Deadlocks in CPU

  

 What do you mean by Deadlock in CPU?

  A  deadlock is a situation  where a group  of processes is permanently  blocked as 

  a result of each process  having acquired  a  set  of  resources  needed  for its

  completion and having to wait  for  the   release of the remaining resources held 

  by  others  thus making it impossible for any of the deadlocked process to proceed.

For example assume a system with one tape drive and one plotter.Process P1 requests

the plotter and process P2 requests   the tape drive. Both requests are granted. Now

Process P1 request the tape drive(without giving the plotter) and process P2 requests

plotter(without releasing tape drive) .Neither request can be granted so both processes

enter a situation called the deadlock situation.

Hey guys before we learn more about deadlocks we must understand about the 

type of resources.

There are mainly two types of resources:

Pre emptive Resources:This process can be taken away from the process with no ill

effects.For eg Memory is an example of a pre-emptable resource   .

Non Pre-emptive Resources:This resource cannot be taken away from the process

without causing ill effect.For example CD Rom

It is to be noted that relocating resources can resolve deadlocks that involve

 pre-emptable resources  while Non pre-emptable resources are difficult to deal with.

 

Characteristics of Deadlock:

There are basically four necessary conditions that  leads to occurence of deadlock.

They are as follows:

1. Mutual Exclusion:

In this the resources used are non shareable At least one resource  must be held in a

non shareable mode.. If another process request that resource the requesting process must 

be delayed  until the resource has been released.

2:Hold and Wait Condition

In this condition a requesting process already holds resources and waits

for the requested resources. A process holding a resource allocated to it waits for

an  additional resources that is/are currently being held by other resources

3.No Preemptive condition:

Resources already allocated to a process cannot be pre-empted.Resources cannot be

removed forcibly from the processes.After completion they will be 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  hold by the next process in the list.