Mutex locks in Operating Systems - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the sentence to explain what a mutex lock does.

Operating Systems

A mutex lock is used to [1] access to a shared resource.

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

Aallow simultaneous

Bprevent simultaneous

Cignore

Dduplicate

Attempts:

3 left

💡 Hint

Common Mistakes

Thinking mutex allows multiple threads to access at the same time.

Confusing mutex with other synchronization tools.

A mutex lock prevents simultaneous access to a shared resource, ensuring only one thread can use it at a time.

2fill in blank

medium

Complete the sentence to describe what happens when a thread tries to lock a mutex already locked by another thread.

Operating Systems

If a thread tries to lock a mutex that is already locked, it will [1] until the mutex is unlocked.

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

Await

Bfail immediately

Cskip the lock

Dlock twice

Attempts:

3 left

💡 Hint

Common Mistakes

Assuming the thread can lock the mutex twice.

Thinking the thread fails immediately without waiting.

The thread will wait (block) until the mutex becomes available to ensure safe access.

3fill in blank

hard

Fix the error in the statement about mutex usage.

Operating Systems

A thread must always [1] a mutex after finishing its work with the shared resource.

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

Aignore

Block

Cunlock

Dduplicate

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to unlock the mutex causing deadlocks.

Trying to lock the mutex again instead of unlocking.

After finishing, a thread must unlock the mutex to allow others to access the resource.

4fill in blank

hard

Fill both blanks to complete the explanation of mutex behavior.

Operating Systems

When a mutex is [1], other threads trying to [2] it will be blocked.

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

Alocked

Bignore

Clock

Dunlock

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing locking and unlocking states.

Assuming threads ignore the mutex state.

When a mutex is locked, other threads trying to lock it will be blocked until it is unlocked.

5fill in blank

hard

Fill all three blanks to complete the dictionary comprehension that maps thread IDs to their lock status if the mutex is locked.

Operating Systems

status = { [1]: [2] for [3] in threads if mutex.is_locked() }

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

Athread_id

Blocked

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same variable for key and value incorrectly.

Not checking if the mutex is locked before creating the dictionary.

This comprehension creates a dictionary with thread IDs as keys and their lock status as values when the mutex is locked.

Practice

1. What is the main purpose of a mutex lock in operating systems?

easy

A. To store data permanently on the disk

B. To speed up the execution of multiple processes simultaneously

C. To allow only one process to access a shared resource at a time

D. To create multiple copies of a resource for each process

Solution

1. Step 1: Understand the role of mutex locks

   Mutex locks are used to control access to shared resources so that only one process can use the resource at a time.

2. Step 2: Identify the correct purpose

   The main goal is to prevent conflicts and errors by ensuring exclusive access, which matches To allow only one process to access a shared resource at a time.

3. Final Answer:

   To allow only one process to access a shared resource at a time -> Option C

4. Quick Check:

   Mutex = exclusive access [OK]

Hint: Mutex means one user at a time [OK]

Common Mistakes:

• Thinking mutex speeds up all processes
• Confusing mutex with data storage
• Believing mutex duplicates resources

2. Which of the following is the correct sequence when using a mutex lock?

easy

A. Acquire the lock, use the resource, then release the lock

B. Release the lock, use the resource, then acquire the lock

C. Use the resource, acquire the lock, then release the lock

D. Use the resource, release the lock, then acquire the lock

Solution

1. Step 1: Recall the correct mutex usage order

   To safely use a shared resource, a process must first acquire the mutex lock before accessing it.

2. Step 2: Complete the sequence

   After using the resource, the process must release the lock so others can access it. This matches Acquire the lock, use the resource, then release the lock.

3. Final Answer:

   Acquire the lock, use the resource, then release the lock -> Option A

4. Quick Check:

   Acquire -> Use -> Release [OK]

Hint: Always lock before use, unlock after [OK]

Common Mistakes:

• Releasing before acquiring
• Using resource without locking
• Locking after using resource

3. Consider this pseudocode using a mutex lock:

mutex.lock()
shared_counter = shared_counter + 1
mutex.unlock()

If two threads run this code simultaneously starting with shared_counter = 5, what is the final value of shared_counter?

medium

A. 6

B. 5

C. Error

D. 7

Solution

1. Step 1: Understand mutex effect on shared_counter

   The mutex lock ensures only one thread increments the counter at a time, preventing race conditions.

2. Step 2: Calculate final value after two increments

   Starting at 5, each thread adds 1 safely, so final value is 5 + 2 = 7.

3. Final Answer:

   7 -> Option D

4. Quick Check:

   Mutex prevents race, count increments twice [OK]

Hint: Mutex ensures both increments count [OK]

Common Mistakes:

• Assuming only one increment happens
• Ignoring mutex effect causing race
• Expecting an error without reason

4. A programmer wrote this code snippet:

mutex.unlock()
use_shared_resource()
mutex.lock()

What is the main problem with this code?

medium

A. The mutex is unlocked before it is locked

B. The code will cause a deadlock

C. The mutex is locked twice without unlocking

D. There is no problem; the code is correct

Solution

1. Step 1: Analyze the order of mutex operations

   The code calls mutex.unlock() before any mutex.lock(), which is incorrect because you cannot release a lock you don't hold.

2. Step 2: Identify the error

   This misuse can cause runtime errors or undefined behavior, matching The mutex is unlocked before it is locked.

3. Final Answer:

   The mutex is unlocked before it is locked -> Option A

4. Quick Check:

   Unlock before lock is wrong [OK]

Hint: Lock first, unlock later; never reverse [OK]

Common Mistakes:

• Unlocking before locking
• Ignoring lock before resource use
• Assuming double lock is okay

5. In a system with multiple threads, which scenario best shows why mutex locks are essential?

hard

A. Two threads perform calculations independently without sharing data

B. Two threads write to the same file simultaneously without locking, causing data corruption

C. A single thread writes to a file without any other threads running

D. Two threads read different files at the same time without any locks

Solution

1. Step 1: Identify when mutex locks are needed

   Mutex locks are essential when multiple threads access and modify the same shared resource to avoid conflicts.

2. Step 2: Match scenario with mutex necessity

   Two threads write to the same file simultaneously without locking, causing data corruption describes simultaneous writes causing corruption, which mutex locks prevent by serializing access.

3. Final Answer:

   Two threads write to the same file simultaneously without locking, causing data corruption -> Option B

4. Quick Check:

   Shared write needs mutex [OK]

Hint: Mutex needed when sharing writable resource [OK]

Common Mistakes:

• Thinking mutex needed for independent reads
• Assuming single thread needs mutex
• Confusing calculation with shared resource access