Deadlock in Operating Systems

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Questions and Answers

What is a characteristic of a deadlock situation involving processes?

  • All processes involved can release resources at any time.
  • Processes can eventually make progress if resources are allocated.
  • Processes are waiting indefinitely for an event that can only be caused by one of them. (correct)
  • There are no conditions causing the waiting processes to halt.

Which of the following correctly defines the condition that can lead to deadlock?

  • Processes can access resources in any order spontaneously.
  • Resource allocation can resume without checks.
  • Preemption of resources is allowed, preventing deadlocks.
  • Circular wait must exist among processes. (correct)

Which method is NOT a common approach to handling deadlocks?

  • Deadlock Avoidance
  • Deadlock Detection
  • Deadlock Ignorance (correct)
  • Deadlock Prevention

Applying the Banker’s algorithm can help prevent deadlock in which way?

<p>By granting resources only if they do not lead to an unsafe state. (C)</p> Signup and view all the answers

What situation does the law mentioned in the Deadlock section illustrate?

<p>A deadlock where progression halts until conditions change. (B)</p> Signup and view all the answers

What is the maximum resource demand of thread T0?

<p>10 (C)</p> Signup and view all the answers

How many resources are currently held by all threads combined?

<p>9 (A)</p> Signup and view all the answers

What will happen if T2 requests one more resource under the current system conditions?

<p>Only T1 can proceed afterward. (A)</p> Signup and view all the answers

Which thread holds the least number of resources currently?

<p>T1 (A)</p> Signup and view all the answers

What is the balance of resources after T1 has been allocated the resources it needs?

<p>5 (A)</p> Signup and view all the answers

What can be said about the system when all threads can complete their tasks one after another safely?

<p>The system is in a safe state. (C)</p> Signup and view all the answers

What is the new balance of resources if T2 is allocated 1 more resource?

<p>3 (A)</p> Signup and view all the answers

What is indicated by the presence of a cycle in a resource allocation graph?

<p>Deadlock is possible, depending on resource instances (B)</p> Signup and view all the answers

In the given scenario, which thread can safely proceed if T2 has made a request for an additional resource?

<p>T1 (D)</p> Signup and view all the answers

What must happen if a process holding resources requests another resource that cannot be immediately allocated?

<p>The process releases all currently held resources. (B)</p> Signup and view all the answers

How many total resources are available after T1 finishes execution and releases its resources?

<p>12 (D)</p> Signup and view all the answers

Which of the following methods prevents deadlock by ensuring that the system never enters a deadlocked state?

<p>Deadlock avoidance (B)</p> Signup and view all the answers

In a situation where each resource type has several instances, what can be concluded from a cycle in the resource allocation graph?

<p>Deadlock may occur but is not guaranteed (C)</p> Signup and view all the answers

What happens to preempted resources when a process is unable to acquire its requested resource?

<p>They are added to the list of resources that other threads are waiting for. (A)</p> Signup and view all the answers

Which resources is preemption generally not applicable to?

<p>Mutex locks. (B)</p> Signup and view all the answers

Which of the following statements about deadlock prevention is false?

<p>It allows the system to enter a deadlock state temporarily (A)</p> Signup and view all the answers

How can circular wait be invalidated in a resource allocation system?

<p>By assigning a unique number to each resource and enforcing an order. (C)</p> Signup and view all the answers

What happens if a resource allocation graph has no cycles?

<p>No deadlock can occur (D)</p> Signup and view all the answers

In an ordered resource acquisition system, how should resources be requested?

<p>In a strictly increasing order based on their assigned numbers. (B)</p> Signup and view all the answers

What is one way an operating system can handle deadlocks?

<p>Detect and recover from deadlocks (C)</p> Signup and view all the answers

What is a characteristic of resources that can be preempted?

<p>Their state can easily be saved and restored. (A)</p> Signup and view all the answers

Why could there be a cycle in a resource allocation graph without resulting in deadlock?

<p>Another thread may release resources, breaking the cycle (D)</p> Signup and view all the answers

What is a possible consequence of invalidating circular wait?

