Chapter 7 – Deadlock
Resources
 Examples of computer resources
 Printers
 Tape drives
 Tables
 Preemptable resources
 Can be taken away from a process with no ill
effects
 Nonpreemptable resources
 Will cause the process to fail if taken away
 Reusable resources
 Used by one process at a time and not
depleted by that use
 Examples: Processors, I/O channels, main and
secondary memory, files, databases, and
semaphores
 Shared and exclusive resources
 Example of shared resource: FILE
 Example of exclusive resource: PRINTER
 Consumable resources
 Created (produced) and destroyed (consumed)
by a process
 Examples: Interrupts, signals, messages, and
information in I/O buffers
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7.1
System Model
 A system consists of a number of resources to be
distributed among a number of competing
processes.
 There are different types of resources R1, R2,...,
Rm.
 CPU cycles, memory space, I/O devices
 Each resource type Ri has Wi instances. For
example, if two CPUs then resource type CPU has
two instances.
Sequence of Events Required to Use a Resource
 Each process utilizes a resource as follows:
 Request a resource:
 Request is made through a system call
 Process must wait if request is denied
 Requesting process may be blocked
 may fail with error code
 Use the resource:
 The process can operate on the resource.
 Release the resource:
 The process releases the resource. A
resource is released through a system call.
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7.2
Deadlock
 Formal Definition
 A set of processes is deadlocked if each
process in the set is waiting for an event that
only another process in the set can cause
 Usually the event is release of a currently held
resource
 None of the processes can …
 Run
 Release resources
 Be awakened
 Involve conflicting needs for resources by two or
more processes
Examples of Deadlock
 Example 1
 System has 2 tape drives. P1 and P2 each
hold one tape drive and each needs another
one.
 Example 2
 Semaphores A and B, initialized to 1
P0
P1
wait (A);
wait(B)
wait (B);
wait(A)
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7.3
 Example 3
 Space is available for allocation of 200K bytes,
and the following sequence of events occur
P0
….
Request 80KB;
…
Request 60KB;
P1
…
Request 70KB;
…
Request 80KB;
 Deadlock occurs if both processes progress to
their second request
Four Conditions for Deadlock
 Deadlock can arise if four conditions hold
simultaneously
 Mutual exclusion condition:
 Only one process at a time can use a
resource (non-shareable resource).
 Each resource is assigned to a process or
is available
 Hold and wait condition:
 A process holding at least one resource
can request for additional resources
 No preemption condition:
 A resource can be released only voluntarily
by the process holding it. That is previously
granted resources cannot be forcibly taken
away.
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7.4
 Circular wait condition:
 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.
Resource-Allocation Graph
 Deadlocks can be described more precisely in terms
of a directed graph, called a system resourceallocation graph.
 This graph consists of a set of vertices V and a set
of edges E.
 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.
 E is partitioned into two types as well:
 Request edge – directed edge P1  Rj
 Assignment edge–directed edge Rj Pi
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7.5
 Different symbols are used to represent processes
and resources as given below:
Process:
Resource type of 4 instances:
Pi requests instance of Rj:
Pi
Rj
Pi is holding an instance of Rj:
Pi
Rj
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7.6
Method of Handling Deadlocks
 Just ignore the problem altogether
 Prevention
 Ensure that the system will never enter a
deadlock state
 Requires negating one of the four necessary
conditions
 Dynamic avoidance
 Require careful resource allocation
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7.7
 Detection and recovery
 Allow the system to enter a deadlock state and
then recover
 We need some methods to determine whether
or not the system has entered into deadlock.
 We also need algorithms to recover from the
deadlock.
The Ostrich Algorithm
 Pretend there is no problem
 The system will eventually stop functioning
 Reasonable if
 Deadlocks occur very rarely
 Cost of prevention is high
 UNIX and Windows takes this approach
 It is a trade off between
 Convenience
 Correctness
Deadlock Prevention
 Prevent/deny Mutual Exclusion condition
 Use shareable resource.
 Impossible for practical system.
 Prevent/Deny Hold and Wait condition
(a) Pre-allocation - Require processes to request
resources before starting
 A process never has to wait for what it
needs
(b) Process must give up all resources and then
request all immediately needed
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7.8
 Problems
 May not know required resources at start
of run
 Low resource utilization – many resources
may be allocated but not used for long time
 Starvation possible – a process may have
to wait indefinitely for popular resources.
 Prevent/deny No Preemption condition
(a) 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.
(b) The required resource(s) is/are taken back
from the process(s) holding it/them and given
to the requesting process
 Problems
 Some resources (e.g. printer, tap drives)
cannot be preempted without detrimental
implications.
 May require the job to restart
 Prevent/Deny Circular Wait
 Order resources (each resource type is
assigned a unique integer) and allow process to
request for them only in increasing order
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7.9
 If a process needs several instances of the
same resource, it should issue a single request
for all of them.
 Alternatively, we can require that whenever a
process requests an instance of a resource
type it has released all the resources which are
assigned a smaller inter value.
 Problem:
 Adding a new resource that upsets
ordering requires all code ever written to
be modified
 Resource numbering affects efficiency
 A process may have to request a
resource well before it needs it, just
because of the requirement that it
must request resources in ascending
order
 An example:
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7.10
Deadlock Avoidance
 OS never allocates resources in a way that could
lead to a deadlock
 Processes must tell OS in advance how many
resources they will request
Some Definitions
 State of a system
 An enumeration of which processes hold, are
waiting for or might request which resource
 Safe state
1. No process is deadlocked, and there exits no
possible sequence of future request in which
deadlock could occur
2. No process is deadlocked and the current state
will not lead to a dead lock state
3. Safe state is where there is at least one
sequence that does not result in deadlock
 Unsafe state
 Is a state that is not safe
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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7.11
Deadlock Avoidance with Resource-Allocation
Graph
 This algorithm can be used if we have only one
instance of each resource type.
