Atomic Operations

2 articles

Software Engineering 17 Sep 2026 4 min read

renameat2 RENAME_EXCHANGE Swaps Two Paths in One Filesystem Operation

renameat2() with RENAME_EXCHANGE changes two existing directory entries as one atomic rename operation. Before the call, each pathname reaches its original object; after a successful call, each pathname reaches the object formerly named by the other path. There is no successful intermediate state in which one of the two names has merely been removed or overwritten. That property is distinct from ordinary rename(). A conventional rename can atomically replace a destination, but replacement discards the destination name from the namespace. Exchange preserves both named objects and swaps their positions.

Software Engineering Linux

Software Engineering 12 Sep 2026 8 min read

The ABA Problem Behind Successful Compare-and-Swap

A compare-and-swap operation answers a narrow question: does a memory location contain the expected bit pattern at the instant of the atomic operation? If it does, the replacement can proceed. The operation does not establish that the location remained unchanged between an earlier read and the later comparison. That distinction creates the ABA problem. A thread observes value A, pauses, and later performs a compare-and-swap expecting A. During the pause, other work changes the location from A to B and then back to A. The comparison succeeds because the current representation matches the expected representation, even though the shared state passed through a transition that may invalidate assumptions attached to the first observation.

Software Engineering Concurrency