--- title: "Deadlock conditions; recovery methods" subject: "Operating System" chapter: 7 key: "semester_6/operating_system:deadlock-resource-conditions-recovery" source: https://www.bctengineer.com/q/semester_6/operating_system/deadlock-resource-conditions-recovery site: BCT Engineering --- # Deadlock conditions; recovery methods Operating System · Chapter 7: Deadlock - Asked in: year not given - Video: [Lec26 Deadlock Recovery in Operating System](https://www.youtube.com/watch?v=bQBt1IYMscI) — Jenny's Lectures CS IT - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/operating_system/deadlock-resource-conditions-recovery - Whole chapter: https://www.bctengineer.com/md/semester_6/operating_system/chapter-7 ## Question Define deadlock. What are the conditions for resource deadlock? Explain some methods for recovery from deadlock. ## Short answer Deadlock occurs when each process in a set waits for a resource held by another in the set. The four conditions are mutual exclusion, hold and wait, no preemption, and circular wait. Recovery methods include resource preemption (taking resources from a process), rollback to checkpoints (restarting a process from a saved state), and killing deadlocked processes (terminating one or all processes in the cycle). ## Answer #### Deadlock A set of processes is deadlocked if each process is waiting for an event (release of a resource) that can be caused only by another process in the set. #### Conditions for resource deadlock (Coffman) 1. **Mutual exclusion:** each resource is either assigned to exactly one process or is available. 2. **Hold and wait:** processes holding earlier-granted resources may request new ones. 3. **No preemption:** resources previously granted cannot be forcibly taken away. 4. **Circular wait:** a circular chain of two or more processes exists, each waiting for a resource held by the next one. #### Recovery from deadlock 1. **Recovery through preemption:** temporarily take a resource away from its owner and give it to another process. This often needs manual intervention and depends on the resource (e.g. suspending a process and giving its printer to another is difficult, but taking memory by swapping is possible). 2. **Recovery through rollback:** processes are checkpointed regularly (state and resources saved in files). On deadlock, a process that owns a needed resource is rolled back to a checkpoint before it acquired it, and the resource is given to a waiting process. 3. **Recovery through killing processes:** the simplest. Kill one process in the cycle, or all of them. The victim should be one that can be rerun without bad effects (e.g. a compile, not a database update). The process is chosen using priority, run time, and resources held. After recovery, care is needed to avoid starvation, so the same victim is not chosen repeatedly. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which defines a deadlocked set of processes? A. One process waits for the CPU for a long time B. Each waits for an event that only another process in the set can cause C. All processes terminate abnormally D. A process waits for user input Answer: B. Deadlock is mutual waiting within a set. (Key point: Deadlock definition) 2. What does 'no preemption' mean? A. Each resource is given to one process only B. A process requests new resources while holding old ones C. Granted resources cannot be forcibly taken away D. Processes form a waiting circle Answer: C. Resources are released only voluntarily. (Key point: No preemption) 3. How does recovery through rollback work? A. All processes are restarted from scratch B. Resources are numbered and requests reordered C. A firewall blocks the cycle D. A process holding a needed resource is rolled back to an earlier checkpoint, freeing the resource Answer: D. Regular checkpoints let the system restart a process from before it acquired the resource. (Key point: Rollback recovery) 4. Which victim is best when killing a process? A. One that can be rerun without bad effects, e.g. a compile rather than a database update B. A database update, as it holds the most C. Always the highest-priority process D. The process that just started Answer: A. Choosing a safely rerunnable process avoids damage. (Key point: Choosing a victim for killing) ## Related in Operating System - [Four deadlock conditions; prevention, recovery](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Adeadlock-conditions-prevention-recovery) — Operating System, chapter 7 - [Deadlock conditions; Banker's with 4 processes](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Afour-conditions-banker-p1-p4) — Operating System, chapter 7 - [Deadlock; necessary conditions with examples](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Adeadlock-necessary-conditions-examples) — Operating System, chapter 7 - [Deadlock conditions and why all necessary](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Adeadlock-conditions-why-necessary) — Operating System, chapter 7 - [Deadlock conditions, detection, action](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Adeadlock-conditions-detection-recovery-action) — Operating System, chapter 7 - [Deadlock conditions; Banker's safe sequence](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Afour-conditions-banker-snapshot-332) — Operating System, chapter 7 ## Related in other subjects - [Distributed deadlock and recovery](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Adistributed-deadlock-note) — Distributed System, chapter 8, asked 3× - [Deadlock and Coffman conditions](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Acoffman-deadlock) — Embedded System, chapter 6, asked 4× - [Deadlock and its conditions](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Adeadlock-conditions) — Microprocessor, chapter 6, asked 2×