GATE CS 2000 Q2.23 — Deadlock

Deadlock prevention schemes work by ensuring that at least one of the four necessary Coffman conditions for deadlock cannot hold:

1. Mutual Exclusion
2. Hold and Wait
3. No Preemption
4. Circular Wait

Let us analyze each option against these conditions:

• Option A: "Release all resources before requesting a new resource."
  This prevents the Hold and Wait condition. Since a process must release all its currently held resources before making a new request, it never holds any resources while waiting for another. Thus, this is a valid deadlock prevention scheme.
• Option B: "Number the resources uniquely and never request a lower numbered resource than the last one requested."
  This is the classic Circular Wait prevention scheme (hierarchical resource ordering). By enforcing a strict total ordering on resource requests, a cycle in the resource allocation graph cannot form. Thus, this is a valid deadlock prevention scheme.
• Option C: "Never request a resource after releasing any resource."
  This rule does not prevent any of the deadlock conditions. For example, process $P_1$ can hold resource $R_1$ and request $R_2$, while process $P_2$ holds $R_2$ and requests $R_1$. Since neither process has released any resource yet, they do not violate this rule, yet they are in a deadlock. Thus, this is not a valid deadlock prevention scheme.
• Option D: "Request and all required resources be allocated before execution."
  This is another way to prevent the Hold and Wait condition. A process must request and receive all its required resources at once before starting execution. If even one resource is unavailable, it gets none. Thus, this is a valid deadlock prevention scheme.

Therefore, C is the correct option.