<p>Elimination of all potential deadlocks. (D)</p> Signup and view all the answers

What occurs when a resource the thread needs is not available?

<p>The thread can be preempted to free up resources. (C)</p> Signup and view all the answers

In the context of deadlocks, which resource is commonly associated with the issue?

<p>Mutex locks. (B)</p> Signup and view all the answers

Flashcards

Deadlock

A situation where two or more processes are stuck waiting for each other indefinitely, preventing any further progress. Each process is holding a resource that the other process needs, hence creation of a circular dependency.

Resource Holding

A situation where a process is blocked because it is waiting for a resource that is currently held by another process.

Resource Request

A condition where a process can request a resource that's currently held by another process.

No Preemption

A condition where a process cannot release resources that it's already holding until it gets all the resources it needs.

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Circular Wait

A condition where a process can only proceed if it gets all the resources it needs at once, in the same order.

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Resource Allocation Graph

A directed graph representation of resource allocation, where nodes represent processes and resources, and edges represent requests and allocations.

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Cycle in Resource Allocation Graph

A condition where a cycle exists in the resource allocation graph, and each process in the cycle is waiting for a resource held by another process in the cycle. This can lead to deadlock.

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Cycle in Graph Without Deadlock

Even if a cycle exists in the resource allocation graph, there might be no deadlock if at least one process in the cycle can release its resources, allowing the cycle to break.

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Deadlock Prevention

A way to handle deadlocks where the system ensures that the deadlock will never occur by preventing one of the necessary conditions for deadlock (mutual exclusion, hold and wait, no preemption, circular wait).

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Deadlock Detection and Recovery

A way to handle deadlocks where the system allows deadlock to occur, detects it, and then recovers by taking actions like preempting resources, killing processes, or rolling back transactions.

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Ignore Deadlock

An approach to handling deadlocks where it is completely ignored, assuming deadlocks are rare or the cost of detection and recovery is too high.

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No Preemption (Deadlock Prevention)

A situation where a process, holding some resources, requests another resource that is not immediately available. To avoid deadlock, the process releases all resources it currently holds and restarts only when it can regain its old resources and the new one it requested.

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Preemptive Deadlock Prevention

A protocol used to prevent deadlock by preempting resources from other waiting threads. This is often applied to resources with easily saved and restored states like CPU registers or database transactions.

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Resource Holding (Deadlock Prevention)

A condition where resources are not released until a process gets all the resources it needs. This can lead to deadlock.

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Resource Request (Deadlock Prevention)

A condition where a process can request resources held by another process. This can contribute to deadlock.

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Circular Wait (Deadlock Prevention)

A solution to prevent deadlock by imposing a total ordering of resource types. Processes must request resources in an increasing order of this defined sequence.

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Invalidate Circular Wait

Circular wait is a common deadlock prevention technique.

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Ordered Resource Acquisition (Deadlock Prevention)

An algorithm that prevents deadlock by assigning each resource a unique number. Processes must acquire resources in an increasing order of their assigned numbers.

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Ordered Resource Acquisition (cont.) (Deadlock Prevention)

Circular wait is broken by imposing an order on resources.

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Resource Ordering (Deadlock Prevention)

A protocol that ensures processes acquire resources in a specific order. This order can be defined by priority, timestamps, or other factors.

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Safe State

A state where the system can allocate resources to each process in a way that allows them to complete their tasks without getting stuck. This means there's a safe sequence where each process can acquire all its required resources and eventually release them.

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Unsafe State

A state where the system cannot guarantee that all processes will be able to acquire all their required resources. This doesn't always mean a deadlock, but it increases the risk.

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Safe Sequence

A sequence of processes where each process can acquire all its required resources and eventually release them, ensuring that all processes can complete their tasks.

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Safe to Unsafe Transition

A state where a system goes from a safe state to an unsafe state. This usually happens when a process requests a resource, and its allocation creates a situation where no safe sequence exists.

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Maximum Need

The maximum number of resources of each type that a process may require during its execution.

