1. Deadlocks
Peng Lu
In a multiprogramming environment, several processes may
compete for a finite number of resources. A process
requests resources; if the resources are not available at
that time, the process enters a wait state.
Waiting processes may never again change state, because
the resources they have requested are held by other
waiting processes. This situation is called a deadlock.
CSCI Dr. John P. Abraham

2. Deadlocks
CSCI Dr. John P. Abraham

3. Deadlocks
“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.”
---The Kansas legislature, early in the 20th century.
Perhaps the best illustration of a deadlock is :
CSCI Dr. John P. Abraham

4. Deadlocks 1.System Model 2.Deadlock Characterization
3.Methods for Handling Deadlocks
4.Deadlock Prevention
5.Deadlock Avoidance
6.Deadlock Detection
7.Recovery from Deadlock
In deadlocks, we will disuses these topics.
CSCI Dr. John P. Abraham

5. 1. System Model
Under the normal mode of operation, a process utilizes a resource as follows:
Request
Use
Release
A process must request a resource before using it, and
must release the resource after using it.
Request: If the request cannot be granted immediately
(for example, the resource is being used by another
process), then the requesting process must wait until
it can acquire the resource.
Use: the process can operate on the resource
(for example, if the resource is a printer, the process
can print on the printer).
Release: the process releases the resource.
CSCI Dr. John P. Abraham

6. 2.Deadlock Characterization
Necessary Conditions:
Mutual exclusion
Hold and wait
No preemption
Circular wait
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
CSCI Dr. John P. Abraham

7. Resource Allocation Graph With A Deadlock
P = Process
R = Resource
CSCI Dr. John P. Abraham

8. 3. Methods for Handling Deadlocks
Ensure that the system will never enter a deadlock state Prevention
Allow the system to enter a deadlock state and then recover Detection
Ignore the problem and pretend that deadlocks never occur in the system
Most operating systems
CSCI Dr. John P. Abraham

9. 4. Deadlock Prevention Mutual Exclusion Hold and Wait No Preemption
Circular Wait
CSCI Dr. John P. Abraham

10. 5. Deadlock Avoidance Safe state Resource-Allocation Graph Algorithm
Banker’s Algorithm
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.
CSCI Dr. John P. Abraham

11. Safe State
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12. Resource-Allocation Graph Algorithm
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13. Unsafe State In Resource-Allocation Graph
CSCI Dr. John P. Abraham

14. Banker’s Algorithm Data Structures:
Available: A vector of length m indicates the number of available resource of each type.
Max: An n x m matrix defines the maximum demand of each process.
Allocation: An n x m matrix defines the number of resources of each type currently allocated to each process.
Need: An n x m matrix indicates the remaining resource need of each process.
The name was chosen because this algorithm
could be used in a banking system to ensure
That the band never allocates it’s available
Cash such that it can no longer satisfy the
Needs of all it’s customers.
CSCI Dr. John P. Abraham

15. 6. Deadlock Detection Single Instance of Each Resource Type
Several Instances of a Resource Type
Detection-Algorithm Usage
SIER Type: If all resources have only a single
instance, the we can define a deadlock detection
algorithm that uses a variant of the
resource-allocation graph, called a wait-for graph.
Detection-Algorithm Usage
When should we invoke the detection algorithm?
The answer depends on two factors:
How often is a deadlocks likely to occur?
How many processes will be affected by deadlock
when it happens?
CSCI Dr. John P. Abraham

16. 7. Recovery from Deadlock
Process Termination
Resource Preemption
When a detection algorithm detecting the deadlock
Exists, one possibility is let the operator deal with
The deadlock manually; the other possibility is to
Let the system recover from the deadlock
Automatically. There two option for breaking a
Deadlock:
CSCI Dr. John P. Abraham

17. Summary of the Deadlock
Prevention or avoidance deadlocks, ensuring that the system will never enter a deadlock state.
Allow the system to enter deadlock state, detect it, and then recover.
Ignore the problem all together, and pretend that deadlocks never occur in the system.
CSCI Dr. John P. Abraham

18. Thanks!
CSCI Dr. John P. Abraham

19. Resource-Allocation Graph
P = Process
R = Resource
CSCI Dr. John P. Abraham

20. Graph With A Cycle But No Deadlock
CSCI Dr. John P. Abraham