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Introduction to Deadlock

Deadlock is a situation where a set of processes are unable to proceed because each process is waiting for a resource held by another process, preventing any of them from proceeding.

Deadlock is a common issue in resource-sharing environments, such as in multi-threaded operating systems, where multiple processes or threads require exclusive access to resources like memory, files, and I/O devices.

Deadlock characterization describes the specific conditions and factors that lead to deadlock in computer systems, particularly within the context of operating systems.

Necessary Conditions for Deadlock in OS (Coffman Conditions) :

The necessary conditions for deadlock, also known as the Coffman conditions, are four conditions that must all hold simultaneously for deadlock to occur in a system.

  • Mutual Exclusion
  • Hold and Wait
  • No Preemption
  • Circular Wait

This condition requires that resources involved in a deadlock must be non-shareable; in other words, only one process can use a resource at any given time. If another process requests a resource currently in use, it must wait until the resource is released by the holding process.

Example:

  • Imagine two processes, P1 and P2, both need access to a single printer to print documents. Since only one process can use the printer at any time, if P1 is using the printer, P2 must wait until P1 is finished.
  • If P2 also needs access to another resource, say a scanner, which P1 is already holding, this waiting can lead to a potential deadlock scenario if other conditions are met.

Implication: To prevent deadlock, some systems try to avoid mutual exclusion by making certain resources shareable. For example, resources like read-only files can be shared among multiple processes without deadlock risk.

This condition occurs when a process holding one or more resources is allowed to request additional resources without releasing the ones it currently holds. This situation can create a chain of dependencies among processes as each one holds resources while waiting for others.

Example:

  • Suppose P1 has acquired a printer and is now waiting for a scanner, while P2 has acquired the scanner and is waiting for the printer.
  • P1 is holding the printer and waiting for the scanner, and P2 is holding the scanner and waiting for the printer. Neither process can proceed until it gets both resources, leading to deadlock if they don’t release the resources they already hold.

Implication: Deadlock can be prevented by not allowing processes to hold resources while waiting. For instance, some systems require processes to request all needed resources at once, which can eliminate the Hold and Wait condition. However, this may lead to reduced resource utilization.

The no-preemption condition specifies that resources cannot be forcibly taken away from processes holding them. A process can only release a resource voluntarily, typically after it has completed its task.

Example:

  • Let’s say P1 is holding a resource like a network socket. If P2 also requires the same socket and P1 has already acquired it, P2 must wait until P1 voluntarily releases the socket. The system cannot forcibly take the socket from P1 to give it to P2.
  • This condition becomes problematic in scenarios where processes hold resources for an extended period or indefinitely, leading to potential deadlock.

Implication: In some systems, preemption is allowed for specific resources to avoid deadlock. For example, a process holding a CPU can be preempted by the operating system to allow another process to run. However, many resources (like files or network connections) cannot be preempted without risking data inconsistency.

Circular wait occurs when a closed chain of processes exists, where each process is waiting for a resource held by the next process in the chain. This circular dependency creates a cycle that prevents any process in the chain from proceeding.

Example:

  • Consider three processes P1, P2, and P3, and three resources R1, R2, and R3.
  • P1 holds R1 and is waiting for R2 (held by P2).
  • P2 holds R2 and is waiting for R3 (held by P3).
  • P3 holds R3 and is waiting for R1 (held by P1).
  • This forms a circular chain: P1 → P2 → P3 → P1, with each process waiting for a resource held by the next. Since no process can release its resource until it acquires the next one, a deadlock occurs.

Implication: Deadlock can be prevented by ensuring that a circular wait cannot happen. One method is to impose a strict ordering on resource requests. For example, processes must request resources in a predefined order (such as numerically or alphabetically). This way, even if a process holds some resources, it will only request resources that come later in the order, preventing a circular dependency.

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