The Three Fences

This lesson gives an overview of the acquire, release, and full fences used in C++ as memory barriers.

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Typically, three kinds of fences are used: full fence, acquire fence and release fence. As a reminder, acquire is a load, and release is a store operation. What happens if I place one of the three memory barriers between the four combinations of load and store operations?

  • Full fence: A full fence std::atomic_thread_fence() between two arbitrary operations prevents the reordering of these operations, but guarantees that it won’t hold for StoreLoad operations. Also, they can be reordered.

  • Acquire fence: An acquire fence std::atomic_thread_fence(std::memory_order_acquire) prevents a read operation before an acquire fence from being reordered with a read or write operation after the acquire fence.

  • Release fence: A release fence std::atomic_thread_fence(std::memory_order_release) prevents a read or write operation before a release fence from being reordered with a write operation after a release fence.

A lot of energy goes into accurately forming the definitions of the acquire and release fence and their consequences for lock-free programming. The subtle differences between the acquire-release semantic of atomic operations are especially challenging to understand. Before I get to that point, I will illustrate the definitions with graphics.

Which kind of operations can cross a memory barrier? Have a look at the following three graphics. If the arrow is crossed with a red bar, the fence prevents this type of operation.

Full fence

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