Deadlocks occur when multiple processes are waiting for resources held by other processes in the set, creating a circular wait. The four conditions for deadlock are mutual exclusion, hold and wait, no preemption, and circular wait. Deadlocks can be addressed through prevention, avoidance, detection, or recovery. Prevention eliminates one of the four conditions. Detection identifies deadlocked processes and resources. Recovery options include terminating all or one deadlocked process at a time until the cycle is broken, or preempting resources from processes until the cycle is broken.

1. Topic : DEAD LOCKS IN OPERATING
SYSTEM
Presented By:
Wani Zahoor
1-MIT-2015

4. When a process requests the resources, the
resources are not available at that time, so the
process enters into the waiting state. The requesting
resources are held by another waiting process, both
are in waiting state, this situation is said to be a. ..
“Deadlock”

5. A set of blocked processes each
holding a resource and waiting to
acquire a resource held by another
process in the set.
o Example
Process-1 requests the printer, gets it
Process-2 requests the tape unit, gets it Process-1 and
Process-1 requests the tape unit, waits Process-2
Process-2 requests the printer, waits Deadlocked!

6. 1. Mutual Exclusion Condition
2. Hold & Wait Condition
3. No Preemption Condition
4. Circular Wait Condition

7. Modeled with directed graphs

8. If we have a resource allocation
system with only one inatance of each
process, a variant of the resource
allocation graph can be used for
deadlock avoidance.

9. R1
R2
R3 R4
P1 P2
P3
Resource Allocation Graph

10.  CIRCLES
 represents processes
 SQUARE
 represents classes of identical resources
 SMALL SQUARES DRAWN INSIDE LARGE SQUARES
 Indicate separate identical resources of each class

11.  If the Resource Allocation graph contains no cycles then
there is no deadlock in the system at that instance.
 If there is a Cycle
o If there is only one instance per resource type than there is a
Deadlock
o If there is more than once instance for some resource type than
there may or may not be a Deadlock
RESOURCE ALLOCATION GRAPH ?

13. • Four major Areas of Interest in Deadlocks:
 Deadlock Prevention
 Deadlock Avoidance
 Deadlock Detection
 Deadlock Recovery

14. • Prevention eliminates one of the four Conditions:
1. Elimination Of Mutual Exclusion
2. Elimination Of Hold & Wait Condition
3. Elimination Of No Preemption Condition
4. Elimination Of Circular Wait Condition

15. • Maximum requirements of
each resource must be stated
in advance by each process.
• Two Approaches
• Two Algorithms
o Resource Allocation Algorithms
o The Banker’s Algorithm

17. • Determines if deadlock has occurred
• Identifies those processes & resources
involved in the deadlock
• Algorithms can incur significant
runtime overhead

18. Detection mechanism of Deadlocks for single instance of resource
type is different. We can detect the dead locks using wait for graph
for single instance resource type and detect using detection
algorithm for multiple instances of resource type.
1. SINGLE INSTANCE OF RESOURCE TYPE : Single instance of resource
type means, the system consisting of only one resource for one type.
We can detect this type of deadlocks withthe help of wait for graph.
DEADLOCK DETECTION MECHANISMS

19. 2. SEVERAL INSTANCE OF RESOURCE TYPE :
The wait for graph is not applicable to several instance
of resource type. So we need another method for this
type, that is “deadlock detection algorithm”. This
algorithm looks like ‘Banker’s algorithm” and it
employees several data structures that are similar to
those used in the …
Banker’s Algorithm.

20. • Process Termination
o Abort all deadlocked processes.
o Abort one process at a time until
the deadlock cycle is eliminated.
• Resource Preemption
o Selecting a victim
o Rollback
o Starvation

21. “Process Termination” it is one method to recover from deadlock. We uses 2
methods for process termination, these are:
a) ABORT ALL DEADLOCKED PROCESS : It means release all the processes in the
deadlocked state, and start the allocation from the starting point. It is a great
expensive method.
b) ABORT ONE BY ONE PROCESS UNTIL THE DEADLOCK CYCLE IS
ELIMINATED : In this method first abort the one of the processes in the
deadlocked state, and allocated the resources to some other process in the
deadlock state then check whether the deadlock breaked or not. If no, abort the
another process from the deadlock state. Continue this process until we recover
from deadlock. This method is also expensive but compare with first one it is
better.

22. To eliminate deadlocks using resource preemption, preempt some resources
from processes and give these resources to other processes until the deadlock cycle is
broken.
There are 3 methods to eliminate the deadlocks using resource preemption.These are
:
a) SELECTING A VICTIM : Select a victim resource from the deadlock state, and
preempt that one.
b) ROLLBACK : If a resource from a process is preempted, what should be done
with that process. The process must be roll backed to some safe state and restart
it from that state.
c) STARVATION : It must be guaranteed that resource will not always be
preempted from the same process to avoid starvation problem.

23. Have a Nice Day !!!