What is deadlock in the context of multithreaded programs?

Deadlock is a situation where two or more threads cannot continue execution because each is waiting for a resource held by another thread in the same group.

The main conditions for the occurrence of deadlock (Coffman's conditions):

1. Mutual Exclusion: A resource can only be used by one thread at a time.
2. Hold and Wait: A thread holding one or more resources waits to acquire additional resources that are being held by other threads.
3. No Preemption: Resources cannot be forcibly taken away from a thread holding them; they can only be released voluntarily.
4. Circular Wait: There exists a circular chain of threads where each thread is waiting for a resource held by the next thread in the chain.

Example of a simple scenario with two threads and two mutexes:

// Execution plan:
// Thread 1 locks mutex1.
// Thread 2 locks mutex2.
// Thread 1 attempts to lock mutex2 (waits).
// Thread 2 attempts to lock mutex1 (waits).
// Both threads end up waiting for each other.

std::mutex mutex1;
std::mutex mutex2;

void thread_func1() {
    mutex1.lock();
    // Do something...
    mutex2.lock(); // Possible deadlock here
    // Do something...
    mutex2.unlock();
    mutex1.unlock();
}

void thread_func2() {
    mutex2.lock();
    // Do something...
    mutex1.lock(); // Possible deadlock here
    // Do something...
    mutex1.unlock();
    mutex2.unlock();
}

Methods to prevent or detect deadlocks:

• Preferred method - avoidance:
  • Always acquire mutexes in a fixed, globally defined order.
  • Use tools that manage the acquisition of multiple mutexes atomically (e.g., std::lock with multiple arguments).
  • Avoid holding mutexes for a long time.
  • Attempt to acquire a mutex with a timeout (try_lock_for).
• Detection and recovery:
  • Develop mechanisms to detect circular wait conditions and forcibly release resources (often difficult to implement correctly in general-purpose multithreaded systems).
  • Use specialized analysis tools (static analysis, dynamic analysis).