Q27

Consider the following transactions with data items $P$ and $Q$ initialized to zero:

$${\begin{array}{|c|l|r|c|}\hline \textbf{$T_1$}& \text{read (P);}\\ & \text{read (Q);} \\ & \text{if P = 0 then Q := Q + 1; }\\ & \text{write (Q)} \\ \hline \textbf{$T_2$}& \text{read (Q);}\\& \text{read (P);} \\ & \text{if Q = 0 then P := P + 1;} \\ & \text{write (P)} \\ \hline \end{array}}$$

Any non-serial interleaving of T1 and T2 for concurrent execution leads to