Multithreaded Deadlock: Diagnosis and Resolution

This problem involves diagnosing and resolving a deadlock scenario in concurrent code. You are given a context where multiple threads acquire shared resources protected by multiple locks, and due to inconsistent lock acquisition orders, deadlocks occasionally occur.

Scenario

Assume two shared resources, A and B, each protected by its own std::mutex (mA and mB respectively). Different threads may attempt to acquire these locks in varying sequences. An example in C++ is provided:

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

1. Identify the Deadlock: Describe the precise sequence of operations (interleaving) between thread1 and thread2 that leads to a deadlock.
2. Propose Code Fixes: Provide specific code changes to the example to prevent deadlocks from occurring.
3. Discuss Techniques and Trade-offs: Elaborate on various strategies for deadlock prevention and resolution, including:
   • Consistent lock ordering
   • Using try_lock with backoff mechanisms
   • Implementing lock timeouts
   • Modifying lock granularity