Q48

Consider two processes $P_1$ and $P_2$ accessing the shared variables $X$ and $Y$ protected by two binary semaphores $S_X$ and $S_Y$ respectively, both initialized to 1. $P$ and $V$ denote the usual semaphore operators, where $P$ decrements the semaphore value, and $V$ increments the semaphore value. The pseudo-code of $P_1$ and $P_2$ is as follows:

$$\begin{array}{|l|l|} \hline P_1: & P_2: \\ \hline \text{While true do \{} & \text{While true do \{} \\ L_1:\dots\dots & L_3:\dots\dots \\ L_2:\dots\dots & L_4:\dots\dots \\ \text{X = X + 1;} & \text{Y = Y + 1;} \\ Y = Y - 1; & X = Y - 1; \\ V(S_X); & V(S_Y); \\ V(S_Y); & V(S_X); \\ \} & \} \\ \hline \end{array}$$

In order to avoid deadlock, the correct operators at $L_1$, $L_2$, $L_3$ and $L_4$ are respectively.