SICP Exercise 3.49 deadlock avoidance does not work

Exercise 3.49.  Give a scenario where the deadlock-avoidance mechanism described above does not work. (Hint: In the exchange problem, each process knows in advance which accounts it will need to get access to. Consider a situation where a process must get access to some shared resources before it can know which additional shared resources it will require.)

EXPLANATION

Consider the case where we have two bank branches in different locations and an account can be accessed at each branch. For fast local access, we allow withdrawals and deposits at each location and design a way to keep the account balance at both locations synchronized. Concurrent processes can access and change the balance in either location. In order to keep the balance the same in both locations, we write a procedure similar to 'synchronized-exchange' but which first updates the local balance and then the remote balance. This procedure acquires a lock on the local account first and then the remote account. This 'sychronized-update' procedure can encounter a deadlock if it runs simultaneously in two locations on the same account.

Our deadlock avoidance mechanism will fail here because we do not know in advance, in what order to acquire locks on the accounts.

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