 left

💡 Hint

Common Mistakes

Using xSemaphoreTakeRecursive() to release

Using xSemaphoreReleaseRecursive() which does not exist

Use xSemaphoreGiveRecursive() to release a recursive mutex.

4fill in blank

hard

Fill both blanks to create and take a recursive mutex with a timeout of 50 ticks.

FreeRTOS

SemaphoreHandle_t xMutex = xSemaphoreCreate[1]Mutex();
if(xSemaphoreTakeRecursive(xMutex, [2]) == pdTRUE) {
    // Critical section
}

Drag options to blanks, or click blank then click option'

ARecursive

B50

C100

DBinary

Attempts:

3 left

💡 Hint

Common Mistakes

Using xSemaphoreCreateBinary() instead of xSemaphoreCreateRecursiveMutex()

Using 100 ticks instead of 50

Create the mutex with xSemaphoreCreateRecursiveMutex() and take it with a 50 tick timeout.

5fill in blank

hard

Fill all three blanks to create a recursive mutex, take it with max delay, and then release it.

FreeRTOS

SemaphoreHandle_t xMutex = xSemaphoreCreate[1]Mutex();
if(xSemaphoreTakeRecursive(xMutex, [2]) == pdTRUE) {
    // Critical section
    xSemaphore[3]Recursive(xMutex);
}

Drag options to blanks, or click blank then click option'

ARecursive

BportMAX_DELAY

CGive

DBinary

Attempts:

3 left

💡 Hint

Common Mistakes

Using xSemaphoreCreateBinary() instead of xSemaphoreCreateRecursiveMutex()

Using 0 instead of portMAX_DELAY for block time

Using xSemaphoreTakeRecursive() to release

Create the recursive mutex, take it with portMAX_DELAY, and release it with xSemaphoreGiveRecursive().

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