Definition
A deadlock is a condition in computer science and concurrent computing systems whereby two or more processes are unable to proceed, as each one is waiting for the other to release a resource. This leads to a standstill where no process can continue its execution.
Examples
- Database Deadlock: Consider two transactions in a database system, Transaction A and Transaction B. If Transaction A locks resource R1 and waits to acquire resource R2 locked by Transaction B, and Transaction B is waiting for resource R1 held by Transaction A, a deadlock occurs.
- Printer and Fax Machine: If one process holds a lock on a printer and waits for a fax machine, while another process holds a lock on the fax machine and waits for the printer, neither can proceed.
- Dining Philosophers Problem: This classic synchronization problem involves philosophers sitting at a table with a fork between each pair. If every philosopher picks up the fork on their right simultaneously, no one can eat, leading to a deadlock.
Frequently Asked Questions
Q1: What are the necessary conditions for a deadlock?
Answer: There are four necessary conditions for a deadlock known as the Coffman conditions:
- Mutual Exclusion: At least one resource must be held in a non-shareable mode.
- 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 pre-empted; they must be released voluntarily by the process holding them.
- Circular Wait: A circular chain of processes exists, where each process holds at least one resource and is waiting for a resource held by the next process in the chain.
Q2: How can deadlocks be prevented?
Answer: Deadlocks can be prevented by ensuring that at least one of the Coffman conditions cannot hold. Methods include:
- Deadlock Prevention: Eliminating the possibility of one of more of the conditions from occurring.
- Deadlock Avoidance: Ensuring that a system will never enter a deadlocked state by careful resource allocation.
- Deadlock Detection and Recovery: Allowing the system to enter a deadlock state and then detecting and recovering from it.
Q3: What are some deadlock detection and recovery techniques?
Answer: Deadlock detection can be performed using resource allocation graphs or wait-for graphs to identify cycles indicating a deadlock. Recovery techniques include:
- Terminating one or more of the processes involved in the deadlock
- Preempting some resources from the processes
Q4: What is the difference between a deadlock and a livelock?
Answer: While both are conditions where progress is not made, a deadlock is a state where processes are stuck waiting indefinitely for each other, and a livelock is a state where processes continue to change state in response to each other but still fail to progress.
Related Terms
- Concurrency: The simultaneous execution of multiple interactions or processes.
- Semaphore: A variable or abstract data type used to control access to a common resource by multiple processes in a concurrent system.
- Critical Section: A part of the program that cannot be concurrently accessed by more than one process.
- Mutex: An abbreviation for “mutual exclusion,” it is a construct that prevents multiple processes from accessing the same resource simultaneously.
Online References
Suggested Books for Further Studies
- Operating Systems: Internals and Design Principles by William Stallings
- Modern Operating Systems by Andrew S. Tanenbaum
- Concurrency in Go: Tools and Techniques for Developers by Katherine Cox-Buday
Fundamentals of Deadlock: Computer Science Basics Quiz
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