Deadlock in Operating Systems: Causes, Prevention, and Detection, Study notes of Operating Systems

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DEADLOCK

 Every process needs some resources to complete its execution. However, the resource is granted in a sequential order.  The process requests for some resource.  OS grant the resource if it is available otherwise let the process waits.  The process uses it and release on the completion.

Introduction to Deadlock

 (^) After some time, P1 demands for R2 which is being used by P2.  (^) P1 halts its execution since it can't complete without R2.  (^) P2 also demands for R3 which is being used by P3.  (^) P2 also stops its execution because it can't continue without R3.  (^) P3 also demands for R1 which is being used by P therefore P3 also stops its execution.  (^) In this scenario, a cycle is being formed among the three processes.  (^) None of the process is progressing and they are all waiting.  (^) The computer becomes unresponsive since all the processes got blocked.

Necessary conditions for Deadlocks

 A resource can only be shared in mutually exclusive manner.  It implies, if two process cannot use the same resource at the same time.

1.Mutual Exclusion

 The process which once scheduled will be executed till the completion.  No other process can be scheduled by the scheduler meanwhile.

3.No preemption

 All the processes must be waiting for the resources in a cyclic manner so that the last process is waiting for the resource which is being held by the first process.

4.Circular Wait

 Deadlock Ignorance is the most widely used approach among all the mechanism.  This is being used by many operating systems mainly for end user uses.  In this approach, the Operating system assumes that deadlock never occurs.  It simply ignores deadlock.  This approach is best suitable for a single end user system where User uses the system only for browsing and all other normal stuff.

1.Deadlock Ignorance

 (^) There is always a tradeoff between Correctness and performance.  The operating systems like Windows and Linux mainly focus upon performance.  (^) However, the performance of the system decreases if it uses deadlock handling mechanism all the time if deadlock happens 1 out of 100 times then it is completely unnecessary to use the deadlock handling mechanism all the time.  (^) In these types of systems, the user has to simply restart the computer in the case of deadlock.  Windows and Linux are mainly using this approach.

 In deadlock avoidance, the operating system checks whether the system is in safe state or in unsafe state at every step which the operating system performs.  The process continues until the system is in safe state.  Once the system moves to unsafe state, the OS has to backtrack one step.  In simple words, The OS reviews each allocation so that the allocation doesn't cause the deadlock in the system.

3.Deadlock avoidance

 This approach lets the processes fall in deadlock and then periodically check whether deadlock occur in the system or not.  If it occurs then it applies some of the recovery methods to the system to get rid of deadlock.

4.Deadlock detection and recovery

 Let's see how we can prevent each of the conditions.

 Mutual exclusion from the resource point of view is the fact that a resource can never be used by more than one process simultaneously which is fair enough but that is the main reason behind the deadlock.  If a resource could have been used by more than one process at the same time then the process would have never been waiting for any resource.  However, if we can be able to violate resources behaving in the mutually exclusive manner then the deadlock can be prevented.

1. Eliminate Mutual Exclusion