[v3,0/5] Add non-blocking ring

  For some users, the rte ring's "non-preemptive" constraint is not acceptable;
for example, if the application uses a mixture of pinned high-priority threads
and multiplexed low-priority threads that share a mempool.

This patchset introduces a non-blocking ring, on top of which a mempool can run.
Crucially, the non-blocking algorithm relies on a 128-bit compare-and-swap, so
it is currently limited to x86_64 machines. This is also an experimental API,
so RING_F_NB users must build with the ALLOW_EXPERIMENTAL_API flag.

The ring uses more compare-and-swap atomic operations than the regular rte ring:
With no contention, an enqueue of n pointers uses (1 + 2n) CAS operations and a
dequeue of n pointers uses 2. This algorithm has worse average-case performance
than the regular rte ring (particularly a highly-contended ring with large bulk
accesses), however:
- For applications with preemptible pthreads, the regular rte ring's worst-case
  performance (i.e. one thread being preempted in the update_tail() critical
  section) is much worse than the non-blocking ring's.
- Software caching can mitigate the average case performance for ring-based
  algorithms. For example, a non-blocking ring based mempool (a likely use case
  for this ring) with per-thread caching.

The non-blocking ring is enabled via a new flag, RING_F_NB. For ease-of-use,
existing ring enqueue/dequeue functions work with both "regular" and
non-blocking rings.

This patchset also adds non-blocking versions of ring_autotest and
ring_perf_autotest, and a non-blocking ring based mempool.

This patchset makes one API change; a deprecation notice will be posted in a
separate commit.

This patchset depends on the non-blocking stack patchset[1].

[1] http://mails.dpdk.org/archives/dev/2019-January/123653.html

v3:
 - Avoid the ABI break by putting 64-bit head and tail values in the same
   cacheline as struct rte_ring's prod and cons members.
 - Don't attempt to compile rte_atomic128_cmpset without
   ALLOW_EXPERIMENTAL_API, as this would break a large number of libraries.
 - Add a helpful warning to __rte_ring_do_nb_enqueue_mp() in case someone tries
   to use RING_F_NB without the ALLOW_EXPERIMENTAL_API flag.
 - Update the ring mempool to use experimental APIs
 - Clarify that RINB_F_NB is only limited to x86_64 currently; ARMv8.1-A builds
   can eventually support it with the CASP instruction.

v2:
 - Merge separate docs commit into patch #5
 - Convert uintptr_t to size_t
 - Add a compile-time check for the size of size_t
 - Fix a space-after-typecast issue
 - Fix an unnecessary-parentheses checkpatch warning
 - Bump librte_ring's library version

Gage Eads (5):
  ring: add 64-bit headtail structure
  ring: add a non-blocking implementation
  test_ring: add non-blocking ring autotest
  test_ring_perf: add non-blocking ring perf test
  mempool/ring: add non-blocking ring handlers

 doc/guides/prog_guide/env_abstraction_layer.rst |   2 +-
 drivers/mempool/ring/Makefile                   |   1 +
 drivers/mempool/ring/meson.build                |   2 +
 drivers/mempool/ring/rte_mempool_ring.c         |  58 ++-
 lib/librte_eventdev/rte_event_ring.h            |   2 +-
 lib/librte_ring/Makefile                        |   3 +-
 lib/librte_ring/rte_ring.c                      |  72 ++-
 lib/librte_ring/rte_ring.h                      | 574 ++++++++++++++++++++++--
 lib/librte_ring/rte_ring_generic_64.h           | 152 +++++++
 lib/librte_ring/rte_ring_version.map            |   7 +
 test/test/test_ring.c                           |  57 ++-
 test/test/test_ring_perf.c                      |  19 +-
 12 files changed, 874 insertions(+), 75 deletions(-)
 create mode 100644 lib/librte_ring/rte_ring_generic_64.h