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

Lock ordering just means that you prevent deadlocks by obtaining locks in a fixed order, and do not obtain locks again after you start unlocking.

I do not think the order of unlocks makes any difference here (in fact, it should be beneficial to release a lock as soon as possible, even if out of order)

Answer 2

Your example isn't going to deadlock with itself ever. Unlocking in reverse order isn't important, it's locking in a consistent order. This will dead lock, even though unlocks are in reverse order

Thread 1

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

Thread 2

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

Answer 3

I do not think deadlock would happen here. The general deadlock concept is one thread waits for some resource locked by other thread, while other thread needs resource locked by first thread to finish and release resource needed by first.

Further reading

Answer 4

The order of unlock will not affect how prone your system is to deadlock, however there's one reasons to think about the order of unlock:

In order to avoid deadlocks you must make sure that your lock/unlocks are paired, in that you never miss an unlock. As a stylistic approach, by conspicuosly having blocks of code that are responsible for a particualr lock it's much easier to visually identifiy that locks and unlocks are paired. The end-effect is that clearly correct code will probably take and release the locks as you describe.