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Currently Held

The number of resources of each type currently held by a process.

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Resource Balance

The difference between the maximum need of a process and the resources currently held by the process.

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Deadlock State

A system where the resources are allocated to processes, but no process can proceed because they are waiting for resources held by other processes in a circular pattern.

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Study Notes

Deadlocks

  • Deadlocks occur when two or more processes are indefinitely waiting for an event that can only be caused by another waiting process.
  • A deadlock can be characterized by four necessary conditions:
    • Mutual exclusion: only one thread can use a resource at a time.
    • Hold and wait: a thread holding at least one resource is waiting to acquire additional resources held by other threads.
    • No preemption: a resource can only be released voluntarily by the thread holding it.
    • Circular wait: there exists a set of waiting threads such that T0 is waiting for a resource held by T1, T1 is waiting for a resource held by T2, etc., and Tn-1 is waiting for a resource held by T0.
  • Deadlocks can be depicted using a resource allocation graph (RAG).
  • A RAG has two types of vertices:
    • Resources
    • Processes
  • Edges in a RAG can be either request or assignment edges.
  • Request edge Ti → Rj indicates that process Ti requests resource Rj.
  • Assignment edge Rj → Ti indicates that resource Rj is allocated to process Ti.
  • If a cycle exists in a RAG, a deadlock has occurred. If the RAG does not have any cycles, there is likely no deadlock.
  • Possible methods for handling deadlocks:
    • Prevention: protocols to prevent the occurrence of deadlocks. This approach ensures the system never enters a deadlocked state.
    • Avoidance: the system is provided information about the future requests to decide whether or not to grant the current request. A safe state exists if an allocation of resources to processes can be completed, which avoids a deadlock situation.
    • Detection: detecting if a deadlock already occurred and recovering.
      • Requires a cycle detection algorithm and a deadlock recovery algorithm.

Deadlock Prevention

  • Preventing deadlocks eliminates one of the four necessary conditions:
    • Mutual Exclusion: Only relevant for non-sharable resources
    • Hold and Wait: Require threads to request and be allocated all resources before execution. Alternatively, a thread must release all held resources before making a new request.
    • No Preemption: If a thread can't acquire a needed resource it will release the resources it currently holds, and then try to acquire the needed and all other resources again.
    • Circular Wait: impose a total ordering on resource types and require that each thread requests resources in an increasing order of enumeration.

Deadlock Avoidance

  • The system is given additional information (i.e., the maximum resources that each process may request) in advance to decide if immediate allocation or delaying the request will lead to a safe state.
  • Techniques for a safe state:
    • Available resources
    • Resource allocation to each thread
    • Future requests (demands) and releases of each thread
  • A safe state means there is a sequence (order) in which all processes can be allocated their requested resources, without causing a deadlock
  • To check if the system is in a safe state, use the Banker's algorithm to look for a safe sequence or series of steps where a process can complete all its requests without preventing another process from completing its requests.

Deadlock Detection

  • An algorithm that examines the system to detect whether a deadlock has occurred.
  • The wait-for graph is built from the resource allocation graph. Resource entries are removed from the graph, and edges are simplified to show waiting relationships.
  • A cycle in the wait-for graph indicates a deadlock.
  • Algorithm:
    • Initialize Work and Finish vectors
    • Initialize Finish[i] to false for all processes
    • Find an unclaimed process i where Finish[i] is false and Request[i] ≤ Work, then update Work to reflect the resources freed by process i
    • Iterate the above until all processes’ Finish variable is true; if Finish[i] is false, a deadlock has occurred.

Recovery from Deadlock

  • Methods to resolve an existing deadlock:
    • Process Termination: Aborting deadlocked processes.
      • Prioritize thread selection
      • Consider resource use, time spent, and how long the thread will take to complete.
      • Prioritize interactive threads which are often more critical
    • Resource Preemption: Deciding which resources and threads to preempt to minimize cost.
      • Include rollback cost in decision making.

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Chapter 8: Deadlocks PDF

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