Thread synchronization with Mutex in Ruby - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new Mutex object.

Ruby

mutex = [1].new

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

AMutex

BThread

CLock

DSync

Attempts:

3 left

💡 Hint

Common Mistakes

Using Thread.new instead of Mutex.new

Using Lock.new which is not a Ruby class

Using Sync.new which does not exist

The Mutex.new creates a new mutex object used for thread synchronization.

2fill in blank

medium

Complete the code to lock the mutex before accessing the shared resource.

Ruby

mutex.[1] do
  # critical section
end

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

Alock

Bsynchronize

Cunlock

Dwait

Attempts:

3 left

💡 Hint

Common Mistakes

Using lock without a block, which requires manual unlock

Using unlock before locking

Using wait which is not a Mutex method

The synchronize method locks the mutex, runs the block, then unlocks it automatically.

3fill in blank

hard

Fix the error in the code to properly unlock the mutex after locking it.

Ruby

mutex.lock
# critical section
mutex.[1]

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

Alock

Bwait

Csynchronize

Dunlock

Attempts:

3 left

💡 Hint

Common Mistakes

Calling lock twice without unlock

Using synchronize without a block

Using wait which is not a Mutex method

After calling lock, you must call unlock to release the mutex.

4fill in blank

hard

Fill both blanks to create a thread-safe increment of a shared counter.

Ruby

counter = 0
mutex = Mutex.new
thread = Thread.new do
  mutex.[1] do
    counter [2] 1
  end
end
thread.join

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

Asynchronize

B+=

C-=

Dlock

Attempts:

3 left

💡 Hint

Common Mistakes

Using lock without unlock for the first blank

Using -= instead of += for increment

Not locking the mutex at all

Use synchronize to lock the mutex during increment, and += to add 1 to the counter.

5fill in blank

hard

Fill all three blanks to create two threads that safely increment a shared variable.

Ruby

counter = 0
mutex = Mutex.new
threads = []
2.times do
  threads << Thread.new do
    mutex.[1] do
      temp = counter
      temp [2] 1
      counter = temp
    end
  end
end
threads.each(&:[3])

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

Asynchronize

B+=

Cjoin

Dlock

Attempts:

3 left

💡 Hint

Common Mistakes

Using lock without unlock for the first blank

Using -= instead of += for increment

Not joining threads causing premature program exit

Use synchronize to lock the mutex, += to increment, and join to wait for threads to finish.

Practice

1. What is the main purpose of using a Mutex in Ruby threads?

easy

A. To create new threads automatically

B. To prevent multiple threads from accessing shared data at the same time

C. To speed up thread execution by running them in parallel

D. To stop threads from running

Solution

1. Step 1: Understand what shared data means in threads

   Shared data is information that multiple threads can access or change.

2. Step 2: Identify the role of Mutex

   Mutex ensures only one thread changes shared data at a time to avoid errors.

3. Final Answer:

   To prevent multiple threads from accessing shared data at the same time -> Option B

4. Quick Check:

   Mutex protects shared data = prevent simultaneous access [OK]

Hint: Mutex locks shared data to avoid conflicts between threads [OK]

Common Mistakes:

• Thinking Mutex speeds up threads
• Believing Mutex creates threads
• Assuming Mutex stops threads completely

2. Which of the following is the correct way to use a Mutex to protect a critical section in Ruby?

easy

A. mutex.synchronize { critical_code }

B. mutex.lock; critical_code

C. mutex.run(critical_code)

D. mutex.start { critical_code }

Solution

1. Step 1: Recall Mutex usage in Ruby

   Ruby's Mutex class provides a synchronize method to wrap critical code safely.

2. Step 2: Compare options

   Only mutex.synchronize { critical_code } is the correct and safe syntax.

3. Final Answer:

   mutex.synchronize { critical_code } -> Option A

4. Quick Check:

   Use synchronize method to protect code [OK]

Hint: Use mutex.synchronize { } to wrap critical code [OK]

Common Mistakes:

• Forgetting to unlock mutex after lock
• Using non-existent methods like run or start
• Not wrapping critical code in a block

3. What will be the output of this Ruby code?

require 'thread'
mutex = Mutex.new
counter = 0
threads = 3.times.map do
  Thread.new do
    2.times do
      mutex.synchronize { counter += 1 }
    end
  end
end
threads.each(&:join)
puts counter

medium

A. 2

B. 3

C. 6

D. 0

Solution

1. Step 1: Count total increments

   There are 3 threads, each increments counter 2 times, total 3*2=6 increments.

2. Step 2: Understand mutex effect

   Mutex ensures increments happen safely without losing counts, so final counter is 6.

3. Final Answer:

   6 -> Option C

4. Quick Check:

   3 threads * 2 increments each = 6 [OK]

Hint: Multiply threads by increments inside synchronize block [OK]

Common Mistakes:

• Ignoring mutex and expecting wrong count
• Counting only one thread's increments
• Confusing thread join effect

4. Identify the problem in this Ruby code using Mutex and how to fix it:

require 'thread'
mutex = Mutex.new
counter = 0
threads = 2.times.map do
  Thread.new do
    mutex.lock
    counter += 1
  end
end
threads.each(&:join)
puts counter

medium

A. Threads are not joined properly; fix by joining inside the thread block

B. Mutex is not needed here; remove mutex.lock

C. counter should be initialized inside the thread

D. Missing mutex.unlock causes deadlock; fix by adding mutex.unlock after increment

Solution

1. Step 1: Check mutex usage

   Mutex is locked but never unlocked, causing threads to wait forever (deadlock).

2. Step 2: Fix by unlocking mutex

   After increment, call mutex.unlock or better use mutex.synchronize to auto-handle locking/unlocking.

3. Final Answer:

   Missing mutex.unlock causes deadlock; fix by adding mutex.unlock after increment -> Option D

4. Quick Check:

   Always unlock mutex after lock to avoid deadlock [OK]

Hint: Always unlock mutex or use synchronize block [OK]

Common Mistakes:

• Forgetting to unlock mutex
• Removing mutex when needed
• Initializing shared data inside threads incorrectly

5. You want to safely update a shared hash from multiple threads in Ruby. Which approach correctly uses Mutex to avoid race conditions?

shared_hash = {}
mutex = Mutex.new
threads = 5.times.map do |i|
  Thread.new do
    10.times do |j|
      # What goes here?
    end
  end
end
threads.each(&:join)
puts shared_hash.size

hard

A. mutex.synchronize { shared_hash[i * 10 + j] = "value" }

B. shared_hash[i * 10 + j] = "value"

C. mutex.lock; shared_hash[i * 10 + j] = "value";

D. Thread.pass; shared_hash[i * 10 + j] = "value"

Solution

1. Step 1: Understand thread safety for shared hash

   Multiple threads writing to shared_hash need protection to avoid data corruption.

2. Step 2: Use mutex.synchronize for safe updates

   mutex.synchronize { shared_hash[i * 10 + j] = "value" } wraps the update inside mutex.synchronize, ensuring only one thread writes at a time.

3. Final Answer:

   mutex.synchronize { shared_hash[i * 10 + j] = "value" } -> Option A

4. Quick Check:

   Use synchronize block to protect shared hash updates [OK]

Hint: Wrap shared data updates inside mutex.synchronize { } [OK]

Common Mistakes:

• Updating shared hash without mutex
• Locking mutex without unlocking
• Using Thread.pass instead of mutex