Fix race condition in concurrent deposit
Company: xAI
Role: Software Engineer
Category: Software Engineering Fundamentals
Difficulty: medium
Interview Round: Technical Screen
## Concurrent bank account debugging (thread safety)
You are given a simple `BankAccount` object that is used concurrently from multiple threads. Two deposits run at the same time:
```python
account = BankAccount(0)
with ThreadPoolExecutor(max_workers=2) as executor:
futures = [
executor.submit(account.deposit, 500),
executor.submit(account.deposit, 700),
]
```
A simplified implementation of `deposit()` looks like this:
```python
class BankAccount:
def __init__(self, balance=0):
self.balance = balance
def deposit(self, amount):
new_balance = self.balance + amount # Read
time.sleep(0.1) # Delay
self.balance = new_balance # Write
```
### Tasks
1. Explain why this code can produce an incorrect final balance (e.g., `700` instead of the expected `1200`).
2. Identify the underlying concurrency bug(s).
3. Propose and describe a correct fix that ensures thread safety for `deposit()`.
4. Mention any important caveats or alternatives (e.g., performance trade-offs, other synchronization approaches).
Overview: This question evaluates understanding of concurrency and thread safety, specifically recognizing race conditions and reasoning about synchronization for shared mutable state in multithreaded programs.
Read the full xAI Software Engineer interview experience this question came from
Community answers
Answer by yuki786
class BankAccount:
def init(self, balance=0):
self.balance = balance
self.lock = threading.lock()
def deposit(self, amount):
with self.lock():
new_balance = self.balance + amount # Read
self.balance = new_balance # Write