multithreading · high

Deadlock & the 4 Coffman Conditions

Two threads lock A and B in opposite orders, forming a circular wait. Watch all four conditions appear - then break one to fix it.

The code

std::mutex A, B;

void t1() {
    std::lock_guard la(A); // T1 locks A
    std::lock_guard lb(B); // T1 then wants B
}
void t2() {
    std::lock_guard lb(B); // T2 locks B
    std::lock_guard la(A); // T2 then wants A
}

What this lesson walks through

1. 01Two threads need the same two mutexes
2. 02T1 locks A
3. 03T2 locks B
4. 04T1 holds A and now wants B
5. 05T2 holds B and now wants A
6. 06Circular wait -> deadlock
7. 07Fix: one global lock order (A before B)

Two threads each need BOTH locks A and B - but they take them in opposite orders. That ordering mismatch is the whole problem.