Deadlock Explanation

Definition

A deadlock is a situation in which two or more computing tasks are each waiting for the other to release a resource, so neither proceeds. This typically occurs when each task locks a resource that the other task needs. Because both tasks are waiting, they are blocked indefinitely.

Conditions for Deadlock

There are four necessary conditions for a deadlock to occur, known as the Coffman conditions:

Mutual Exclusion: At least one resource must be held in a non-sharable mode; i.e., only one task at a time can use the resource. If another task requests that resource, the requesting task must be delayed until the resource has been released.
Hold and Wait: A task must hold at least one resource and be waiting to acquire additional resources that are currently being held by other tasks.
No Preemption: Resources cannot be forcibly taken away from a task. A resource can only be released voluntarily by the task holding it after that task has completed its task.
Circular Wait: A set of waiting tasks exists such that task 1 is waiting for a resource held by task 2, task 2 is waiting for a resource held by task 3, and so on, until task n is waiting for a resource held by task 1.

If all four of these conditions hold simultaneously in a system, then a deadlock can occur.

Example

Consider two tasks (Task A and Task B) and two resources (Resource X and Resource Y).

Task A acquires Resource X.
Task B acquires Resource Y.
Task A now requests Resource Y, but it is currently held by Task B. So, Task A waits.
Task B now requests Resource X, but it is currently held by Task A. So, Task B waits.

At this point, neither task can proceed because each is waiting for the other to release a resource. This is a deadlock.

import threading
import time

# Define two locks
lock_a = threading.Lock()
lock_b = threading.Lock()


def task_a():
    print("Task A: Trying to acquire lock_a...")
    with lock_a:
        print("Task A: Acquired lock_a")
        time.sleep(1)  # Simulate some work

        print("Task A: Trying to acquire lock_b...")
        with lock_b:
            print("Task A: Acquired lock_b")
            print("Task A: Finished")


def task_b():
    print("Task B: Trying to acquire lock_b...")
    with lock_b:
        print("Task B: Acquired lock_b")
        time.sleep(1)  # Simulate some work

        print("Task B: Trying to acquire lock_a...")
        with lock_a:
            print("Task B: Acquired lock_a")
            print("Task B: Finished")


# Create and start the threads
thread_a = threading.Thread(target=task_a)
thread_b = threading.Thread(target=task_b)

thread_a.start()
thread_b.start()

thread_a.join()
thread_b.join()

print("Both tasks finished (or deadlocked)")

In this example:

task_a acquires lock_a and then tries to acquire lock_b.
task_b acquires lock_b and then tries to acquire lock_a.

If task_a acquires lock_a and task_b acquires lock_b simultaneously, both tasks will block indefinitely, waiting for the other to release the lock. This results in a deadlock.

Deadlock Prevention

Mutual Exclusion: Cannot be completely avoided as some resources are inherently non-sharable.
Hold and Wait: Ensure that tasks request all required resources before they begin execution. This can be achieved by requiring tasks to release all currently held resources before requesting new ones.
No Preemption: If a task is holding some resources and requests another resource that cannot be immediately allocated to it, then all resources currently being held by the task are preempted. These resources are implicitly released, thus allowing them to be allocated to other waiting tasks. The preempted task will be restarted only when it can regain its old resources, as well as the new ones that it requested.
Circular Wait: Impose a total ordering of all resource types and require that each task requests resources in an increasing order of enumeration. For example, if a task needs Resource X and Resource Y, and X is ordered before Y, the task must request X before requesting Y.

Deadlock Detection and Recovery

Detection: The operating system can periodically check for circular wait conditions among tasks. This involves maintaining a resource allocation graph.
Recovery: Once a deadlock is detected, the system can recover by:
- Task Termination: Terminate one or more tasks involved in the deadlock.
- Resource Preemption: Forcibly take resources away from some tasks and give them to others until the deadlock is broken.

Conclusion

Deadlocks are a significant concern in concurrent systems, and understanding their causes and prevention/recovery methods is crucial for building reliable software.

What is a deadlock?