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.)

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As the hint says, any situation in which the resources on which locks 
must be obtained are not all known in advance can be subject to deadlock 
problems which require more advanced techniques to resolve.