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
### Which of the following is NOT a necessary condition for a deadlock?
- [x] Preemption is possible.
- [ ] Mutual exclusion exists.
- [ ] Hold and wait.
- [ ] Circular wait exists.
> **Explanation:** For a deadlock to occur, preemption must not be possible. The resources cannot be forcibly taken from the processes holding them.
### What is a simple way to prevent circular wait in resource allocation?
- [ ] Allow processes to wait indefinitely.
- [x] Enforce a strict ordering of resource acquisition.
- [ ] Allow resource preemption.
- [ ] Increase the number of resources.
> **Explanation:** By enforcing a strict ordering of resource acquisition, circular wait can be prevented as processes will always request resources in the pre-defined order, thus avoiding circular dependencies.
### Which of the following conditions can't be true if deadlock is prevented?
- [ ] Hold and wait.
- [ ] Circular wait exists.
- [ ] Mutual exclusion.
- [x] Preemption of resources must occur.
> **Explanation:** Preemption must not be possible for a deadlock to occur. If resources can be preempted, deadlock is prevented.
### What is a livelock?
- [ ] A state where one or more processes are terminated forcibly.
- [ ] A state where multiple processes are blocked indefinitely waiting for resources held by each other.
- [x] A state where processes keep changing states without making progress.
- [ ] A state where a process decrements its own priority.
> **Explanation:** Livelock occurs when processes keep changing their states in response to each other without making progress, unlike deadlock, where processes do not change state waiting for resources.
### What graphical model is used to detect deadlocks?
- [ ] State machine diagrams.
- [ ] Use case diagrams.
- [ ] Data flow diagrams.
- [x] Resource allocation graphs or wait-for graphs.
> **Explanation:** Resource allocation graphs or wait-for graphs are used to model the allocation of resources and detect any cycles that indicate a potential deadlock.
### Which strategy is NOT used for deadlock recovery?
- [ ] Process termination.
- [ ] Resource preemption.
- [x] Enforcing strict priority rules.
- [ ] Killing all processes involved in the deadlock.
> **Explanation:** Strategies for deadlock recovery typically involve terminating processes or preempting resources rather than enforcing strict priority rules, which is a preventative measure.
### What happens when the "Hold and Wait" condition is eliminated?
- [ ] Processes release resources after allocation.
- [ ] Processes will not be able to hold resources while waiting for others.
- [ ] Mutual exclusion conditions are negated.
- [x] A process cannot hold on to resources while waiting to acquire more.
> **Explanation:** Eliminating "Hold and Wait" forces processes to release any resources they hold if they need to request new ones, making it impossible for processes to hold resources while waiting for others.
### In which system is deadlock most likely to occur?
- [ ] Single-tasking systems.
- [x] Multi-tasking systems.
- [ ] Real-time systems.
- [ ] Embedded systems.
> **Explanation:** Deadlock is more likely to occur in multi-tasking systems where multiple processes run simultaneously and compete for shared resources.
### Deadlock detection algorithms are based on which kind of checks?
- [ ] Random sampling.
- [ ] Prioritization schemes.
- [ ] Time-stamp methods.
- [x] Cycle detection algorithms.
> **Explanation:** Deadlock detection algorithms typically use cycle detection to identify circular dependencies in resource allocation graphs.
### What happens to processes involved in a deadlock after the system detects it?
- [ ] They are terminated.
- [x] They are either preempted or forcefully terminated.
- [ ] They run to completion.
- [ ] They are paused indefinitely.
> **Explanation:** Typically, processes involved in a deadlock are either preempted (resources are forcibly taken from them) or forcefully terminated to break the deadlock.
Thank you for diving into our analysis of deadlock scenarios and tackling our quiz challenges. Continue to sharpen your understanding of concurrent systems and resource management!
