The document explains deadlocks in operating systems, defining a deadlock as a situation where processes are blocked due to resource holding and waiting. It outlines the conditions necessary for a deadlock to occur, including mutual exclusion, hold and wait, no preemption, and circular wait. Additionally, it discusses methods for handling deadlocks through prevention, avoidance, and recovery strategies.

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Dead-Locks in
Operating Systems
An Unconditional Waiting Situation
by
VENKATA SREERAM

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What is a Dead-Lock ?
Deadlock is a situation where a set of processes are
blocked because each process is holding a resource
and waiting for another resource acquired by some
other process.
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Realtime-Example of Deadlock
• Two Trains are Travelling on different Tracks. If a Crossing came, A Train
Must Hold or wait for sometime to continue the Journey. That waiting
situation Is Referred as “Deadlock” Situation.
• While, playing Chess – CHECKMATE is the deadlock situation. Where, there
is no chance of escaping and have to hold for sometime to move. 3

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Deadlock Situation in Data-Base
• No Two Transactions takes place at a time in the Database. It hold one
transaction In the Queue to Proceed.
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Conditions for Deadlock
What are the conditions that a Process Undergone Deadlock
situation. How can we Identify that a Process is in Deadlock
Situation ?
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Conditions for Deadlock in OS
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Mutual
Exclusion
Hold and
Wait
No
Preemption
Circular
Wait

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Mutual-Exclusion
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• When a Process is Accessing a
Shared Variable, the Process is
said to be in a CRITICAL
SITUATION
• One Process at a time can Use a
resource.
p1
p2
R1
R2

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Hold and Wait
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• A Process holding at least one resource is waiting to acquire
additional resources held by another processes.
• Printer needs scanned copy to Print and scanner wants a copy to
scan.

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No Preemption
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• A Resource can be released only voluntarily by the process holding
it, after that process has completed its task.
• Number of resources can be feasibly removed from a processor
holding it.

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Circular Wait
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• There exists a set { p1,p2,p3…pn }
of waiting processes such that p1
is waiting for a resource that Is
held by p2, p2 is waiting for a
resource that is held by p3.
• Pn is waiting for the resource pn-1

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Resource-Allocation Graph
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Resource Allocation Without cycles
• If Graph contains no cycles
 No Deadlock
• If graph contains a Cycle
•  If only one instance per
resource type, then deadlock.
•  If several Instances per
resource type, possibility of
dead lock.

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Resource Allocation Graph
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 Resource Allocation
graph contains cycles & but,
no deadlock.

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How to Handle the Deadlocks ?
Deadlock Recovery
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Deadlock Detection
Deadlock Avoidance
Deadlock Prevention

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Deadlock Prevention
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At least one of 4 deadlock conditions is
never satisfied.
Elimination of
 Mutual Exclusion
 Hold & Wait Condition
 No Preemption Condition
 Circular Wait

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Deadlock Prevention
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Mutual
Exclusion
We can deny this situation by simple protocol i.e.,
“ Convert All non-sharable resources to sharable Resources “

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Deadlock Prevention
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We can deny this situation with the following Protocols:
 A Process can Request the Resources only when the
Process has None.
 Each Process to request and be allocated all its Resources
before it begins Execution.
Hold and
Wait

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Deadlock Prevention
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To ensure that this condition does not hold, we use the following Protocol:
 We preempt the desired resources from the waiting process
and allocate them to the requesting Process.
No
Preemption

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Deadlocks Prevention
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We ensure that circular wait must not happened if we apply a simple solution:
 Numbering all the Resources types and each Process
Request resources in an Increasing order of enumeration.
Circular
Wait

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Deadlock Avoidance
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At least one of 4 deadlock conditions is
never satisfied.
Elimination of
 Mutual Exclusion
 Hold & Wait Condition
 No Preemption Condition
 Circular Wait

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Deadlock can be avoided if certain
information about processes are available to
the operating system before allocation of
resources.
For every resource request, the system sees
whether granting the request will cause the
system to enter an unsafe state that means
this state could result in deadlock.
The system then only grants the requests that
will lead to safe states.
Deadlock Avoidance

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2121
Deadlock Avoidance

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Deadlock Avoidance

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Deadlock Avoidance

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Deadlock Detection
Allow System to enter deadlock state
Detection algorithm
Recovery Scheme

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Deadlock Detection

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Deadlock Detection
Example for Detection Algorithm

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Deadlock Detection

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Deadlock Detection
Once Deadlock has been
detected, Some Strategy is
needed for recovery. The
various approaches of
recovering from deadlocks are:
• Process Termination
• Resource Preemption

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Deadlock Recovery
Process termination
occurs when
the process is terminated
The exit() system call is
used by most operating
systems for process
termination.
This process leaves the
processor and releases all
its resources.

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Deadlock Recovery

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Deadlocks-Concept Overview
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Beyond the concepts of Deadlocks, It was very easy to understand with Realtime concepts where
the resources has been allocated to the processors while performing operations in Operating
System.
We must ensure that the recovery Strategies of deadlocks and prevention, avoidance Algorithms
well by giving suitable examples.

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“
Knowledge Increases by
Sharing but not by Saving ”
- Venkata Sreeram
18U91A584
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😁Thank you !