This document presents information about deadlocks, including examples, causes, characterization, and methods for handling deadlocks. It discusses the four conditions required for deadlock (mutual exclusion, hold and wait, no preemption, and circular wait). Methods for handling deadlocks include prevention, avoidance (such as using a resource allocation graph or banker's algorithm), and recovery through process termination or resource preemption. The document provides details on how these techniques work and considerations for choosing a victim process during recovery.

1. ORIENTAL INSTITUTE OF SCIENCE AND TECHNOLOGY,
BHOPAL
Department of Information Technology
SELF STUDY
Presentation
On
Deadlock
Presented By:-
DEEPAK YADAV
0105IT131027
Semester V09/03/15 1
Oriental Institute Of Science & Technology, Bhopal

2. The Deadlock Problem
EXAMPLES:

"It takes money to make money".

You can't get a job without experience; you can't get
experience without a job.
BACKGROUND:
The cause of deadlocks: Each process needing what another
process has. This results from sharing resources such as
memory devices, links.
Under normal operation, a resource allocations proceed like
this:

Request a resource (suspend until available if necessary.

Use the resource.

Release the resource.
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2Oriental Institute Of Science & Technology,
Bhopal

3. Deadlock Characterization
Deadlock can arise if four conditions hold simultaneously.

Mutual exclusion: only one process at a time can use a
resource

Hold and wait: a process holding at least one resource is
waiting to acquire additional resources held by other processes

No preemption: a resource can be released only voluntarily by
the process holding it, after that process has completed its task
 Circular wait: there exists a set {P0, P1, …, Pn} of waiting
processes such that P0 is waiting for a resource that is held by
P1, P1 is waiting for a resource that is held by P2, …, Pn–1 is
waiting for a resource that is held by Pn, and Pn is waiting for a
resource that is held by P0.09/03/15 3Oriental Institute Of Science & Technology,
Bhopal

4. Methods for Handling Deadlocks

Ensure that the system will never enter a
deadlock state – deadlock prevention

Allow the system to enter a deadlock state
and then recover

Ignore the problem and pretend that
deadlocks never occur in the system; used
by most operating systems, including UNIX
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5. Do not allow one of the four conditions to occur.
Mutual exclusion:
a) Automatically holds for printers and other non-sharables.
b) Shared entities (read only files) don't need mutual exclusion
(and aren’t susceptible to deadlock.)
c) Prevention not possible, since some devices are intrinsically
non-sharable.
Hold and wait:
a) Collect all resources before execution.
b) A particular resource can only be requested when no others
are being held. A sequence of resources is always collected
beginning with the same one.
c) Utilization is low, starvation possible.
Deadlock Prevention
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6. Example (Cont.)

The content of the matrix Need is defined to be Max –
Allocation
Need
A B C
P0 7 4 3
P1 1 2 2
P2 6 0 0
P3 0 1 1
P4 4 3 1
 The system is in a safe state since the sequence < P1,
P3, P4, P2, P0> satisfies safety criteria
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Oriental Institute Of Science & Technology,
Bhopal

7. NOTE: All deadlocks are unsafe, but all unsafes are
NOT deadlocks.
SAFE
DEADLOCK
UNSAFE
Only with luck will
processes avoid
deadlock.
O.S. can avoid
deadlock.
Deadlock Avoidance
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8. 
Single instance of a resource type

Use a resource-allocation graph

Multiple instances of a resource
type

Use the banker’s algorithm
Avoidance algorithms
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9. Recovery from Deadlock:
Process Termination

Abort all deadlocked processes

Abort one process at a time until the deadlock cycle is
eliminated

In which order should we choose to abort?

Priority of the process

How long process has computed, and how much
longer to completion

Resources the process has used

Resources process needs to complete

How many processes will need to be terminated

Is process interactive or batch?
09/03/15 9Oriental Institute Of Science & Technology,
Bhopal

10. Recovery from Deadlock:
Resource Preemption

Selecting a victim – minimize cost

Rollback – return to some safe state,
restart process for that state

Problem: starvation – same process may
always be picked as victim, include number
of rollback in cost factor
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Bhopal

11. 09/03/15 11Oriental Institute Of Science & Technology,
Bhopal