GATE Overflow Test Series | Operating Systems | Test 1 | Question: 5

Question

Which of the following is/are correct about deadlocks in an Operating System? (Mark all the appropriate choices)

• A. If a system is in a safe state, it will never enter a deadlock after a sequence of resource allocations. 
• B. If a system is in an unsafe state, it will eventually enter a deadlock.
• C. Deadlock prevention is stricter that deadlock avoidance.
• D. Deadlock prevention ensures better resource utilization than deadlock avoidance.

Comments

• If a system is in a safe state $\implies$ no deadlocks
• If a system is in an unsafe state $\implies$ possibility of deadlock
• Deadlock Avoidance $\implies$ ensures that a system will never enter an unsafe state.

Ref:https://www.cs.odu.edu/~cs471w/spring10/lectures/Deadlocks.htm 

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Actually Safe state means we have a safe sequence following which our system can never enter into deadlock state.
But what if we allocate resources in a way other than safe sequence , then deadlock may happen. Considering this option A is right here .

Answer 1

A safe state means there is a safe sequence of possible resource allocations which guarantees deadlock freedom. But if we follow an unsafe sequence, there can be a deadlock.

If a system is in an unsafe state, then there is not any possible safe sequences for resource allocation. But this still does not guarantee that there is a deadlock or there will be a deadlock. For example a process may free up a resource even without finishing its execution. So, both options A and B are false.

Deadlock prevention is more stricter than deadlock avoidance. In deadlock prevention, we need to ensure one of the four necessary conditions of deadlock does not occur. So, it may be the case that a resource request might be rejected even if the resulting state is safe. (One example, is when we impose a strict ordering for the processes to request resources).

Deadlock avoidance is less restrictive than deadlock prevention. Deadlock avoidance is like a police man and deadlock prevention is like a traffic light. The former is less restrictive and allows more concurrency. For the same reason, deadlock prevention ensures lesser resource utilization than deadlock avoidance. Option C is true and option D false.

Correct answer: C

Comments

@ayush.5 option A is an implication  i.e if system is in safe state then ..(other statement) . why to consider unsafe state if already it is conditional that upon the safe state condition is satisfied , system will never enter a deadlock after a sequence of resource allocations.  option A should be true .

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How A is incorrect?

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A safe state is one in which there is at least one sequence of resource allocations to processes that does not result in a deadlock (i.e., all of the processes can be run to completion). An unsafe state is, of course, a state that is not safe.

- O.S. William Stallings

A state is safe if the system can allocate resources to each process (up to its maximum) in some order and still avoid a deadlock. More formally, a system is in a safe state only if there exists a safe sequence.

- O.S. Galvin

All resource allocation sequences in a safe state may not be “safe sequences”. So even if a system is in the safe state there may exists a sequence which leads to unsafe state, which in turn, may lead to deadlock.