Deadlock

   A deadlock is a situation when a process in the system has acquired some resources and waiting for more resources which are acquired by some other process which in turn is waiting for the resources acquired by this process. Hence, none of them can proceed and OS can’t do any work.

Necessary and Sufficient deadlock conditions: 

Coffman (1971) identified four (4) conditions that must hold simultaneously for there to be a deadlock.

1. Mutual Exclusion Condition

The resources involved are non-shareable.

Explanation: At least one resource (thread) must be held in a non-shareable mode, that is, only one process at a time claims exclusive control of the resource. If another process requests that resource, the requesting process must be delayed until the resource has been released.

2. Hold and Wait Condition

Requesting process hold already, resources while waiting for requested resources.

Explanation: There must exist a process that is holding a resource already allocated to it while waiting for additional resource that are currently being held by other processes.

3. No-Preemptive Condition

Resources already allocated to a process cannot be preempted.

Explanation: Resources cannot be removed from the processes are used to completion or released voluntarily by the process holding it.

4. Circular Wait Condition

The processes in the system form a circular list or chain where each process in the list is waiting for a resource held by the next process in the list.

As an example, consider the traffic deadlock in the following figure                                   

                    

 Submitted By: Deeksha Pundir

                                              B.Sc. 2^nd year(4^th sem.)

                                               Roll no: 2321