Identify and fix deadlock in locked code

Q: Identify and fix deadlock in locked code

This question evaluates understanding of concurrency control, deadlock diagnosis, and mutex-based synchronization in multithreaded programs. It is commonly asked to assess reasoning about inter-thread interactions and trade-offs in system design, is categorized under System Design and concurrent programming, and primarily tests practical application of debugging and mitigation strategies rather than purely conceptual theory.

Q: How do I approach System Design interview questions?

System Design questions require understanding of core concepts and practice. PracHub provides solutions with explanations to help you master system design interviews.

Q: What difficulty level is this interview question?

This is a medium difficulty System Design question, commonly asked during Onsite rounds at Box.

Q: What role is this question designed for?

This question is commonly asked for Software Engineer candidates at Box during technical interviews.

Question

Multithreaded Deadlock: Diagnose and Fix

Context

You have concurrent code that acquires multiple locks and occasionally deadlocks. The underlying issue is likely inconsistent lock acquisition order across threads. The goal is to identify the exact deadlock scenario and propose robust fixes. Assume two shared resources protected by two locks (A and B), and that different threads may acquire them in different orders.

Example Code (C++)

std::mutex mA;
std::mutex mB;

void thread1() {
  std::unique_lock<std::mutex> lA(mA);
  // ... do some work
  std::unique_lock<std::mutex> lB(mB);
  // critical section using A and B
}

void thread2() {
  std::unique_lock<std::mutex> lB(mB);
  // ... do some work
  std::unique_lock<std::mutex> lA(mA);
  // critical section using B and A
}

Tasks

Identify the precise deadlock scenario (the interleaving leading to deadlock).
Propose code changes to prevent deadlocks.
Discuss techniques and trade-offs: consistent lock ordering, try-lock with backoff, timeouts, and changing lock granularity.

Identify and fix deadlock in locked code

Quick Overview

Multithreaded Deadlock: Diagnose and Fix

Context

Example Code (C++)

Tasks

Solution

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