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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 pseudo code 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

fails as L can overflow

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

works correctly but may starve some processes

works correctly without starvation