Operating Systems - Deadlocks
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Questions and Answers

What must a thread do if its request for a resource cannot be granted immediately?

  • Release any resources currently held
  • Request a different resource
  • Wait for the resource to become available (correct)
  • Terminate the process
  • Which of the following actions is NOT a part of the resource utilization process by a thread?

  • Using the resource
  • Releasing the resource
  • Preemptively blocking resources (correct)
  • Requesting the resource
  • What system calls are examples for the request and release of resources?

  • allocate() and deallocate()
  • get() and return()
  • open() and flush()
  • request() and release() (correct)
  • What does the wait() operation on a semaphore typically involve?

    <p>Blocking the process until the semaphore is available</p> Signup and view all the answers

    What happens if a thread requests more resources than are available in the system?

    <p>The request will be denied until sufficient resources are available</p> Signup and view all the answers

    What occurs when a thread requests resources that are not available?

    <p>The thread enters a waiting state.</p> Signup and view all the answers

    In a deadlock situation, what is true about the processes involved?

    <p>Each process is waiting for an event that can only be caused by another process.</p> Signup and view all the answers

    Which of the following best describes the system model in the context of deadlocks?

    <p>The system has a finite number of resources distributed among competing threads.</p> Signup and view all the answers

    What can be classified as a resource type in a system model?

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

    What is one primary concern related to deadlock situations?

    <p>The acquisition and release of resources.</p> Signup and view all the answers

    How many instances does each resource type have in a system model?

    <p>A finite number specific to each type.</p> Signup and view all the answers

    What happens to threads in a deadlocked state?

    <p>They wait indefinitely for resources held by others.</p> Signup and view all the answers

    What is not a characteristic of deadlock?

    <p>All processes are actively running.</p> Signup and view all the answers

    Signup and view all the answers

    Study Notes

    Operating Systems - Deadlocks

    • Deadlocks occur in multi-programming environments, where several threads compete for a finite number of resources.
    • A thread requests resources; if they're unavailable, the thread enters a waiting state.
    • Sometimes, a waiting thread can't change state because the requested resources are held by other waiting threads.
    • A deadlock occurs when every process in a set of processes is waiting for an event that can only be caused by another process in the set.
    • A set of threads is in a deadlocked state when each thread waits for an event caused by another thread in the set.

    System Model

    • Systems have a finite number of resources to be distributed among competing threads.
    • Resource types are R1, R2, ..., Rm.
    • Examples of resources include CPU cycles, memory, I/O devices.
    • Synchronization tools (e.g., mutex locks, semaphores) are also resources.
    • Each resource type (e.g., R₁) has a number of instances (e.g., W₁).
    • Processes utilize resources in these steps:
      • Request: The thread asks for a resource.
      • If the request can't be granted immediately, the thread waits.
      • Use: The thread operates on the resource.
      • Release: The thread releases the resource after usage.

    Deadlock Characterization

    • Deadlocks arise when these four conditions hold simultaneously:
      • Mutual Exclusion: Only one process can use a resource at a time.
      • Hold and Wait: A process holding at least one resource is waiting to acquire additional resources held by other processes.
      • No Preemption: Resources cannot be taken away from a process holding them until the process releases them.
      • Circular Wait: There exists a cycle of waiting processes, where each process is waiting for a resource held by the next process in the cycle.

    Deadlock Prevention

    • Invalidate one of the four conditions to prevent deadlocks.

      • Mutual Exclusion: Break mutual exclusion for those resources that can be shared.
      • Hold and Wait: Require a process to request all its needed resources at once, preventing it holding some while waiting for others.
      • No Preemption: Allow the system to preempt resources from a process that's waiting and give them to a process that's requesting them.
      • Circular Wait: Impose a total ordering on all the resources. Requiring each process to seek resources in an increasing order prevents circular waits.

    Deadlock Avoidance

    • Systems must have a priori information about how resources are to be requested.
    • Deadlock avoidance algorithms dynamically examine the resource allocation state to prevent circular waits.
    • Resource allocation states use available and allocated resources and maximum demands of processes.

    Safe State

    • A system is in a safe state if there exists a safe sequence of all processes (e.g., P₁, P₂, ..., Pn).
    • The resources needed by P₁ can be granted using current available resources and resources of processes with indices < i.
    • If a process requests resources when they are unavailable in the safe state, the process can wait until the other processes have completed and can obtain the resources it requires in a safe order.

    Unsafe State

    • If a system is in an unsafe state, there is a possibility of deadlock.
    • However, not all unsafe states lead to deadlocks.

    Resource Allocation Graph

    • Deadlocks can be analyzed using a system resource allocation graph, which consists of a set of vertices (V) and edges (E) representing threads (T) and resource instances (R).
    • A request edge (T1 → R;) signifies that T1 has requested R₁ and is waiting.
    • An assignment edge (R; → T1) indicates that R₁ has been allocated to T₁.
    • Cycle in the graph might imply potential deadlock.
    • If there are no cycles, then the system is in a safe state.

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    Description

    This quiz explores the concept of deadlocks in operating systems, particularly in multi-programming environments. You'll learn about how threads compete for resources and the conditions under which deadlocks occur. Test your understanding of resource allocation and synchronization tools.

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