 In addition to the request and assignment edges, a
claim edge is also introduced.
 Claim edge Pi  Rj indicated that process Pj may
request resource Rj in future; represented by a
dashed line.
 Claim edge converts to request edge when a
process requests a resource.
 When a resource is released by a process,
assignment edge reconverts to a claim edge.
 Resources must be claimed a priori in the system.
That is, before a process starts executing, all of its
claim edges must already appear in the resourceallocation graph.
 Suppose that process Pi requests resource Rj. The
request can be granted only if converting the
request edge if converting the request edge PiRj
to an assignment edge does not result in a cycle in
the resource-allocation graph. That is we use a
cycle detection algorithm is used. If no cycle exits,
the process Pi will have to wait.
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7.12
Resource-allocation graph for deadlock avoidance
An unsafe state in the resource-allocation graph
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7.13
Banker’s Algorithm
 Applicable to system with multiple instances of
resource types.
 Each process must a priori claim maximum use.
 When a process requests a resource it may have to
wait.
 When a process gets all its resources it must return
them in a finite amount of time.
 Banker’s algorithm runs each time:
 A process requests resource – Is it sage?
 A process terminates – Can I allocate released
resources to a suspended process waiting for
them?
 A new state is safe if and only if every process
can complete after allocation is made
 Make allocation and then check system
state and deallocate if unsafe
Data Structures for Banker’s algorithm
 Let n = number of processes, and m = number of
resources types.
 Available: Vector of length m. If available [j] = k,
there are k instances of resource type Rj available.
 Max: n x m matrix. Max [i,j] = k mean that process
Pi may request at most k instances of Rj.
 Allocation: n x m matrix. If Allocation[i,j] = k then
Pi is currently allocated k instances of Rj.
 Need: n x m matrix. If Need[i,j] = k, then Pi may
need k more instances of Rj to complete its task.
Need [i,j] = Max[i,j] – Allocation [i,j].
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7.14
Safety Algorithm
1. Let Work and Finish be vectors of length m and n,
respectively.
Initialize: Work = Available
Finish [i]=false for i=1,3, …, n.
2. Find and i such that both:
(a) Finish [i] = false
(b) Needi  Work
If no such i exists, go to step 4.
3. Work = Work + Allocationi
Finish[i] = true
go to step 2.
4. If Finish [i] == true for all i, then the system is in a safe state.
Resource-Request algorithm for Process Pi
Request = request vector for process Pi. If Requesti [j] = k then
process Pi wants k instances of resource type Rj.
1. If Requesti  Needi go to step 2. Otherwise, raise error
condition, since process has exceeded its maximum claim.
2. If Requesti  Available, go to step 3. Otherwise Pi must wait,
since resources are not available.
3. Pretend to allocate requested resources to Pi by modifying the
state as follows:
Available = Available = Requesti;
Allocationi = Allocationi + Requesti;
Needi = Needi – Requesti
• If safe  the resources are allocated to Pi.
• If unsafe  Pi must wait, and the old resource-allocation
state is restored
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7.15
Example of Banker’s Algorithm
5 processes P0 through P4; 3 resource types
A (10 instances),
B (5 instances), and
C (7 instances).
Snapshot at time T0:
P0
P1
P2
P3
P4
Allocation
ABC
010
200
302
211
002
Max
ABC
753
322
902
222
433
Available
ABC
332
The content of the matrix. Need is defined to be Max –
Allocation.
Process
P0
P1
P2
P3
P4
Need
ABC
743
122
600
011
431
The system is in a safe state since the sequence
<P1,P3,P4,P2,P0> satisfies safety criteria.
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7.16
Example P1 Request (1,0,2)
 Check that Request  Available
that is, (1,0,2)  (3,3,2)  true.
Process
P0
P1
P2
P3
P4
Allocation
ABC
010
302
301
211
002
Need
ABC
743
020
600
011
431
Available
ABC
230
 Executing safety algorithm shows that sequence
<P1, P3, P4, P0, P2> satisfies safety requirement.
 Can request for (3,3,0) by P4 be granted?
 Can request for (0,2,0) by P0 be granted?
Deadlock Detection Recovery
 Allow system to enter deadlock state
 Need a detection algorithm
 Need a recovery algorithm
How to Detect a Deadlock Using a Resource-Graph?
 If each resource type has exactly one instance and
the graph has a cycle then a deadlock has occurred.
Or if the cycle involves only a set of resource types,
each of which has only a single instance, then the
deadlock has occurred.
 Therefore, a cycle in the graph is both a
necessary and sufficient condition for the
existence of a deadlock.
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7.17
Examples:
Resource-allocation graph with a deadlock
Recovery from Deadlocks – 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.
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7.18
 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?
Recovery from Deadlocks – Resource Preemption
 Selecting a victim – minimize cost.
 Rollback – return to some safe state, restart process
for that state.
 Starvation – same process may always be picked
as victim, include number of rollback in cost factor.
Combined Approach to Deadlock Handling
 Combine the three basic approaches
 prevention
 avoidance
 detection
allowing the use of the optimal approach for each of
resources in the system.
 Partition resources into hierarchically ordered
classes.
 Use most appropriate technique for handling
deadlocks within each class.
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7.19