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.</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.</p> Signup and view all the answers

    What is the maximum resource demand of thread T0?

    <p>10</p> Signup and view all the answers

    How many resources are currently held by all threads combined?

    <p>9</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.</p> Signup and view all the answers

    Which thread holds the least number of resources currently?

    <p>T1</p> Signup and view all the answers

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

    <p>5</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.</p> Signup and view all the answers

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

    <p>3</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</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</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.</p> Signup and view all the answers

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

    <p>12</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</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</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.</p> Signup and view all the answers

    Which resources is preemption generally not applicable to?

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

    What happens if a resource allocation graph has no cycles?

    <p>No deadlock can occur</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.</p> Signup and view all the answers

    What is one way an operating system can handle deadlocks?

    <p>Detect and recover from deadlocks</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.</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</p> Signup and view all the answers

    What is a possible consequence of invalidating circular wait?

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

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

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

    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

    Description

    This quiz tests your understanding of deadlock situations in operating systems, including their characteristics, conditions for occurrence, and methods for handling them. Questions cover concepts like the Banker’s algorithm, resource allocation, and safe states among threads.

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