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Chapter 7: Deadlock (System Model (Each process utilized a resource as…

- - - - only one process at a time can use a resource
    - - a process holding at least one resource is waiting to acquire additional resources held by other processes
    - - there exists a set {P0, P1, …, Pn} of waiting processes such that P0 is waiting for a resource that is held by P1, P1 is waiting for a resource that is held by P2, …, Pn–1 is waiting for a resource that is held by Pn, and Pn is waiting for a resource that is held by P0.
    - - a resource can be released only voluntarily by the process holding it, after that process has completed its task
  - - - No deadlock
    - - 1 instance per resource type (no deadlock)
      - several instances per resource type (may occur deadlock)
- - - - Three strategies
        
        Preemption
        
        We can take a resource from one process and give it to other. (may causes problems)
        
        Rollback
        
        System can periodically make a record of the state of each process and when deadlock occurs, roll everything back to the last checkpoint, and restart, but allocating resources differently so that deadlock does not occur.
        
        Kill one or more processes
        
        simplest way, but it works.
    - - Avoiding at least one of the four conditions
        
        Mutual Exclusion
        
        Resources shared such as read-only files do not lead to deadlocks but resources, such as printers and tape drives, requires exclusive access by a single process.
        
        Hold and Wait
        
        In this condition processes must be prevented from holding one or more resources while simultaneously waiting for one or more others.
        
        No Preemption
        
        Preemption of process resource allocations
        
        Circular Wait
        
        Circular wait can be avoided if we number all resources, and require that processes request resources only in strictly increasing(or decreasing) order.