Question

Consider a monitor with 5 condition variables, C1-C5. Assume that no process has entered the monitor yet and C1-C5 are free. Briefly describe, the effect of the following sequence of five operations by processes P1-P2. State any assumption you make. (In other words, for each operation in the sequence, state what happens on its execution, given the state of the monitor at that time.) Assume that there are no other processes in the system except P1 and P2. P1: C1.wait(); P2: C2.wait(); P1: C2.wait(); P2: C2.signalo; P1: C3.wait();

Answer 1

There are only two processes in the system which are P1 and P2. And we have 5 condition variables C1-C5.
We will start with the given sequence of operations.

P1 : C1.wait(); : As it is given that all the condition variables are free, this wait operation will acquire the lock related to C1 variable and allow the the process to P1 to use the resource that it requires.

P2 : C2.wait(); : This will acquire the lock of C2 variable and allow P2 to use the required resources.

P1 : C2.wait(); : Due to the fact that process P2 just acquired this lock, process P1 will not be able to do the same, unless someone performs a signal(). So the P1 process will now have to wait until this lock is released so it will go to sleep.
(Signal() is used to release the acquired lock so that the process which went to sleep would wake up and use the resources by acquiring this lock.)

P2 : C2.signal(); : This will release the lock of C2 variable the process which are in queue for this lock can acquire it. Here process P1 needed this lock, which is currently put to sleep. This sleep() will wake up P1 so that it can continue with its execution.

P1 : C3.wait(); : This will simply acquire the lock of another condition variable which is C2 and allow P1 to use the required resources.