Operating Systems | Process Management | Question 6
A shared variable x, initialized to zero, is operated on by four concurrent processes W, X, Y, Z as follows. Each of the processes W and X reads x from memory, increments by one, stores it to memory, and then terminates. Each of the processes Y and Z reads x from memory, decrements by two, stores it to memory, and then terminates. Each process before reading x invokes the P operation (i.e., wait) on a counting semaphore S and invokes the V operation (i.e., signal) on the semaphore S after storing x to memory. Semaphore S is initialized to two. What is the maximum possible value of x after all processes complete execution? (GATE CS 2013)
Explanation: Processes can run in many ways, below is one of the cases in which x attains max value
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Semaphore S is initialized to 2 Process W executes S=1, x=1 but it doesn't update the x variable. Then process Y executes S=0, it decrements x, now x= -2 and signal semaphore S=1 Now process Z executes s=0, x=-4, signal semaphore S=1 Now process W updates x=1, S=2 Then process X executes X=2
So correct option is (D).
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