concurrency - Would you explain lock ordering?

Question

I learned that I should unlock reverse order to lock order. For example.

A.lock();
B.lock();
B.unlock();
A.unlock();

But, what happen if I did like this :

A.lock();
B.lock();
A.unlock();
B.unlock();

I try to make a deadlock scenario, but if I always lock A earlier then B, then I don't know how deadlock would happen. Would you help me?

Answer 1

In the simple case given, unlocking in the reverse order is not necessary to avoid a deadlock.

However, as the code gets more complicated, unlocking in the reverse order helps you maintain proper lock ordering.

Consider:

A.lock();
B.lock();
Foo();
A.unlock();
Bar();
B.unlock();

If Bar() attempts to reacquire A, you've effectively broken your lock ordering. You're holding B and then trying to get A. Now it can deadlock.

If you unlock in the reverse order style (which is very natural if you use RAII):

A.lock();
B.lock();
Foo();
B.unlock();
Bar();
A.unlock();

then it doesn't matter if Bar() attempts to take a lock, as lock ordering will be preserved.