Non-Blocking Synchronization: Lock-Free Data Structures and Atomic CAS Primitives

NON-BLOCKING SYNCHRONIZATION: LOCK-FREE DATA STRUCTURES AND ATOMIC CAS PRIMITIVES To prevent data corruption in multi-threaded programs, developers traditionally use locks (mutexes) to synchronize access to shared memory. However, locks introduce overhead, priority inversion, and the risk of deadlocks. Non-blocking synchronization avoids these issues by implementing lock-free data structures using atomic CPU primitives. COMPARE-AND-SWAP (CAS) PRIMITIVES The foundation of lock-free programming is the atomic Compare-and-Swap instruction, supported at the hardware level by modern CPUs (e.g., CMPXCHG on x86). CAS takes three arguments: - Address (A) of the variable. - Expected old value (O). - New value (N). The CPU checks if the value at address A equals O. - If YES, it writes N to address A and returns true. - If NO, it leaves the memory unchanged and returns false. This comparison and write are executed as a single, indivisible atomic hardware instruction. THE LOCK-FREE STACK IMPLEMENTATION Consider pushing a node onto a lock-free stack. Instead of locking the stack pointer: 1. Read the current head of the stack (Expected Head). 2. Set the new node's next pointer to this Expected Head. 3. Call CAS(&stack_head, Expected Head, New Node). 4. If CAS returns true, the push is successful. If false (meaning another thread modified the stack head in the meantime), repeat from step 1. THE ABA PROBLEM A primary challenge in lock-free programming is the ABA problem. This occurs when a thread reads a value A from an address, another thread changes the value to B and then back to A, and the original thread executes CAS, succeeding because the value is still A. However, the internal state of the stack may have changed, leading to memory corruption. The ABA problem is mitigated by using double-width CAS to write a version tag (or sequence number) alongside the pointer, ensuring uniqueness for every modification. Reference: Ray Dawson, "Programming in ANSI C", Section 10: Pointers & CPU operations.