compareAndSet

compareAndSet is a fundamental atomic operation commonly found in concurrent programming. It is used to atomically update a variable to a new value if and only if the variable's current value is equal to a specified expected value. This operation is crucial for implementing lock-free data structures and algorithms where multiple threads might attempt to modify the same shared data simultaneously.

The core idea behind compareAndSet is to avoid the need for traditional locks. Instead of acquiring a lock, modifying the data, and then releasing the lock, compareAndSet allows a thread to attempt an update directly. It first checks if the current value of the variable matches the expected value. If they match, it proceeds to set the variable to the new value. If they do not match, meaning another thread has already modified the variable, the operation fails and the variable remains unchanged.

This atomic nature ensures that the check and the update happen as a single, indivisible operation. No other thread can intervene between the check and the update. If the compareAndSet operation fails, the calling thread typically retries the operation, often in a loop, until it succeeds. This retry mechanism is the cornerstone of many lock-free algorithms. Common implementations of compareAndSet are provided by hardware instructions on many modern processors, making them highly efficient.