Deadlocks occur when a set of processes are blocked because each process is holding a resource and waiting for another resource held by another process in the set. For example, two processes P1 and P2 each hold one of two disk drives and each needs the other disk drive. Deadlock can arise when four conditions hold simultaneously: mutual exclusion, hold and wait, no preemption, and circular wait. A resource-allocation graph with vertices for processes and resources and edges for requests and assignments can model resource usage and identify if a deadlock cycle exists. The presence of a cycle indicates the potential for deadlock.

1. Deadlocks
Prepared By: Mr. Sangram A. Patil
Assistant Professor PVPIT,Budhgaon

2.  Definition
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
process.
 Example
 System has 2 disk drives.
 P1 and P2 each hold one disk drive and each needs another one.
 Example
 semaphores A and B, initialized to 1
P0
wait (A);
wait (B);
P1
wait(B)
wait(A)

3. System Model
 System consists of
1. Finite number of resources shared between number of competing processes.
Resources are partitioned into several types such as CPU cycles, files and IO
devices(printers and drives) .
Each resource type consisting of several instances.
2. process must request a resource before using it and release a resource after using it.
process can request as many as resources required to it to carry out designated
task
number of resource request cannot exceed.
Process request a resource allocation of any instance of resource satisfies the
request.

4.  Each process utilizes a resource as follows:
 request
 use
 release

5. Deadlock Characterization
 Deadlock can arise if four conditions hold simultaneously.
1. Mutual exclusion: only one process at a time can use a resource.
2. Hold and wait: a process holding at least one resource is waiting to
acquire additional resources held by other processes.
3. No preemption: resources can not be preempted a resource can be
released only voluntarily by the process holding it, after that process has
completed its task.
4. Circular wait: there exists a set {P0, P1, …, Pn} of waiting processes such
that P0 is waiting for a resource that is held by P1, P1 is waiting for a resource
that is held by P2, …, Pn–1 is waiting for a resource that is held by Pn, and Pn is
waiting for a resource that is held by P0.

6. Resource-Allocation Graph
 A set of vertices V and a set of edges E.
 V is partitioned into two types:
 P = {P1, P2, …, Pn}, the set consisting of all active processes in the
system.
 R = {R1, R2, …, Rm}, the set consisting of all resource types in the system.
 request edge – directed edge Pi  Rj
 assignment edge – directed edge Rj  Pi

7. Resource-Allocation Graph (Cont.)

8. Example of a Resource Allocation Graph
• Consists of sets P,R,E
P={P1,P2,P3}
R= {R1,R2,R3,R4}
E={P1->R1,P2->R3,R1->P2,R2->P2,R2->P1,
R3->P3}
• Resource instances:
• Process states
If graph contains no cycle then no process in the
system is deadlocked
If graph does contain a cycle then deadlock may
exists.

9. Resource Allocation Graph With A
Deadlock
 Two cycles
P1->R1->P2->R3->P3->R2->P1
P2->R3->P3->R2->P2

10. Graph With A Cycle But No Deadlock
 There is cycle but no deadlock
P1->R1->P3->R2->P1

11. Basic Facts
 If graph contains no cycles  no deadlock.
 If graph contains a cycle 
 if only one instance per resource type, then deadlock.
 if several instances per resource type, possibility of deadlock.