Deadlock

✅ Deadlock

requirement for system resource order is mixed up

• when more than two processes/threads cannot be run bc there is no resource
  
• wait infinite for resource
  
• occurs when several processes/threads need to work with limited resource
  
• 둘 이상의 프로세스가
• 서로가 점유한 자원을 서로 기다림
• 무한 대기에 빠진다.

situation: process 1 and 2 both need resource 1 and 2
t1: process 1 gets resource 1 and process 2 gets resource 2
t2: process 1 waits for resource 2 and process 2 waits for resource 1

✅ Conditions for Deadlock

✔️ mutual exclusion

• only one process can use resource at a time
• if other process wants to use the resource, has to wait

✔️ hold and wait

• there should be a process with minimum one resource,
• waiting for resource to be allocated, that is already posessed by another process

✔️ no preemption

• Cannot take resource from another process forcefully

✔️ circular wait

• Resource wait in circular way

✅ Solution for Deadlock

1. Prevention
   

• prevent mutual exclusion
• prevent hold and wait
• prevent no preemption
• prevent circular wait

1. Avoidance
   

• only allow resource with deadlock vulnerability in safe state

• do not allow unsafe allocation(situation with deadlock possibility)

• Safe state: safe sequence(order for allocating resource without deadlock)

  • every process can end

• Unsafe state: situation with deadlock possibility

  • 💊 은행원 알고리즘
  • 💊 자원 할당 알고리즘
• In order for avoidance, following conditions should be met
• fixed number of process
• fixed number of type, number of process
• knowledge of process maximum required resource
• process should return resource after use

1. Detection, Recovery
   

• detect deadlock

• 👎🏻 need to continuously check, overhead
  

• recovery
• A. 사용자 처리
• user forcefully stops one process
• B. 시스템 처리
• stops process
• stops all process in deadlock
• stops process on by one until deadlock is solved
• take resource from process, and allocate to other processes until deadlock is solved