A deadlock is a state where processes are blocked, each waiting for resources held by others, requiring key conditions like mutual exclusion and circular wait to be met. Methods to handle deadlocks include prevention, recovery, and ignoring the problem, with prevention strategies focused on eliminating the conditions that lead to deadlocks. Recovery can involve terminating processes or preempting resources to resolve the deadlock situation.

1. DEAD LOCKS
S.SaiNishitha
22011A0527

2. what is a DEADLOCK?
• Deadlock is a situation in which group of processes are
blocked.
• Each process acquires a resource needed for its completion
• Simultaneously the processes waits for the resources to be
released by others – thus making impossible for any process to
proceed.

3. CONDITIONS FOR DEADLOCK TO OCCUR
 Mutual Exclusion: At least one resource must be held in a non-
sharable mode, meaning only one process can use it at a time.
 Hold and Wait: A process must be holding at least one resource is
waiting to acquire additional resources held by another processes.
 No Preemption: Resources cannot be forcibly taken away from a
process; they must be released voluntarily.
 Circular Wait: A circular chain of two or more processes exists,
where each process is waiting for a resource held by the next
process in the chain.

4. METHODS FOR HANDLING DEADLOCKS
 Ensure that the system will never enter a deadlock state
 Allow the system to enter a deadlock and then recover
 Ignore the problem and pretend that deadlock never occurred in the
system.This is the simplest and most used in OS.
 The first and second methods need techniques to be developed and
implemented.These methods are Deadlock Prevention and Deadlock
Avoidance.

7. DEADLOCK PREVENTION
INDIRECT :-
prevent the
occurrence of
Mutual Exclusion, Hold
and Wait, No
Preemption
DIRECT :-
prevent the
occurrence of Circular
Wait

8. 1. Mutual exclusion – not required for sharable resources,must
hold for non sharable resources.
2. Hold and wait – must guarantee that whenever a process
request a resource,it does not hold any other resources.
3.No Preemption – can be prevented in many ways.
• If a process that is holding some resources requests
another resources that cannot be immediately allocated to
it,then all resources currently being held are released

9. 4.Circular wait -- impose a total ordering of resources types and require that each
process requests resources in an increasing order of enumeration.
• 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 resources that it is requesting

10. RECOVERY FROM DEADLOCK
1.PROCESS TERMINATION ->
- Abort all deadlock processes
-Abort one process at a time until deadlock cycle is eliminated
-In partial termination process choosen to abort
o Priority of process
o How long process has completed and how much longer
to completion
o Resources process has used
o How many resources process need to complete

11. 2.RESOURCE PREEMPTION ->
- successively preempt resources from processes and give it to
other processes until deadlock is broken
- issues to be considered
o SELECTING A VICTIM : minimize cost
o ROLL BACK : return to some safe
state,restart process for that state
o STARVATION : same process may always be
picked as victim,include a number of rollback
in cost factor