Q11a

A system of four concurrent processes, $P, Q, R$ and $S$, use shared resources $A, B$ and $C$. The sequences in which processes, $P, Q, R$ and $S$ request and release resources are as follows:

$$\begin{array}{ll} \text{ Process P: } & \text{1.}& \text{ P requests A } && \\ \text{ } & \text{2.}& \text{ P requests B } && \\ \text{ } & \text{3.}& \text{ P releases A } &&\\ \text{} & \text{4.}& \text{ P releases B } && \\ \text{ Process Q: } & \text{1.}& \text{ Q requests C } && \\ \text{ } & \text{2.}& \text{ Q requests A } && \\ \text{ } & \text{3.}& \text{ Q releases C } &&\\ \text{} & \text{4.}& \text{ P releases A } &&\\ \text{ Process R: } & \text{1.}& \text{ R requests B } \\ \text{ } & \text{2.}& \text{ R requests C } \\ \text{ } & \text{3.}& \text{ R releases B } &&\\ \text{} & \text{4.}& \text{ R releases C } &&\\ \text{ Process S: } & \text{1.}& \text{ S requests A } && \\ \text{ } & \text{2.}& \text{ S requests C } && \\ \text{ } & \text{3.}& \text{ S releases A } &&\\ \text{} & \text{4.}& \text{ S releases C } && \end{array}$$

If a resource is free, it is granted to a requesting process immediately. There is no preemption of granted resources. A resource is taken back from a process only when the process explicitly releases it. Can the system of four processes get into a deadlock? If yes, give a sequence (ordering) of operations (for requesting and releasing resources) of these processes which leads to a deadlock. Will the processes always get into a deadlock? If your answer is no, give a sequence of these operations which leads to completion of all processes. What strategies can be used to prevent deadlocks in a system of concurrent processes using shared resources if preemption of granted resources is not allowed?