On x86_64 reading a 128-bit value requires multiple instructions.
Under some conditions if the counted pointer counter is read before
the item pointer the fifo can be left in an inconsistent state. This
commit forces the read of the counter to always be read first.
The fifo does not appear to suffer from the same race.
It is possible the compiler can reorder the read of the head item and
the head itself. This could lead to a situation where the item
returned was not really the head item.
- Rename opal_atomic_lifo_t to opal_lifo_t to reflect both atomic and
non-atomic usage. Added new routines (opal_lifo_*_st) for non-atomic
usage as well as routines conditioned off opal_using_threads(). The
atomic versions are always thread safe and the non-atomic are always
not thread safe.
- Add a new atomic lifo implementation that makes use of 128-bit
compare-and-swap. The new implementation should scale better with
larger numbers of threads.
- Add threading unit test for opal_lifo_t.
