Deadlock occurs when a set of processes are blocked because each process is holding a resource and waiting for another resource held by another process, forming a circular chain. There are four conditions for deadlock: mutual exclusion, where a resource can only be held by one process; hold and wait, where a process can hold resources while requesting more; no preemption, where a resource cannot be forcibly removed from a process; and circular wait, where there is a circular chain of processes each waiting on a resource held by the next process. An example is given of a traffic deadlock where cars block each other in a circular formation on a single-lane road.

1. Unit 3 – Part 1
Deadlock

2. Deadlock
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). 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.

4. Deadlock can arise if following four
Coffman Conditions
A deadlock occurs if the four Coffman conditions
hold true. But these conditions are not mutually
exclusive.
The Coffman conditions are given as follows
• Mutual Exclusion
• Hold and Wait
• No Preemption
• Circular Wait

5. Mutual Exclusion
There should be a resource that can only be held by one
process at a time. In the diagram below, there is a single
instance of Resource 1 and it is held by Process 1 only.
The resources involved are non-shareable
Explanation: At least one resource (thread) must be held
in a non-shareable mode. that is, only one process at a
time claims exclusive control of the resource.
If another process requests that resource, the requesting
process must be delayed until the resource has been
released.

6. Hold and Wait
A process can hold multiple resources and still
request more resources from other processes which
are holding them. In the diagram given below,
Process 2 holds Resource 2 and Resource 3 and is
requesting the Resource 1 which is held by Process 1.

7. No Preemption
A resource cannot be preempted from a process by force.
A process can only release a resource voluntarily. In the
diagram below, Process 2 cannot preempt Resource 1
from Process 1. It will only be released when Process 1
give it up voluntarily after its execution is complete.

8. Circular Wait
A process is waiting for the resource held by the
second process, which is waiting for the resource
held by the third process and so on, till the last
process is waiting for a resource held by the first
process. This forms a circular chain. For example:
Process 1 is allocated Resource2 and it is requesting
Resource 1. Similarly, Process 2 is allocated Resource
1 and it is requesting Resource 2. This forms a
circular wait loop.

9. As an example, consider the traffic
deadlock in the following figure

10. Consider each section of the street as a resource.
• Mutual exclusion condition applies, since only one
vehicle can be on a section of the street at a time.
• Hold-and-wait condition applies, since each vehicle is
occupying a section of the street, and waiting to move
on to the next section of the street.
• No-preemptive condition applies, since a section of the
street that is a section of the street that is occupied by a
vehicle cannot be taken away from it.
• Circular wait condition applies, since each vehicle is
waiting on the next vehicle to move. That is, each
vehicle in the traffic is waiting for a section of street
held by the next vehicle in the traffic.