Deadlock and cycle in a resource allocation graph

Question

Here is a resource allocation graph asked in my Operating Systems Theory midterm. The question is, "Is there a deadlock here? Explain your answer in detail"

Ra and Rb are resource sets and every dot inside of them are resources. Circles are processes. An arrow from process to a resource set means that process is requesting a resource from that set. An arrow from resource set to process means that process owns a resource from that resource set.

I want to have your opinions on this, because the lecturer's answer is conflicting with mine. Lecturer says there is a deadlock here. But my answer was, since Py and Pz are not requesting a resource, they will simply continue their execution and terminate, releasing their resources. Then Px and Pw can obtain their requested resources and keep executing. It is obvious there is a cycle in this graph as Px-Pw but this doesn't conclude us to a deadlock. Thus I can't see a way to make "there is a deadlock here"conclusion.

So is there a deadlock here?

Answer 1

I did some thinking and then searching and all RAG conditions of this example leads to conclusion that there is no deadlock (altho there is a cycle here).

Beside after searching I found this where you can find this:

Which clearly stands Graph With A Cycle But No Deadlock

Are you sure that you have your graph correct here?

Answer 2

Here no deadlock occurs because p2 and p4 are holding a resource. So they can release it so that p1 or p3 can acquire it to avoid deadlock

Answer 3

Your analysis makes sense, and there is no deadlock.

Some basic facts about RAG:

If the graph contains no cycles, then no deadlock

If the graph contains a cycle:

• If only one instance per resource type, then deadlock
• If several instances per resource type, there is the possibility of deadlock

Hence it is inappropriate to conclude a deadlock simply because of cycle, when resources hold mutiple instances.

Answer 4

By all means, your answer is correct: judging by the RAG, Py and Pz have all resources they need to complete; so, once they finish, there will be resources available to Px and Pw finish their own tasks. Therefore, there is no deadlock. Simple like that.

Please, check this answer, provided by Neelesh Shukla.

The only possibility for your answer become wrong: the graph the lecturer referred to is NOT the one depicted here!!

Answer 5

there is no deadlock because there are more than one instance per resource type even though there is cycle in the graph.