GATE Technical: Computer Science and Information Tech - CS questions with solutions

Q1. Fetch_And_Add(X,i) is an atomic Read-Modify-Write instruction that reads the value of memory location X, increments it by the value i, and returns the old value of X. It is used in the pseudocode shown below to implement a busy-wait lock. L is an unsigned integer shared variable initialized to 0. The value of 0 corresponds to lock being available, while any non-zero value corresponds to the lock being not available. AcquireLock(L){ while (Fetch_And_Add(L,1)) L = 1; } ReleaseLock(L){ L = 0; } This implementation

1. fails as L can overflow
2. fails as L can take on a non-zero value when the lock is actually available
3. works correctly but may starve some processes
4. works correctly without starvation

Answer: fails as L can take on a non-zero value when the lock is actually available

The implementation fails because the Fetch_And_Add operation increments L even when it is already non-zero, which means that the lock can appear to be held (non-zero) even when it is actually available (zero). This can lead to situations where a process believes the lock is not available when it is, causing incorrect behavior.