Anh Trần Tuấn1. Concept
Deadlock is a frequent issue in multithreaded programming where two or more processes or threads become stuck, each waiting for the other to finish, creating a standstill that prevents any progress.
Example of Deadlock:
Imagine you have two processes, A and B, and two resources, X and Y. The processes request resources in the following sequence:
Process A acquires resource X and requests resource Y.
Process B acquires resource Y and requests resource X.
In this scenario, process A cannot proceed because it is waiting for resource Y, while process B is holding resource Y and waiting for resource X. Conversely, process B cannot proceed because it is waiting for resource X, which process A is currently holding. As a result, both processes are unable to continue and are stuck in a deadlock state.
2. Deadlock Conditions
To better understand deadlock, consider the necessary conditions for a deadlock to occur:
Mutual Exclusion : This means that only one process can use a resource at a time. If multiple processes try to access the same resource simultaneously, a deadlock can occur.
Hold and Wait : A process is holding at least one resource and is waiting to acquire additional resources that are currently being held by other processes. This creates a dependency between processes.
No Preemption : The operating system cannot forcibly take a resource away from a process that is currently holding it. This prevents the system from breaking the deadlock.
Circular Wait : A circular chain of processes exists where each process is waiting for a resource held by the next process in the chain. This creates a deadlock because no process can proceed.
3. Deadlock Handling
3.1 Detection and Recovery
Detection and recovery is a method that aims to detect a deadlock state when it occurs and then take measures to restore the system to a normal operating state. This process typically involves:
+ Monitoring and Detection : The system continuously monitors processes and resources to detect signs of a deadlock. This is often done by building resource and process graphs or using deadlock detection algorithms such as the Wait-For Graph algorithm.
+ Recovery : When a deadlock is detected, the system will take recovery measures to resolve the situation. Recovery measures may include:
+ Process Termination : Terminating one or more processes to release the resources they are holding. This can lead to data loss or unfinished work.
+ Resource Preemption : Automatically reclaiming resources from blocked processes, which may require saving the process state and restoring it after the resource is released.
3.2 Deadlock Avoidance
Deadlock avoidance is a method that aims to prevent deadlocks from occurring by designing the system so that the conditions that cause deadlocks cannot occur. One of the most common ways to avoid deadlocks is to use the Banker's algorithm:
This is a safe resource allocation algorithm that checks whether granting a resource request will lead to an unsafe state. Before allocating a resource, the system checks if the current state can lead to a deadlock, and only allocates the resource if the system remains in a safe state.
3.3 Deadlock Prevention
Deadlock prevention is a method of designing a system so that the necessary conditions for a deadlock never occur. Some strategies include:
+ Resource Ordering : Ensuring that all processes request resources in the same order. For example, if process A requests resource X before Y, then all other processes must also request resource X before Y. This helps to avoid circular wait, one of the necessary conditions for a deadlock.
+ Periodic Resource Allocation : Periodic resource allocation involves managing resources systematically to reduce the risk of deadlocks:
Resource Management : Using periodic resource management mechanisms to ensure resources are allocated and released in a specific order. This may involve establishing resource allocation rules, dividing resources into different types, and ensuring that resources are allocated and released in an orderly manner to minimize the risk of deadlocks.
4. Java Example
public class DeadlockDemo {
private static final Object lock1 = new Object();
private static final Object lock2 = new Object();
public static void main(String[] args) {
Thread thread1 = new Thread(() -> {
synchronized (lock1) {
System.out.println("Thread 1: Locked lock1");
try { Thread.sleep(100); } catch (InterruptedException e) {}
synchronized (lock2) {
System.out.println("Thread 1: Locked lock2");
}
}
});
Thread thread2 = new Thread(() -> {
synchronized (lock2) {
System.out.println("Thread 2: Locked lock2");
try { Thread.sleep(100); } catch (InterruptedException e) {}
synchronized (lock1) {
System.out.println("Thread 2: Locked lock1");
}
}
});
thread1.start();
thread2.start();
}
}
Code Introduction:
Creating Resources: lock1 and lock2 are two objects used as synchronization resources (locks) in the code.
Two Threads: Two threads (thread1 and thread2) attempt to acquire the resources in different orders:
Thread 1: First acquires lock1, then tries to acquire lock2.
Thread 2: First acquires lock2, then tries to acquire lock1.
Explanation of Deadlock:
Thread 1 starts and acquires lock1.
Meanwhile, Thread 2 starts and acquires lock2.
Thread 1 continues and tries to acquire lock2 but is blocked because lock2 is already held by Thread 2.
Thread 2 continues and tries to acquire lock1 but is blocked because lock1 is already held by Thread 1.
As a result, both threads are unable to proceed because they are waiting for each other to release the resource they need, leading to a deadlock situation.
5. Conclude
Deadlock is a serious issue in multithreading programming that can cause system congestion and reduced performance. Understanding the conditions that lead to deadlocks and applying prevention and resolution techniques is crucial to ensuring a smooth and efficient system operation.
If you have any questions about deadlocks or how to address them, feel free to ask!
