Deadlocks occur when two or more processes are waiting for resources held by each other, resulting in none of the processes making progress. There are four conditions required for deadlock: mutual exclusion, hold and wait, no preemption, and circular wait. Techniques to prevent deadlocks include attacking each of these conditions, such as not allowing simultaneous access to resources, having processes request all resources up front before starting, allowing preemption of resources, and imposing a global numbering system on resource requests.

1. DEADLOCKS
CHAPTER – 7

2. DEADLOCK
 Definition:
 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 reason, why earliest computer OS ran only one
program
at a time.
 A set of processes is deadlocked if each process in
the set is waiting for an event that only another
process can trigger.

3. CONTINUE...
 As all the processes in the set are waiting for an
another process to trigger an event that could wake
up any of the other members of the set, all the
processes would wait for ever.
 None of the processes can run, none of them can
release any resource, and thus none can be awakened.
 The no. of processes and the no. and kind of
resources processed and requested are unimp..
This kind of request holds for any kind of resource
and is called Resource Deadlock.

4. CHARACTERISTICS OF DEADLOCK
 A deadlock situation can arise if the following four
conditions hold simultaneously in a system:
1. Mutual Exclusion
2. Hold & Wait
3. No Pre-emption
4. Circular wait.

5. PREVENTION TECHNIQUES
 The methods to prevent the Deadlock
are:
1. Attacking the Mutual Exclusion
Condition.
2. Attacking the Hold & Wait Condition.
3. Attacking the No Pre-emption
Condition.
4. Attacking the Circular Wait Condition.

6. ATTACKING THE MUTUAL EXCLUSION
CONDITION
 Mutual Exclusion: For two processes, if any one
process is accessing the common resource then the
another process is excluded from accessing the
resource.
 Thus, each resource is either currently allocated to
exactly one process or it is available for other
process to access it. Two processes can’t
simultaneously access the common resource.

7. EXAMPLE
 It is clear that by allowing two processes to write on the
printer
simultaneously will lead to chaos.
 Actually, the only process which requests the physical
printer is the printer daemon.
 Printer daemon never requests any other resource, so
the deadlock can be eliminated and also it prints only
after the complete o/p file is available.
 But this could in itself lead to deadlock for two processes
having finished their part, but not all, of their o/p, and can’t
continue.
 Thus, avoiding to assign a resource unless not
absolutely necessary and try to make less
processes request for the resource.

8. ATTACKING THE HOLD & WAIT
CONDITION
 Hold and Wait: In this scenario, processes currently
holding resource can request new resource to
complete its task.
 If the process requests all the resources before
starting the execution, deadlocks can be eliminated.
 I.E. if everything is available, process will be
allocated whatever it needs and can run to
completion.
 Whereas, if one or more of the requested resource
is/are busy, then nothing will be allocated and the
process would just wait for the resource to become
free and enter the critical region.
 The problem that how many resources does a
process may need can’t be known until it has started

9. EXAMPLE
 Consider a process, that reads data from an i/p tape,
analyzes it for an hour, and then writes an o/p tape as
well as plotting the results.
 Thus, if all resources are requested in adv, the process
will tie up the o/p tape and the plotter for an hour.
 Thus, this method wastes the resources as for an
hour no other process would be able to use the
plotter and the o/p tape.

10. ATTACKING THE NO PRE-EMPTION CONDITION
 No Pre-emption: Once a process holds a resource, it
can’t be taken away by another process or the kernel
and resource can only be voluntarily released by the
process.
 Example: if a process is assigned a printer and is in the
middle of printing its output, forcibly taking away the
printer because the plotter needed isn’t available is not
possible.
 Thus, a resource can be used only if the process
holding it, releases it voluntarily.

11. ATTACKING THE CIRCULAR WAIT
CONDITION
 Circular wait: Each process is waiting to obtain a
resource which is held by another process.
 The circular wait can be eliminated by a rule that if a
process is holding a resource at the moment then it
can’t request for other resource, and if it wants to
access the other resource then it must release the first
one.
 Another way to avoid the circular wait is to provide a
global numbering of all the resources.
 A process can request whenever it wants, but all the
request
must be made in numerical order.

12. EXAMPLE
 Consider the following resources numbered as below:
1. Image setter
2. Scanner
3. Plotter
4. Tape-Drive
5. CD-ROM drive.
• A process may request first a printer and then a tape drive, but it can’t request
first a plotter and then a printer.

13. Example
 Here, if A requests for j ;
i>j, then A is not allowed
to request j because
that is lower than what it
already has.
 Similarly, if B requests
for i; i<j, then B is not
allowed to request for i.
 Thus, either way
deadlock
is not possible.
A
i
B
j