DeadLock

A deadlock is a situation in which some process wait for each other actions indefinitely. Process involved in a deadlock remains blocked permanently.

Operating system handles only deadlock cause by sharing of resources in the system.

Deadlock in resource allocation: -

          ·         Request: -     A process requests a resource through a system call. If the resources are available then the kernel allocates it to the process immediately; otherwise it changes the state of the process to blocked i.e., if the request cannot be granted immediately then the requesting process must wait until it can acquire the resource.

          ·         Allocation:-     The process becomes the holder of the resource allocated to it. The resource state information is updated and the state of the process is changed to ready.

          ·         Release: -     A process releases the resource through a system call. If some processes are blocked on the allocation event for the resource, the kernel uses some tie breaking rule.

  v Deadlock prevention: -

      a.      Mutual Exclusion (Non Sharable Resource): -     This condition must hold for non-sharable resources. Ex: - printer cannot be simultaneously share by several processes.

            If many processes attempt to open a read only file at the same time they can be granted simultaneous access to the file. A process never needs to wait for a sharable resource.

            Generally deadlock cannot be prevented by denying this condition.

      b.     Hold and Wait: -     To stop this condition two protocol can be used.

1.     Each process to request and allocated all its resource before it begins execution.

2.     A process can request any resource it must release all its currently allocated resources.

These protocols have two disadvantages:-

1.     Resource utilization should be low (protocol 1).

2.     A process that needs many resources may have to wait indefinitely (protocol 2).

      c.      No pre-emption: -     If a process requesting a resource that can’t be currently allocated to it, then all currently held resources are pre-empted. The process will be restarted only when it regains its old as well new resources it is requesting.

      d.     Circular wait: -     Suppose we have three resources:  tape drive, disk drive and printer. A process which meant to access disk drive and printer should firstly access disk drive and then printer. That is resources are allocated according to their numbering.

  

  v Deadlock avoidance: -

              a.      Safe state

              b.     Resource allocation graph method

              c.      Banker’s algorithm

  v Deadlock detection: -     If a system doesn’t apply with deadlock prevention or deadlock avoidance then a deadlock may occur. Deadlock can be detected by using RAG algorithm and Banker’s algorithm.

  v Deadlock resolution or recovery: -      When a deadlock is detected a system can be recovered from the deadlock automatically or manually (by rebooting the system).

     There are two options to break a deadlock.

1.     We have to terminate some process so that a circular wait condition breaks and this way some resources also become free for the running process.

           2.   We have to add some new units of resource required.

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