Explain Read-Copy-Update (RCU) synchronization.

RCU is a synchronization mechanism optimized for read-heavy workloads. Readers access data without locks (fast). Writers create modified copy, update pointer atomically, wait for all current readers to finish (grace period), then free old version. Readers are never blocked by writers. Used extensively in Linux kernel for routing tables, firewall rules. Challenges: determining grace period (quiescent state-based), memory overhead of keeping old versions. Provides excellent read scalability.