The RCU (Read-Copy-Update) mechanism is a synchronization technique used in the Linux kernel to improve the performance and scalability of shared data structures that are frequently read but infrequently modified. The RCU mechanism allows multiple threads to concurrently read a shared data structure without any locking, while still allowing for efficient updates.
The basic idea behind RCU is to maintain multiple copies of the shared data structure, each corresponding to a different version of the data. Whenever a thread wants to modify the data structure, it creates a new version of the data structure and updates a pointer to the new version. Threads that are still using the old version of the data structure can continue to do so until they finish reading, at which point they switch to the new version. This allows for highly concurrent reads and infrequent updates.
RCU works by maintaining three different states for each version of the data structure: active, grace period, and quiescent. When a thread modifies the data structure, it creates a new version and sets the old version to the grace period state. Threads that are currently reading the old version can continue to do so during the grace period, but new threads will use the new version. Once the grace period has passed and all threads have finished reading the old version, the old version is freed and enters the quiescent state.
The RCU mechanism is used extensively in the Linux kernel for shared data structures, such as per-CPU data structures, file descriptor tables, and network socket tables. RCU allows for high levels of concurrency and scalability, without the need for locking and with minimal overhead. The RCU mechanism is implemented in the kernel using a set of specialized macros and APIs, such as rcu_read_lock(), rcu_read_unlock(), synchronize_rcu(), and call_rcu().
One important consideration when using RCU is the need for careful design of the shared data structures and their update paths. RCU works best when updates are infrequent and can be made in a way that avoids contention with readers. Additionally, RCU requires careful memory management, as each version of the data structure must be allocated and freed correctly to prevent memory leaks and corruption.