This document discusses deadlocks in operating systems. It defines a deadlock as when a set of processes are blocked because each process is holding a resource and waiting for another resource held by another process. Four conditions must be met simultaneously for a deadlock to occur: mutual exclusion, hold and wait, no preemption, and circular wait. An example of a deadlock between two processes, Process 1 and Process 2, is provided. Methods for handling deadlocks include prevention/avoidance, detection and recovery, and ignoring the problem. Prevention negates one of the necessary conditions, while avoidance uses strategies like Banker's algorithm to ensure deadlocks are avoided.

1. DEADLOCK IN OPERATING
SYSTEMS
BY
S.JAMUNA
ASSISTANT PROFESSOR
DEPARTMENT OF COMPUTER APPLICATIONS
BON SECOURS COLLEGE FOR WOMEN
THANJAVUR.

2. DEADLOCK IN OPERATING SYSTEMS
• A process in operating systems uses different
resources and uses resources in following way.
1) Requests a resource
2) Use the resource
2) Releases the resource

3. DEADLOCK IN OPERATING SYSTEMS
Deadlock is a situation where a set of processes are blocked
because each process is holding a resource and waiting for
another resource acquired by some other process.
-Consider an example when two trains are coming toward
each other on same track and there is only one track, none of
the trains can move once they are in front of each other.
-Similar situation occurs in operating systems when there are
two or more processes hold some resources and wait for
resources held by other(s).

4. DEADLOCK IN OPERATING SYSTEMS
For example, in the below diagram, Process 1 is holding Resource 1 and waiting for
resource 2 which is acquired by process 2, and process 2 is waiting for resource 1.

5. DEADLOCK IN OPERATING SYSTEMS
Deadlock can arise if following four conditions hold
simultaneously (Necessary Conditions)
Mutual Exclusion: One or more than one resource are non-
sharable (Only one process can use at a time)
Hold and Wait: A process is holding at least one resource and
waiting for resources.
No Pre-emption: A resource cannot be taken from a process
unless the process releases the resource.
Circular Wait: A set of processes are waiting for each other in
circular form.

6. DEADLOCK IN OPERATING SYSTEMS
Methods for handling deadlock
There are three ways to handle deadlock
1) Deadlock prevention or avoidance: The idea is to not let the system
into deadlock state.
One can zoom into each category individually, Prevention is done by
negating one of above mentioned necessary conditions for deadlock.
Avoidance is kind of futuristic in nature.
By using strategy of “Avoidance”, we have to make an assumption. We
need to ensure that all information about resources which process will
need are known to us prior to execution of the process.
We use Banker’s algorithm (Which is in-turn a gift from Dijkstra) in
order to avoid deadlock.

7. DEADLOCK IN OPERATING SYSTEMS
2) Deadlock detection and recovery: Let deadlock occur, then do
pre-emption to handle it once occurred.
3) Ignore the problem all together: If deadlock is very rare, then
let it happen and reboot the system.

8. DEADLOCK IN OPERATING SYSTEMS
Deadlock Starvation
1 Deadlock is a situation where no process got
blocked and no process proceeds
Starvation is a situation where the low priority process
got blocked and the high priority processes proceed.
2 Deadlock is an infinite waiting. Starvation is a long waiting but not infinite.
3 Every Deadlock is always a starvation. Every starvation need not be deadlock.
4 The requested resource is blocked by the other
process.
The requested resource is continuously be used by the
higher priority processes.
5 Deadlock happens when Mutual exclusion, hold
and wait, No preemption and circular wait
occurs simultaneously.
It occurs due to the uncontrolled priority and resource
management.
Difference between Starvation and
Deadlock

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