Example Of Deadlock In Operating System

Before we discuss about the example of DEADLOCK.

We need to know what is DEADLOCK?

DEADLOCK is a set of processes is in a deadlock state if every process in the set is waiting for an event (release)  that can only be caused by some other process in the same set. A deadlock is a situation in which two computer programs sharing the same resource are effectively preventing each other

from accessing the resource, resulting in both programs ceasing to function. The earliest computer operating systems ran only one program at a time

 

The earliest computer operating systems ran only one program at a time. All of the resources of the system were available to this one program. Later, operating systems ran multiple programs at once, interleaving them. 

 Programs were required to specify in advance what resources they needed so that they could avoid conflicts with other programs running at the same time. Eventually some operating systems offered dynamic allocation of resources. Programs could request further allocations of resources after they had begun running. This led to the problem of the deadlock

Example of DEADLOCK

Example Of Deadlock In Operating System

Process-1 requests the printer, gets it
Process-2 requests the tape unit, gets it Process-1 and
Process-1 requests the tape unit, waits Process-2 are
Process-2 requests the printer, waits deadlocked!

• A process must request a resource before using it. It must release the resource after
using it. (request 􀃆 use 􀃆 release)

• A process cannot request a number more than the total number of resources available in
the system.

For the resources of the system, a resource table shall be kept, which shows whether each
process is free or if occupied, by which process it is occupied. For every resource, queues
shall be kept, indicating the names of processes waiting for that resource.

A deadlock occurs if and only if the following four conditions hold in a system
simultaneously:

1. Mutual Exclusion: At least one of the resources is non-sharable (that is; only a limited
number of processes can use it at a time and if it is requested by a process while it is
being used by another one, the requesting process has to wait until the resource is
released.).

2. Hold and Wait: There must be at least one process that is holding at least one
resource and waiting for other resources that are being hold by other processes.

3. No Preemption: No resource can be preempted before the holding process
completes its task with that resource.

4. Circular Wait: There exists a set of processes: {P1, P2, ..., Pn} such that

P1 is waiting for a resource held by P2
P2 is waiting for a resource held by P3
...
Pn-1 is waiting for a resource held by Pn
Pn is waiting for a resource held by P1