Deadlock is a situation where a process or a set of processes is blocked, waiting for some other resource that is held by some other waiting process. It is an undesirable state of the system. The following are the four conditions that must hold simultaneously for a deadlock to occur.
- Mutual Exclusion –
A resource can be used by only one process at a time. If another process requests for that resource then the requesting process must be delayed until the resource has been released.
- Hold and wait –
Some processes must be holding some resources in non shareable mode and at the same time must be waiting to acquire some more resources, which are currently held by other processes in non-shareable mode.
- No pre-emption –
Resources granted to a process can be released back to the system only as a result of voluntary action of that process, after the process has completed its task.
- Circular wait –
Deadlocked processes are involved in a circular chain such that each process holds one or more resources being requested by the next process in the chain.
Methods of handling deadlocks :
There are three approaches to deal with deadlocks.
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1. Deadlock Prevention 2. Deadlock avoidance 3. Deadlock detection
These are explained as following below.
1. Deadlock Prevention :
The strategy of deadlock prevention is to design the system in such a way that the possibility of deadlock is excluded. Indirect method prevent the occurrence of one of three necessary condition of deadlock i.e., mutual exclusion, no pre-emption and hold and wait. Direct method prevent the occurrence of circular wait.
Prevention techniques –
Mutual exclusion – is supported by the OS.
Hold and Wait – condition can be prevented by requiring that a process requests all its required resources at one time and blocking the process until all of its requests can be granted at a same time simultaneously. But this prevention does not yield good result because :
No pre-emption – techniques for ‘no pre-emption are’
- If a process that is holding some resource, requests another resource that can not be immediately allocated to it, the all resource currently being held are released and if necessary, request them again together with the additional resource.
- If a process requests a resource that is currently held by another process, the OS may pre-empt the second process and require it to release its resources. This works only if both the processes do not have same priority.
Circular wait One way to ensure that this condition never hold is to impose a total ordering of all resource types and to require that each process requests resource in an increasing order of enumeration, i.e., if a process has been allocated resources of type R, then it may subsequently request only those resources of types following R in ordering.
2. Deadlock Avoidance :
This approach allows the three necessary conditions of deadlock but makes judicious choices to assure that deadlock point is never reached. It allows more concurrency than avoidance detection
A decision is made dynamically whether the current resource allocation request will, if granted, potentially lead to deadlock. It requires the knowledge of future process requests. Two techniques to avoid deadlock :
- Process initiation denial
- Resource allocation denial
Advantages of deadlock avoidance techniques :
- Not necessary to pre-empt and rollback processes
- Less restrictive than deadlock prevention
- Future resource requirements must be known in advance
- Processes can be blocked for long periods
- Exists fixed number of resources for allocation
3. Deadlock Detection :
Deadlock detection is used by employing and algorithm that tracks the circular waiting and killing one or more processes so that deadlock is removed. The system state is examined periodically to determine if a set of processes is deadlocked. A deadlock is resolved by aborting and restarting a process, relinquishing all the resources that the process held.
- This technique doe not limit resources access or restrict process action.
- Requested resources are granted to processes whenever possible.
- It never delays the process initiation and facilitates online handling.
- The disadvantage is the inherent pre-emption losses.