The document discusses the concept of deadlock in operating systems, providing definitions and examples, including a real-world analogy with trains. It outlines methods for deadlock prevention, avoidance, detection, and recovery, as well as the use of the Banker's algorithm for safe resource allocation. Various examples are presented to illustrate safe and unsafe states in resource allocation scenarios.

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BY THE NAME OF ALLAH

2. GROUP: MOUNTAIN OF
LIGHT
 MD. Saidur Rahman Kohinoor [142-15-3669]
 Md. Mudasser-ul Haque [142-15-3688]
 Obaidul Hassan [142-15-3559]
 Israt ZaHan Esha [142-15-3604]
 Md. Hamia [142-15-3458]
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3. DEADLO
CK
Operating System
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4. WHAT IS DEADLOCK?
o An example from US Kansas law:
o “When two trains approach each other at a
crossing, both shall come to a full stop and
neither shall start up again until the other
has gone.”
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5. REAL WORLD
DEADLOCKS!
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• Truck A has to wait
for truck B to
move
• Not
deadlocked
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6. REAL WORLD DEADLOCKS!
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Neither truck can proceed
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7. GRAPH WITHOUT A
DEADLOCK
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R1
P1
P2
P3
R2
P4
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8. GRAPH WITH A DEADLOCK
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R1
P1 P2 P3
R3
R2
R4
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9. DEALING WITH DEADLOCKS
o Stop a deadlock ever occuring
o deadlock prevention
o disallow at least one of the necessary
conditions
o deadlock avoidance
o see a deadlock coming and alter the
process/resource allocation strategy
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10.  Deadlock detection and recovery
 Ignore the problem
done by most OS, including UNIX
cheap solution
infrequent, manual reboots may be
acceptable
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11. DEADLOCK PREVENTION
 Eliminate one (or more) of:
mutual exclusion
hold and wait
no preemption (i.e. have preemption)
circular wait
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12. ELIMINATE MUTUAL
EXCLUSION
 Shared resources do not require mutual
exclusion
e.g. read-only files
 But some resources cannot be shared (at the
same time)
e.g. printers
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13. DEADLOCK AVOIDANCE
 In deadlock avoidance, the necessary
conditions are untouched.
 Instead, extra information about resources is
used by the OS to do better forward planning
of process/resource allocation
indirectly avoids circular wait
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14. SAFE STATES
• An OS is in a safe state if there is a
safe sequence of process executions
<P1, P2, …, Pn>.
• A safe state cannot lead to deadlock.
• An unsafe state may lead to deadlock.
• Deadlock is avoided by always keeping
the system in a safe state
– this may reduce resource utilization 14
Deadlock
Unsafe
Safe
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15. EXAMPLE 1
 Max no. of resources: 12 tape drives
 Max needs Current Allocation
P0 10 5
P1 4 2
P2 9 2
 Currently, there are 3 free tape drives
 The OS is in a safe state, since <P1, P0, P2> is a
safe sequence.
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16. EXAMPLE 2
 Same as last slide, but P2 now has 4 tape
drives allocated currently.
 Max needs Current Allocation
P0 10 5
P1 4 2
P2 9 4
 The OS is in an unsafe state.
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17. BANKER’S ALGORITHM
 Assume that:
a resource can have multiple instances
the OS has N processes, M resource types
 Initially, each process must declare the
maximum no. of resources it will need.
 Calculate a safe sequence if possible.
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18. WAIT-FOR GRAPH
• Assume that each resource has only one
instance.
• Create a wait-for graph by removing the
resource types nodes from a resource allocation
graph.
• Deadlock exists if and only if the wait-for graph
contains a cycle.
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19. EXAMPLE
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R1
P2
R3 R4
R2 R5
P1 P3
P5
P4
P2P1 P3
P5
P4
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20. BANKER’S ALGORITHM
VARIATION
 If a resource type can have multiple instances,
then an algorithm very similar to the banker’s
algorithm can be used.
 The algorithm investigates every possible
allocation sequence for the processes that
remain to be completed.
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21. EXAMPLE
 Resource Type Instances
A 7
B 2
C 6
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22. Allocation Request Available
A B C A B C A B C
P0 0 1 0 0 0 0 0 0 0
P1 2 0 0 2 0 2
P2 3 0 3 0 0 0
P3 2 1 1 1 0 0
P4 0 0 2 0 0 2
 The OS is not in a deadlocked state since
<P0, P2, P3, P1, P4> is a safe sequence.
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23. EXAMPLE 2
 Change P2 to request 1 C instance
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AllocationAllocation RequestRequest AvailableAvailable
A B CA B C A B CA B C A B CA B C
PP00 0 1 00 1 0 0 0 00 0 0 0 0 00 0 0
PP11 2 0 02 0 0 2 0 22 0 2
PP22 3 0 33 0 3 0 00 0 11
PP33 2 1 12 1 1 1 0 01 0 0
PP44 0 0 20 0 2 0 0 20 0 2
The OSThe OS isis deadlocked.deadlocked.
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24. DEADLOCK RECOVERY
 Tell the operator
 System-based recovery:
abort one or more processes in the circular
wait
preempt resources in one or more deadlocked
processes
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25. PROCESS TERMINATION
 Abort all deadlocked processes, or
 Abort one process at a time until the
deadlocked cycle disappears
not always easy to abort a process
choice should be based on minimum cost
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26. RESOURCE PREEMPTION
 Issues:
how to select a resource
(e.g. by using minimum cost)
how to rollback the process which has just
lost its resources
avoiding process starvation
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