19.2.5 Deadlock
If the necessary synchronization requires acquiring more than one lock, there are some special considerations that need to be taken into account. For example, the code below implements the transfer of funds from one bank account to another. The code assumes there is a separate semaphore lock for each account and since it needs to adjust the balance of two accounts, it acquires the lock for each account. Consider what happens if two customers try simultaneous transfers between their two accounts. The top customer will try to acquire the locks for accounts 6005 and 6004. The bottom customer tries to acquire the same locks, but in the opposite order. Once a customer has acquired both locks, the transfer code will complete, releasing the locks. But what happens if the top customer acquires his first lock (for account 6005) and the bottom customer simultaneously acquires his first lock (for account 6004). So far, so good, but now each customer will be not be successful in acqu...