The document summarizes key topics about deadlocks in operating systems including deadlock characterization, prevention, and avoidance. It defines deadlock as a set of blocked processes where each process is waiting for a resource held by another in the set. Four conditions must be met for deadlock to occur: mutual exclusion, hold and wait, no preemption, and circular wait. Deadlock prevention methods ensure systems never enter a deadlock state while avoidance uses a safe state algorithm and resource ordering to dynamically prevent unsafe states.

1. Operating System
Lecture-8
Presented By
Khadija Islam (Eti)
Lecturer, CSE
Sonargaon University
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2. Key topics:
 System Model
 Deadlock Characterization
 Methods for Handling Deadlocks
 Deadlock Prevention
 Deadlock Avoidance
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3. The Deadlock Problem
 Deadlock: A set of blocked processes each holding a
resource and waiting to acquire a resource held by another
process in the set.
 Example
 System has 2 tape drives.
 P1 and P2 each hold one tape drive and each needs
another one.
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4. Bridge Crossing Example
 Traffic only in one direction.
 Each section of a bridge can be viewed as a resource.
 If a deadlock occurs, it can be resolved if one car backs up
(preempt resources and rollback).
 Several cars may have to be backed up if a deadlock occurs.
 Starvation is possible.
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5. System Model
 Resource types R1, R2, . . ., Rm
CPU cycles, memory space, I/O devices
 Each resource type Ri has Wi instances.
 Each process utilizes a resource as follows:
 request
 use
 Release
 A set of processes is in a deadlock state when every process
in the set is waiting for an event that can be caused only by
another process in the set.
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6. Deadlock Characterization
 Mutual exclusion: only one process at a time can use a resource. If
another process requests that resource, the requesting process must
be waited until the resource has been released.
 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, …, P0} 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 P0 is waiting for a resource that is
held by P0.
Deadlock can arise if four conditions hold simultaneously.
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7. Resource-Allocation Graph
 V is partitioned into two types:
 P = {P1, P2, …, Pn}, the set consisting of all the processes in
the system.
 R = {R1, R2, …, Rm}, the set consisting of all resource types in
the system.
 request edge – directed edge P1  Rj
 assignment edge – directed edge Rj  Pi
A set of vertices V and a set of edges E.
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8. Resource-Allocation Graph
(Cont.)
 Process
 Resource Type with 4 instances
 Pi requests instance of Rj
 Pi is holding an instance of Rj
Pi
Pi
Rj
Rj
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9. Basic Facts
 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
deadlock.
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10. Example of a Resource Allocation Graph
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11. Resource Allocation Graph With A
Deadlock
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12. Resource Allocation Graph With A Cycle But No
Deadlock
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13. Methods for Handling
Deadlocks
 Ensure that the system will never enter a deadlock state----
System can use either a deadlock prevention or a deadlock-
avoidance scheme.
 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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14. Deadlock Prevention
 Mutual Exclusion – not required for sharable resources; must hold for
non-sharable resources. For example: a printer cannot be simultaneously
shared by several processes. Again if several processes attempts to open
a read-only file at the same time, they can be granted simultaneous
access to the file.
 Hold and Wait – must guarantee that whenever a process requests a
resource, it does not hold any other resources.
 One protocol that can be used require process to request and be
allocated all its resources before it begins execution
 An alternative protocol allows process to request resources only
when the process has none.
 Two problem: i) Low resource utilization; ii) starvation possible.
Deadlock prevention is a set of methods for ensuring that at least one of the
necessary conditions cannot hold.
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15. Deadlock Prevention (Cont.)
 No Preemption –
 If a process that is holding some resources requests another resource that
cannot be immediately allocated to it, then all resources currently being
held are released.
 Preempted resources are added to the list of resources for which the
process is waiting.
 Process will be restarted only when it can regain its old resources, as
well as the new ones that it is requesting.
 Alternatively, if a process requests some resources, we first check whether
they are available. If they are, we allocate them.
 If they are not available, we check whether they are allocated to some
other process that is waiting for additional resources. If so, we
preempted the desired resources from the waiting process and allocate
them to the requesting process.
 If the resources are not either available or held by a waiting process, the
requesting process must wait.
 While it is waiting, some of its resources may be preempted but only if
another process requests them.
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16. Deadlock Prevention (Cont.)
 Circular Wait – impose a total ordering of all resource types,
and require that each process requests resources in an
increasing order of enumeration.
 Let R= {R1, R2,…..,Rm} be the set of resources types.
 We assign to each resource type a unique integer number,
which allows us to compare two resources and to
determine whether one precedes another in our ordering.
 Each process can request resources only in an increasing
order of enumeration. That is, a process can initially request
any number of instances of a resource type, say Ri.
 After that, the process can request instances of resource
type Rj, if and only if F(Rj)>F(Ri).
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17. Deadlock Avoidance
 Simplest and most useful model requires that each process
declare the maximum number of resources of each type that it
may need.
 The deadlock-avoidance algorithm dynamically examines the
resource-allocation state to ensure that there can never be a
circular-wait condition.
 Resource-allocation state is defined by the number of available
and allocated resources, and the maximum demands of the
processes.
Requires that the system has some additional a priori information
available.
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18. Safe State
 A state is safe if the system can allocate resources to each process
in some order and still avoid a deadlock.
 When a process requests an available resource, system must
decide if immediate allocation leaves the system in a safe state.
 System is in safe state if there exists a safe sequence of all
processes.
 Sequence <P1, P2, …, Pn> is safe if for each Pi, the resources that Pi
can still request can be satisfied by currently available resources +
resources held by all the Pj, with j<I.
 If Pi resource needs are not immediately available, then Pi can
wait until all Pj have finished.
 When Pj is finished, Pi can obtain needed resources, execute,
return allocated resources, and terminate.
 When Pi terminates, Pi+1 can obtain its needed resources, and
so on.
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19. Basic Facts
 If a system is in safe state  no deadlocks.
 If a system is in unsafe state  possibility of deadlock.
 Avoidance  ensure that a system will never enter an
unsafe state.
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20. Safe, unsafe , deadlock state
spaces
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21. Continue….
 Consider a system with 12 magnetic tape drives and 3
processes: P1, P2 and P3. Process P1 requires 10 tape drive,
process P2 requires 4 and process P3 requires 9 tape drives.
Suppose that, at time t0, process p1 is holding 5 tape drive,
P2 is holding 2 and P3 is holding 2 tape drives. (thus there
are 3 free tape drives)
 At time t0, the system is in safe state. The safe sequence is
<P2,P1,P3>.
 A system may go from a safe state to an unsafe state.
 Suppose at time t1 process P3 requests and is allocated
1 more tape drive. The system is no longer in safe state.
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22. End of Today’s Lecture
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