[tveita]

The Linux kernel seems to leave one element free, which surprised me, but it does have this interesting note about it:

https://www.kernel.org/doc/Documentation/circular-buffers.tx...

  Note that wake_up() does not guarantee any sort of barrier unless something
  is actually awakened.  We therefore cannot rely on it for ordering.  However,
  there is always one element of the array left empty.  Therefore, the
  producer must produce two elements before it could possibly corrupt the
  element currently being read by the consumer.  Therefore, the unlock-lock
  pair between consecutive invocations of the consumer provides the necessary
  ordering between the read of the index indicating that the consumer has
  vacated a given element and the write by the producer to that same element.