Unit 3 part 1(DEADLOCK)
- 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.