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GATE CSE 2024 Session 2 Q46: Possible outcomes of two threads using two semaphores

Question

Consider a multi-threaded program with two threads T1 and T2. The threads share two semaphores: s1 (initialized to 1) and s2 (initialized to 0). The threads also share a global variable x (initialized to 0). The threads execute the code shown below.

// code of T1
wait(s1);
x = x+1;
print(x);
wait(s2);
signal(s1);

// code of T2
wait(s1);
x = x+1;
print(x);
signal(s2);
signal(s1);

Which of the following outcomes is/are possible when threads T1 and T2 execute concurrently?

Try it: multiple select: one or more correct options

Select one or more options
Select all correct options. No negative marking, no partial marks.
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Answer: B, C (official GATE 2024 final answer key)

Concept used: Waiting on a semaphore while holding a mutex that the signalling thread needs causes deadlock.

First step: If T1 gets s1 first, it blocks on wait(s2) still holding s1, and T2 can never pass wait(s1).

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Source: official GATE 2024 Session 2 CS question paper and final answer key, organised by IISc Bangalore. Question paper · Answer key

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