Spinlocks

Spinlocks are a synchronization primitive used to protect shared data in multi-threaded or multi-core systems. They rely on busy-waiting: a thread repeatedly checks a lock flag until it becomes available, instead of sleeping or yielding the processor.

Acquisition is typically implemented with atomic operations such as test-and-set or compare-and-swap. A thread attempting to acquire the lock spins until it observes the lock as free, then atomically sets it to held and proceeds into the critical section. Releasing the lock clears the flag and may include memory barriers to ensure proper ordering of memory operations.

Variants of spinlocks address different concerns. Simple spinlocks use a single boolean flag and can cause cache line bouncing under contention. Ticket spinlocks grant access in FIFO order, providing fairness but with additional overhead. Queue-based spinlocks, such as MCS or CLH locks, place waiting threads in a local queue, reducing cache contention and improving scalability on many-core systems.

Advantages of spinlocks include low overhead for very short critical sections and the avoidance of context switches or sleeping in environments where blocking is expensive or disallowed, such as some kernels or real-time systems. Disadvantages include wasted CPU cycles during contention, potential starvation or unfairness in some variants, cache thrashing, and poor performance for long-held locks or highly contended data. They are generally unsuitable for user-space threads that may sleep, or for workloads with long critical sections.

Implementation considerations include the use of atomic primitives, memory ordering guarantees, and, in interrupt contexts, disabling interrupts to prevent deadlock. Backoff strategies are often employed to reduce contention. Spinlocks remain common in low-level systems and performance-critical paths where blocking is undesirable.