LLX

LLX refers to a set of lock-free synchronization primitives used in concurrent data structures to obtain a consistent view of an object in the presence of other threads performing updates. The LLX family, often described together with SCX, is designed to support atomic multi-field updates without locking, enabling higher concurrency for complex structures such as trees and linked data structures.

An LLX operation attempts to read a node and produce a snapshot that reflects a point-in-time view of its fields. If the node or its related structure has been part of an in-progress update, the LLX may fail or return a result that indicates the modification. In many designs, the operation can temporarily “freeze” the node to prevent concurrent changes while the snapshot is validated.

LLX is typically used in conjunction with SCX (store-conditional extended) to perform atomic multi-field updates. After successfully obtaining valid LLX results on a set of nodes, an SCX can atomically replace selected fields, conditional on the LLX results remaining valid. This pairing enables complex updates to occur as if they were atomic, without the use of locks.

Applications of LLX/SCX include non-blocking implementations of data structures such as concurrent trees and linked structures where updates affect several pointers. The approach emphasizes high concurrency and predictable performance under contention, albeit with implementation complexity and varying practical performance depending on workload and hardware.

In summary, LLX is a theoretical and practical tool in lock-free synchronization used to read consistent snapshots and, together with SCX, to perform atomic multi-field updates in non-blocking data structures. It appears mainly in academic literature and specialized libraries focused on lock-free algorithms.