RCU stands for Read-Copy-Update, which is a powerful synchronization mechanism used in the Linux kernel. Unlike traditional locking mechanisms that block all access during updates, RCU allows multiple readers to access data structures concurrently without any locks. When a writer needs to update the data, it creates a new version while readers continue accessing the old version safely. This approach significantly improves performance in read-heavy workloads common in operating systems.
RCU operates through three fundamental phases that work together to provide safe concurrent access. First, the read-side critical sections allow multiple readers to access data structures without any locking overhead. Second, the update phase enables writers to create new versions of data while readers continue accessing the old version. Finally, the grace period mechanism ensures that all existing readers have completed their access before the old data is safely freed. This three-phase approach is what makes RCU so efficient for read-heavy workloads.
The grace period mechanism is the heart of RCU's safety guarantee. It works by tracking when all CPUs in the system have passed through a quiescent state. A quiescent state occurs during context switches, system call returns, idle periods, or interrupt handling. The kernel maintains a record of each CPU's quiescent state and only declares the grace period complete when all CPUs have reported at least one quiescent state. This ensures that no reader can still be accessing the old data structure, making it safe to free the memory. The grace period detection is implemented efficiently across multiple CPUs using per-CPU data structures and careful synchronization.
The RCU API provides several key functions that map directly to the three phases we discussed. For readers, rcu_read_lock and rcu_read_unlock mark the boundaries of read-side critical sections, while rcu_dereference safely accesses RCU-protected pointers. For writers, rcu_assign_pointer publishes new data structures atomically. The synchronize_rcu function blocks until a grace period completes, ensuring all readers have finished. Alternatively, call_rcu schedules a callback to run after the grace period, allowing asynchronous cleanup. These functions work together to provide the lock-free synchronization that makes RCU so efficient for read-heavy workloads in the Linux kernel.
The complete RCU processing flow demonstrates how all the components work together seamlessly. The process begins when readers enter their critical sections and access RCU-protected data without any locking. Simultaneously, writers can create new versions of data structures and publish them using rcu_assign_pointer. The grace period mechanism then tracks all CPUs to ensure they reach quiescent states. Once the grace period completes, the system can safely reclaim the old memory. This entire workflow allows multiple readers and writers to operate concurrently, making RCU extremely efficient for scenarios where reads vastly outnumber writes, which is common in operating system kernels.