Which of the following is not a valid deadlock prevention scheme ?

Question

Accepted Answer

The correct option is (3):
Deadlock prevention is achieved by restricting the way processes request resources to ensure that at least one of the four necessary conditions for deadlock (Mutual Exclusion, Hold and Wait, No Preemption, and Circular Wait) can never occur. The policy "Never request a resource after releasing any resource" is not a standard, valid prevention scheme. This rule is overly restrictive, effectively making it impossible for a process to continue with a multi-phase execution that requires different resources at different times. It does not directly map to negating any of the four fundamental deadlock conditions in a sensible or practical operating system design, which is why it is the one that is not a valid scheme.

The incorrect options are (1), (2), and (4) because they are all established deadlock prevention methods:

The incorrect option is (4):
The strategy "Request and all required resources be allocated before execution" is a valid technique to prevent the **Hold and Wait** condition. The Hold and Wait condition states that a process must be holding at least one resource while simultaneously waiting for another. By forcing a process to request all of its necessary resources at the beginning of its execution, and only starting the process once all are available, the system ensures that the process never enters a state where it is holding resources while waiting for an additional one.

The incorrect option is (2):
The scheme "Number the resources uniquely and never request a lower numbered resource than the last one requested" is a classical method to prevent the **Circular Wait** condition. This method imposes a total ordering on all resource types in the system. A process is only allowed to request resources in an increasing order of enumeration. This one-way street for resource requests breaks any possibility of forming a circular chain of processes, which is a key requirement for deadlock.

The incorrect option is (1):
The rule "Release all resources before requesting a new resource" is another valid mechanism used to prevent the **Hold and Wait** condition. This is an alternative to the 'request all at once' scheme. Under this policy, if a process already holds some resources but needs another one, it must first release *all* currently held resources. It then re-requests the full set of resources (the ones it just released plus the new one) in a single atomic operation. This ensures that the process is not 'holding' resources while 'waiting' for a new one.