Ordering without a value
A fence, also called a memory barrier, is an operation that restricts reordering of memory accesses around it without itself reading or writing a shared location. Where acquire release attach ordering to a specific atomic access, a fence is a standalone ordering point.
Kinds of fences
Hardware and languages expose several flavors, conceptually controlling which pairs of access types may cross.
- A load load or acquire fence stops later loads from moving above it.
- A store store or release fence stops earlier stores from moving below it.
- A full fence orders all access types in both directions and is the most expensive.
Fence versus atomic order
A standalone fence can be paired with relaxed atomic operations to recreate acquire release semantics, which is useful when you want one fence to order several accesses at once rather than tagging each. The tradeoff is that fences are coarser: they order everything of the relevant kind, not just one variable, so they can be heavier than a targeted acquire or release.
Key idea
A fence is a standalone barrier that orders classes of memory accesses around it, and combined with relaxed atomics it can substitute for per operation acquire release at a coarser granularity.