Deadlock Prevention in Operating System

In this tutorial, we will elaborate on the deadlock prevention approach.

As we are already familiar with all the necessary conditions for a deadlock. In brief, the conditions are as follows:

• Mutual Exclusion

• Hold and Wait

• No preemption

• Circular Wait

Deadlock Prevention in Operating System

let us take an example of a chair, as we know that chair always stands on its four legs. Likewise, for the deadlock problem, all the above given four conditions are needed. If anyone leg of the chair gets broken, then definitely it will fall. The same is the situation with the deadlock if we become able to violate any condition among the four and do not let them occur together then there can be prevented from the deadlock problem.

We will elaborate deadlock prevention approach by examining each of the four necessary conditions separately.

Mutual Exclusion

This condition must hold for non-sharable resources. For example, a printer cannot be simultaneously shared by several processes. In contrast, Sharable resources do not require mutually exclusive access and thus cannot be involved in a deadlock. A good example of a sharable resource is Read-only files because if several processes attempt to open a read-only file at the same time, then they can be granted simultaneous access to the file.

A process need not to wait for the sharable resource. Generally, deadlocks cannot be prevented by denying the mutual exclusion condition because there are some resources that are intrinsically non-sharable.

Hold and Wait

Hold and wait condition occurs when a process holds a resource and is also waiting for some other resource in order to complete its execution. Thus if we did not want the occurrence of this condition then we must guarantee that when a process requests a resource, it does not hold any other resource.

There are some protocols that can be used in order to ensure that the Hold and Wait condition never occurs:

• According to the first protocol; Each process must request and gets all its resources before the begining of its execution.

• The second protocol allows a process to request resources only when it does not occupy any resource.

Let us illustrate the difference between these two protocols:

We will consider a process that mainly copies data from a DVD drive to a file on disk, sorts the file, and then prints the results to a printer. If all the resources must be requested at the beginning of the process according to the first protocol, then the process requests the DVD drive, disk file, and printer initially. It will hold the printer during its entire execution, even though the printer is needed only at the end.

While the second method allows the process to request initially only the DVD drive and disk file. It copies the data from the DVD drive to the disk and then releases both the DVD drive and the disk file. The process must then again request the disk file and printer. After copying the disk file to the printer, the process releases these two resources as well and then terminates.

Disadvantages of Both Protocols

• Utilization of resources may be low, since resources may be allocated but unused for a long period. In the above-given example, for instance, we can release the DVD drive and disk file and again request the disk file and printer only if we can be sure that our data will remain on the disk file. Otherwise, we must request all the resources at the beginning of both protocols.

• There is a possibility of starvation. A process that needs several popular resources may have to wait indefinitely because at least one of the resources that it needs is always allocated to some other process.

No Preemption

The third necessary condition for deadlocks is that there should be no preemption of resources that have already been allocated. In order to ensure that this condition does not hold the following protocols can be used :

• According to the First Protocol: "If a process that is already holding some resources requests another resource and if the requested resources cannot be allocated to it, then it must release all the resources currently allocated to it."

• According to the Second Protocol: "When a process requests some resources, if they are available, then allocate them. If in case the requested resource is not available then we will check whether it is being used or is allocated to some other process waiting for other resources. If that resource is not being used, then the operating system preempts it from the waiting process and allocate it to the requesting process. And if that resource is being used, then the requesting process must wait".

The second protocol can be applied to those resources whose state can be easily saved and restored later for example CPU registers and memory space, and cannot be applied to resources like printers and tape drivers.

Circular Wait

The Fourth necessary condition to cause deadlock is circular wait, In order to ensure violate this condition we can do the following:

Assign a priority number to each resource. There will be a condition that any process cannot request for a lesser priority resource. This method ensures that not a single process can request a resource that is being utilized by any other process and due to which no cycle will be formed.

Example: Assume that R5 resource is allocated to P1, if next time P1 asks for R4, R3 that are lesser than R5; then such request will not be granted. Only the request for resources that are more than R5 will be granted.