Concurrency considerations in LLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to acquire a lock before accessing shared data.

LLD

lock.[1]()  # Acquire the lock before critical section

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

Aacquire

Brelease

Cwait

Dnotify

Attempts:

3 left

💡 Hint

Common Mistakes

Using release() before acquire() causes errors.

Confusing wait() or notify() with lock acquisition.

To safely access shared data, you must acquire the lock first using acquire().

2fill in blank

medium

Complete the code to safely release the lock after the critical section.

LLD

try:
    # critical section code
    pass
finally:
    lock.[1]()  # Release the lock

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

Await

Bacquire

Crelease

Dnotify

Attempts:

3 left

💡 Hint

Common Mistakes

Calling acquire() again instead of release().

Forgetting to release the lock causing deadlocks.

After finishing the critical section, always release the lock with release() to allow others to proceed.

3fill in blank

hard

Fix the error in the code to avoid race conditions when updating a shared counter.

LLD

def increment():
    global counter
    [1]  # Missing synchronization here
    counter += 1
    lock.release()

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

Alock.notify()

Block.release()

Clock.wait()

Dlock.acquire()

Attempts:

3 left

💡 Hint

Common Mistakes

Releasing the lock before acquiring it.

Using wait() or notify() incorrectly.

Before modifying shared data, acquire the lock to prevent race conditions.

4fill in blank

hard

Fill both blanks to implement a thread-safe queue using locks.

LLD

def enqueue(item):
    [1]  # Acquire lock
    queue.append(item)
    [2]  # Release lock

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

Alock.acquire()

Block.release()

Clock.wait()

Dlock.notify()

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping acquire and release calls.

Using wait() or notify() instead of acquire/release.

Acquire the lock before modifying the queue and release it after to ensure thread safety.

5fill in blank

hard

Fill all three blanks to implement a safe read-modify-write operation with locks.

LLD

def update_value(key, delta):
    [1]  # Acquire lock
    current = data.get([2], 0)
    data[[3]] = current + delta
    lock.release()

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

Alock.acquire()

Bkey

Dlock.release()

Attempts:

3 left

💡 Hint

Common Mistakes

Not acquiring the lock before reading and writing.

Using different variables for key in get and set.

Acquire the lock first, then use the key to get and update the value safely.

Practice

1. What is the main purpose of using locks in concurrent systems?

easy

A. To allow unlimited access to shared resources

B. To prevent multiple threads from accessing shared data simultaneously

C. To speed up the execution of a single thread

D. To reduce memory usage in the system

Solution

1. Step 1: Understand concurrency risks

   When multiple threads access shared data at the same time, it can cause errors or inconsistent results.

2. Step 2: Role of locks

   Locks ensure only one thread accesses the shared data at a time, preventing conflicts and data corruption.

3. Final Answer:

   To prevent multiple threads from accessing shared data simultaneously -> Option B

4. Quick Check:

   Locks protect shared data = C [OK]

Hint: Locks protect shared data from simultaneous access [OK]

Common Mistakes:

• Thinking locks speed up single-thread execution
• Believing locks allow unlimited resource access
• Confusing locks with memory optimization

2. Which of the following is the correct way to acquire a lock in a typical low-level design?

easy

A. lock.notify() before accessing shared data

B. lock.release() before accessing shared data

C. lock.wait() after accessing shared data

D. lock.acquire() before accessing shared data

Solution

1. Step 1: Understand lock usage order

   To safely access shared data, a thread must first acquire the lock to block others.

2. Step 2: Correct method to acquire lock

   The method lock.acquire() is used to obtain the lock before accessing shared data.

3. Final Answer:

   lock.acquire() before accessing shared data -> Option D

4. Quick Check:

   Acquire lock first = A [OK]

Hint: Acquire lock before shared data access [OK]

Common Mistakes:

• Releasing lock before access
• Using wait or notify incorrectly
• Confusing acquire with release

3. Consider this pseudocode for two threads incrementing a shared counter without locks:

Thread 1: temp = counter
          temp = temp + 1
          counter = temp

Thread 2: temp = counter
          temp = temp + 1
          counter = temp

What is the possible final value of counter if it starts at 0?

medium

A. 2

B. Any negative number

C. 1

D. 0

Solution

1. Step 1: Analyze concurrent increments without locks

   Both threads read the same initial value 0, increment it to 1, and write back 1, causing one increment to be lost.

2. Step 2: Determine final counter value

   Because of race condition, the counter may only increase once, resulting in final value 1 instead of 2.

3. Final Answer:

   1 -> Option C

4. Quick Check:

   Race condition causes lost update = 1 [OK]

Hint: Without locks, increments can overwrite each other [OK]

Common Mistakes:

• Assuming both increments always succeed
• Ignoring race conditions
• Thinking counter can be negative here

4. In the following code snippet, what is the main concurrency issue?

lock.acquire()
shared_data.append(1)
# Missing lock.release()

medium

A. Deadlock due to missing lock release

B. Data race on shared_data

C. Syntax error in lock usage

D. No issue, code is safe

Solution

1. Step 1: Identify missing lock release

   The code acquires a lock but never releases it, so other threads waiting for the lock will block forever.

2. Step 2: Understand deadlock impact

   This causes a deadlock where threads cannot proceed, halting system progress.

3. Final Answer:

   Deadlock due to missing lock release -> Option A

4. Quick Check:

   Missing release causes deadlock = A [OK]

Hint: Always release locks after acquiring [OK]

Common Mistakes:

• Thinking it's a syntax error
• Assuming no issue without release
• Confusing deadlock with data race

5. You design a system where multiple threads read and write a shared cache. To improve performance, you want to allow multiple readers but only one writer at a time. Which concurrency control mechanism fits best?

hard

A. Use a read-write lock allowing concurrent reads but exclusive writes

B. Use a simple mutex lock for all access

C. Use no locks and rely on thread scheduling

D. Use a semaphore with count 1 for all operations

Solution

1. Step 1: Understand concurrency needs for readers and writers

   Multiple readers can safely access shared data simultaneously, but writers need exclusive access to avoid conflicts.

2. Step 2: Choose appropriate lock type

   A read-write lock allows many readers at once but only one writer, balancing concurrency and safety efficiently.

3. Final Answer:

   Use a read-write lock allowing concurrent reads but exclusive writes -> Option A

4. Quick Check:

   Read-write lock fits multiple readers, single writer = B [OK]

Hint: Read-write locks allow many readers, one writer [OK]

Common Mistakes:

• Using simple mutex reduces concurrency
• Ignoring need for exclusive write access
• Relying on no locks causes data races